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{{Short description|Class of echinoderms, marine animal}}
{{Redirect2|Seastar|Sea Star||SeaStar (disambiguation)|and| Starfish (disambiguation)}}
{{Redirect|Asteroidea|the Asteraceae subfamily|Asteroideae}}
{{Featured article}}
{{Protection padlock|small=yes}}
{{Use Oxford spelling|date = August 2018}}
{{Use dmy dates|date=October 2020}}
{{Automatic taxobox
| fossil_range = {{Fossil range|Ordovician|Recent}}
| image = Starfish montage.png
| image_caption = Clockwise from top left: ''[[Linckia laevigata]]'', ''[[Echinaster serpentarius]]'', ''[[Protoreaster nodosus]]'', and ''[[Hymnaster pellucidus]]''. Not to scale.
| taxon = Asteroidea
| authority = [[Henri Marie Ducrotay de Blainville|Blainville]], 1830
| subdivision_ranks = Child taxa and [[Order (biology)|orders]]
| subdivision = *Infraclass [[Concentricycloidea]]
**[[Xyloplax|Peripodida]]
*Superorder [[Forcipulatacea]]
**[[Brisingida]]
**[[Forcipulatida]]
*Superorder [[Spinulosacea]]
**[[Spinulosida]]
*Superorder [[Valvatacea]]
**[[Notomyotida]]
**[[Paxillosida]]
**[[Valvatida]]
*[[Velatida]]<ref name="Asterozoa">{{cite web |url=http://palaeo.gly.bris.ac.uk/Palaeofiles/Fossilgroups/asteroz2/index_f/mod_fm.html |date=22 November 2005 |title=Fossil Groups: Modern forms: Asteroids: Extant Orders of the Asteroidea |publisher=University of Bristol |last=Sweet |first=Elizabeth |access-date=2016-05-31 |archive-url=https://web.archive.org/web/20070714073619/http://palaeo.gly.bris.ac.uk/Palaeofiles/Fossilgroups/asteroz2/index_f/mod_fm.html |archive-date=14 July 2007 }}</ref>
† [[Calliasterellidae]] <br/>
† [[Trichasteropsida]]<ref name=Knott/>
| diversity = 1,900+ species
}}

'''Starfish''' or '''sea stars''' are a class of [[marine invertebrates]] generally shaped like a [[star polygon]]. (In common usage, these names are also often applied to [[brittle star|ophiuroids]], which are correctly referred to as [[brittle star]]s or basket stars.) Starfish are also known as '''asteroids''' because they form the [[Class (biology)|taxonomic class]] '''Asteroidea''' ({{IPAc-en|ˌ|æ|s|t|ə|ˈ|r|ɔɪ|d|i|ə}}). About 1,900 species of starfish live on the [[seabed]], and are found in all the world's [[ocean]]s, from warm, [[tropics|tropical zones]] to frigid, [[polar regions of Earth|polar regions]]. They can occur from the [[intertidal zone]] down to [[abyssal zone|abyssal]] depths, at {{cvt|6000|m|ft}} below the surface.

Starfish are [[echinoderm]]s and typically have a central disc and usually five arms, though some species have a larger number of arms. The aboral or upper surface may be smooth, granular or spiny, and is covered with overlapping plates. Many species are brightly coloured in various shades of red or orange, while others are blue, grey or brown. Starfish have [[tube feet]] operated by a [[water vascular system|hydraulic system]] and a mouth at the centre of the oral or lower surface. They are [[opportunistic]] feeders and are mostly [[Predation|predators]] on [[benthic invertebrate]]s. Several species have specialized feeding behaviours including eversion of their stomachs and [[suspension feeding]]. They have complex [[biological life cycle|life cycles]] and can reproduce both sexually and [[asexual reproduction in starfish|asexually]]. Most can [[regeneration (biology)|regenerate]] damaged parts or lost arms and they can shed arms as a means of defense.

The Asteroidea occupy several significant [[ecological niche|ecological roles]]. Some, such as the [[ochre sea star]] (''Pisaster ochraceus'') and the [[Stichaster australis|reef sea star]] (''Stichaster australis''), serve as [[keystone species]], with an outsize impact on their environment. The tropical [[crown-of-thorns starfish]] (''Acanthaster planci'') is a voracious predator of [[coral]] throughout the Indo-Pacific region, and the [[Northern Pacific seastar]] is on a list of the [[100 of the World's Worst Invasive Alien Species|Worst Invasive Alien Species]].

The [[fossil]] record for starfish is ancient, dating back to the [[Ordovician]] period around 450 million years ago, but it is rather sparse, as starfish tend to disintegrate after death. Only the [[Ossicle (echinoderm)|ossicles]] and spines of the animal are likely to be preserved, making remains hard to locate. With their appealing symmetrical shape, starfish have played a part in literature and legend. They are sometimes collected as [[Collectable|curios]], used in design or as logos, and in some cultures they are eaten.

==Anatomy==
Most starfish have five arms that radiate from a central disc, but the number varies with the group. Some species have six or seven arms and others have 10–15 arms.<ref name="arms">{{cite arXiv |eprint=1202.2219|last1=Wu|first1=Liang|title=The advantages of the pentameral symmetry of the starfish|last2=Ji|first2=Chengcheng|last3=Wang|first3=Sishuo|last4=Lv|first4=Jianhao|class=q-bio.PE|year=2012}}</ref> In Antarctic ''[[Labidiaster annulatus]],'' the number of arms can exceed fifty.<ref>{{cite book|author=Prager, Ellen|year=2011|title=Sex, Drugs, and Sea Slime: The Oceans' Oddest Creatures and Why They Matter|publisher=University of Chicago Press|page=74|isbn=978-0-2266-7872-6}}</ref> Evidence from [[gene expression]] finds that the starfish body corresponds to a head externally (with lips attached to the tube feet) and a [[torso]] internally.<ref>{{cite journal|last1=Lacalli|first1=T|year=2023|title=A radical evolutionary makeover gave echinoderms their unusual body plan|journal=Nature|volume=623|issue=7987|pages=485–486|doi=10.1038/d41586-023-03123-1|bibcode=2023Natur.623..485L}}</ref> Starfish possess two [[Blood vessel|vascular]] systems, one for transport of water to support locomotion and other functions, and another for circulation of blood.

===Body wall===
The body wall layers include a thin cuticle, an [[Epidermis (zoology)|epidermis]] consisting of a single layer of cells, a thick [[dermis]] formed of [[connective tissue]], a thin [[coelom]]ic [[Myoepithelial cell|myoepithelial]] layer for the muscles, and a [[peritoneum]]. The dermis contains an [[endoskeleton]] of [[calcium carbonate]] components known as ossicles. These are honeycomb-like structures composed of [[calcite]] microcrystals arranged in a lattice.<ref name="Ruppert876">Ruppert et al., 2004. pp. 876–880</ref> They vary in form, from flat plates to granules to spines, and cover the aboral surface.<ref name="Glossary">{{cite web |url=http://www.sms.si.edu/IRLSpec/Phyl_Echino_Glossary.htm |title=Glossary of terms: Phylum Echinodermata |author=Sweat, L. H. |date=2012-10-31 |publisher=Smithsonian Institution |access-date=2013-05-12|archive-url=
https://web.archive.org/web/20160304203303/http://www.sms.si.edu/IRLSpec/Phyl_Echino_Glossary.htm|archive-date=4 March 2016}}</ref> Some are specialised structures such as the [[madreporite]] (the entrance to the water vascular system), [[pedicellaria]]e, and [[Paxilla (ossicle)|paxillae]]. Paxillae are umbrella-like structures found on starfish that live buried in substrate. The edges of adjacent paxillae meet to form a false cuticle with a water cavity beneath in which the madreporite and delicate gill structures are protected. The ossicles are located under the epidermal layer, even those emerging externally.<ref name=Ruppert876/>

Several groups of starfish, including [[Valvatida]] and [[Forcipulatida]], possess pedicellariae.<ref name="Ruppert889" /> These are scissor-like ossicles at the tip of the spine which displace organisms from resting on the starfish's surface.<ref name="synthesis">{{cite book | title=The Invertebrates: a new synthesis | author=Barnes, R. S. K. |author2=Callow, P. |author3=Olive, P. J. W.  | year=1988 | publisher=Blackwell Scientific Publications | location=Oxford | isbn=978-0-632-03125-2 | pages=158–160}}</ref><ref name="Carefoot">{{cite web | url=http://www.asnailsodyssey.com/LEARNABOUT/SEASTAR/seasPedi.php | title=Pedicellariae | publisher=A Snail's Odyssey | work=Sea Stars: Predators & Defenses | access-date=2013-05-11 | author=Carefoot, Tom | archive-url=https://web.archive.org/web/20130316010647/http://www.asnailsodyssey.com/LEARNABOUT/SEASTAR/seasPedi.php | archive-date=16 March 2013 }}</ref> Some species like ''Labidiaster annulatus'' and ''[[Novodinia antillensis]]'' use their pedicellariae to catch prey.<ref>{{cite book|title=Starfish: Biology and Ecology of the Asteroidea |author = Lawrence, J. M. | contribution = The Asteroid Arm |url=https://books.google.com/books?id=yY-5OsDZ9H8C&pg=PA15 | pages=15–23|isbn = 978-1-4214-0787-6 |date = 2013-01-24 |publisher = JHU Press }} in Lawrence (2013)</ref> There may also be [[papula]]e, thin-walled protrusions of the body cavity that reach through the body wall into the surrounding water. These serve a [[aquatic respiration|respiratory]] function.<ref name="Lander">{{cite web |url=http://lanwebs.lander.edu/faculty/rsfox/invertebrates/asterias.html |title=''Asterias forbesi'' |author= Fox, Richard |date=2007-05-25 |work=Invertebrate Anatomy OnLine |publisher=Lander University |access-date=2012-05-19}}</ref> The structures are supported by collagen fibres set at right angles to each other and arranged in a three-dimensional web with the ossicles and papulae in the [[wikt:interstice|interstices]]. This arrangement enables both easy flexion of the arms and the rapid onset of stiffness and rigidity required for some actions performed under stress.<ref>{{cite journal |author=O'Neill, P. |year=1989 |title=Structure and mechanics of starfish body wall |journal=Journal of Experimental Biology |volume=147 |issue=1 |pages=53–89 |doi=10.1242/jeb.147.1.53 |pmid=2614339 |bibcode=1989JExpB.147...53O }}</ref>

<gallery mode="packed" heights="150px">
File:Luidia maculata, Ras Sedr, Egypt.jpg|<!--image is misnamed on Commons-->''[[Luidia maculata]]'', a seven armed starfish
File:Astropecten aranciacus Naxos08 1775 dett.jpg|''[[Astropecten aranciacus]]'' [[Ossicle (echinoderm)|ossicles]]
File:Pédicellaires d' Acanthaster Planci.JPG|Pedicellariae and retracted papulae among the spines of ''[[Acanthaster planci]]''
File:Asterias forbesi pedicellaria and papulae.jpg|Pedicellaria and papulae of ''[[Asterias forbesi]]''
</gallery>

===Water vascular system===
The water [[Blood vessel|vascular]] system of the starfish is a [[Hydraulics|hydraulic system]] made up of a network of fluid-filled canals and is concerned with locomotion, adhesion, food manipulation and [[gas exchange]]. Water enters the system through the [[madreporite]], a porous, often conspicuous, sieve-like ossicle on the aboral surface. It is linked through a [[calcareous]]-lined canal called the stone canal, to a ring canal around the mouth opening. A set of radial canals branch off from the ring canal; one radial canal runs along the [[wikt:ambulacral|ambulacral]] groove in each arm. There are short lateral canals branching off alternately to either side of the radial canal, each ending in an ampulla. These bulb-shaped organs are joined to tube feet (podia) on the exterior of the animal by short linking canals that pass through ossicles in the ambulacral groove. There are usually two rows of tube feet but in some species, the lateral canals are alternately long and short and there appear to be four rows. The interior of the whole canal system is lined with [[cilium|cilia]].<ref name="Ruppert883">Ruppert et al., 2004. pp. 879–883, 889</ref>

Water is pushed into the tube face when longitudinal muscles in the ampullae contract, and shut the valves in the lateral canals. This causes the tube feet to stretch and touch the [[Substrate (biology)|substrate]].<ref name="Ruppert883"/> Although the tube feet resemble suction cups in appearance, the gripping action is a function of adhesive chemicals rather than suction.<ref>{{cite journal |author1=Hennebert, E. |author2=Santos, R. |author3=Flammang, P. |year=2012 |title=Echinoderms don't suck: evidence against the involvement of suction in tube foot attachment |journal=Zoosymposia |volume=1 |pages=25–32 |url=http://www.mapress.com/zoosymposia/content/2012/v7/f/v007p025-032f.pdf |doi=10.11646/zoosymposia.7.1.3 }}</ref> Other chemicals and relaxation of the ampullae allow for release from the substrate. The tube feet latch on to surfaces and move in a wave, with one arm section attaching to the surface as another releases.<ref name=Dorit/><ref>{{cite journal|doi=10.1007/BF00210108 |title=Specializations for excitation-contraction coupling in the podial retractor cells of the starfish ''Stylasterias forreri'' |year=1981 |last1=Cavey |first1=Michael J. |last2=Wood |first2=Richard L. |journal=Cell and Tissue Research |volume=218 |issue=3 |pages=475–485 |pmid=7196288 |s2cid=21844282 }}</ref> To expose the sensory tube feet and the eyespot to external stimuli, some starfish turn up the tips of their arms while moving.<ref>{{cite web | url=http://www.asnailsodyssey.com/LEARNABOUT/SEASTAR/seasTube.php | title=Tube feet | publisher=A Snail's Odyssey | work=Sea Stars: Locomotion | access-date=2013-05-11 | author=Carefoot, Tom | archive-url=https://web.archive.org/web/20131021181550/http://www.asnailsodyssey.com/LEARNABOUT/SEASTAR/seasTube.php | archive-date=21 October 2013 }}</ref>

Having descended from [[bilateria|bilateral]] organisms, starfish may move in a bilateral fashion, particularly when hunting or threatened. When crawling, certain arms act as the leading arms, while others trail behind. When a starfish finds itself upside down, two adjacent arms and an opposite arm press against the ground to lift up the two remaining arms; the opposite arm leaves the ground as the starfish turns over and recovers its normal stance.<ref name="Chengcheng2012">{{cite journal |author1=Chengcheng, J. |author2=Wu, L. |author3=Zhoa, W. |author4=Wang, S. |author5=Lv, J. |year=2012 |title=Echinoderms have bilateral tendencies |journal=PLOS ONE |volume=7 |issue=1 |article-number=e28978 |doi=10.1371/journal.pone.0028978 |pmid=22247765 |pmc=3256158 |arxiv=1202.4214 |bibcode=2012PLoSO...728978J |doi-access=free }}</ref>

Apart from their function in locomotion, the tube feet act as accessory gills. The water vascular system serves to transport [[oxygen]] from, and carbon dioxide to, the tube feet and nutrients from the gut to the muscles involved in locomotion. Fluid movement is bidirectional and initiated by cilia.<ref name="Ruppert883" />

<gallery mode="packed" heights="150px">
File:Seastar.webm|Video showing the tube feet movement of a starfish
File:Détail bras d'étoile de mer.jpg|alt=Arm tip with tube feet|Arm tip of ''[[Leptasterias polaris]]'' showing tube feet and eyespot
</gallery>

===Digestive system and excretion===
[[File:Asterias rubens, dissection.svg|right|thumb|260px|alt=Diagram of starfish anatomy|Aboral view of partially dissected [[Asterias rubens|starfish]]:
{{image key
|list type=ordered
|Pyloric stomach
|Intestine and anus
|Rectal sac
|Stone canal
|Madreporite
|Pyloric caecum
|Digestive glands
|Cardiac stomach
|Gonad
|Radial canal
|Ambulacral ridge}}]]

The gut of a starfish fills most of the central disc and extends into the arms. The mouth occupies the centre of the oral surface, where it is surrounded by a tough [[wikt:peristomial|peristomial]] membrane and closed with a [[sphincter]]. A short [[oesophagus]] connects the mouth to a [[stomach]], which consists of an [[wikt:eversible|eversible]] cardial portion and a smaller [[Pylorus|pyloric]] portion. The cardial stomach is glandular and pouched, and is supported by [[ligament]]s attached to ossicles in the arms so it can be pulled back into position after it has been everted. The pyloric stomach has two extensions into each arm: the pyloric caeca. These are long, hollow tubes lined by a series of glands which secrete digestive [[enzyme]]s and absorb nutrients from the food. A short [[intestine]] and [[rectum]] run from the pyloric stomach to the [[anus]] at the apex of the aboral surface of the disc.<ref name="Ruppert885">Ruppert et al., 2004. p. 885</ref>

Primitive starfish, such as ''Astropecten'' and ''Luidia'', swallow their [[Predation|prey]] whole, and start to digest it in their cardial stomachs, spitting out hard material like shells. The semi-digested fluid flows into the caeca for more digestion as well as absorption.<ref name="Ruppert885" /> In more advanced species of starfish, the cardial stomach can be everted from the organism's body to engulf and digest food, which is passed to the pyloric stomach.<ref name="Carefootfeeding">{{cite web | url=http://www.asnailsodyssey.com/LEARNABOUT/SEASTAR/seasAdul.php | title=Adult feeding | publisher=A Snail's Odyssey | work=Sea Stars: Feeding, growth, & regeneration | access-date=2013-07-13 | author=Carefoot, Tom | archive-url=https://web.archive.org/web/20130512053338/http://www.asnailsodyssey.com/LEARNABOUT/SEASTAR/seasAdul.php | archive-date=12 May 2013 }}</ref><ref name="Dorit"/> The retraction and contraction of the cardial stomach is activated by a [[neuropeptide]] known as NGFFYamide.<ref>{{cite journal|author1=Semmens, Dean C. |author2=Dane, Robyn E. |author3=Pancholi, Mahesh R. |author4=Slade, Susan E. |author5=Scrivens, James H. |author6=Elphick, Maurice R. |year=2013|title=Discovery of a novel neurophysin-associated neuropeptide that triggers cardiac stomach contraction and retraction in starfish |journal=Journal of Experimental Biology |doi=10.1242/jeb.092171 |volume=216 |issue=21 |pages=4047–4053 |pmid=23913946 |s2cid=19175526 |doi-access=free }}</ref>

The main nitrogenous waste product is [[ammonia]], which is removed via [[diffusion]] through the tube feet, papulae and other thin-walled areas. Other waste material include [[urate]]s. The body fluid contains [[phagocyte|phagocytic]] cells called [[coelomocyte]]s, which are also found within the hemal and water vascular systems. These cells engulf waste material, and eventually migrate to the tips of the papulae, where a portion of body wall is nipped off and ejected into the surrounding water.<ref name="Ruppert887">Ruppert et al., 2004. pp. 886–887</ref>

Starfish keep their body fluids at the same salt concentration as the surrounding water, the lack of an osmoregulation system probably explains why starfish are not found in fresh water and rarely in [[estuarine]] environments.<ref name=Ruppert887/>

===Sensory and nervous systems===

Although starfish do not have many well-defined sense organs, they do perceive touch, light, temperature, orientation and the status of the water around them. The tube feet, spines and pedicellariae are sensitive to touch. The tube feet, especially those at the tips of the rays, are also sensitive to chemicals, enabling the starfish to detect odour sources such as food.<ref name=Carefootfeeding/> There are eyespots at the ends of the arms, each one made of 80–200 simple [[ocellus|ocelli]] composed of [[pigment]]ed epithelial cells. Individual [[photoreceptor cells]] are present in other parts of their bodies and respond to light. Whether they advance or retreat depends on the species.<ref name="Ruppert884">Ruppert et al., 2004. pp. 883–884</ref>

While a starfish lacks a [[Central nervous system|centralized brain]], it has a complex [[nervous system]] with a nerve ring around the mouth and a radial nerve running along the ambulacral region of each arm parallel to the radial canal. The peripheral nerve system consists of two nerve nets: one in the epidermis and the other in the lining of the coelomic cavity, which are the sensory and motor systems respectively. Neurons passing through the dermis join the two. Both the ring and radial nerves function in movement and sensory. The sensory component is supplied with information from the sensory organs while the motor nerves control the tube feet and musculature. If one arm detects something attractive, it becomes dominant and temporarily over-rides the other arms to initiate movement towards it.<ref name="Ruppert884"/>

===Circulatory and gas exchange system===

The body cavity contains the [[circulatory system|circulatory]] or haemal system. The vessels form three rings: one around the mouth (the hyponeural haemal ring), another around the digestive system (the gastric ring), and the third near the aboral surface (the genital ring). The heart beats about six times a minute and is at the apex of a vertical channel (the axial vessel) that connects the three rings. Blood does not contain a pigment such as [[heme]], but is probably used to transport nutrients around the body.<ref name="Ruppert887"/> Gas exchange mainly takes place through [[gills]] known as papulae, which are thin-walled bulges along the aboral surface of the arms. Oxygen is transferred from these to the [[Coelom#Coelomic fluid|coelomic fluid]], which moves gas around the body.<ref name="Ruppert883" />

===Secondary metabolites===
Starfish produce a large number of [[secondary metabolite]]s in the form of [[lipid]]s, including [[steroid]]al derivatives of [[cholesterol]], and [[fatty acid]] [[amide]]s of [[sphingosine]]. The steroids are mostly [[saponin]]s, known as asterosaponins, and their [[sulfation|sulphated]] derivatives. They vary across species and are typically formed from up to six sugar molecules (usually [[glucose]] and [[galactose]]) connected by up to three [[glycosidic]] chains. Long-chain fatty acid amides of sphingosine occur frequently, with some having known [[biological activity]]. Starfish also contain various [[ceramide]]s and a small number of [[alkaloid]]s. These chemicals in the starfish may function in defence and communication. Some are feeding deterrents used by the starfish to discourage predation. Others are [[wikt:antifouling|antifoulants]] and supplement the pedicellariae to prevent other organisms from settling on the starfish's aboral surface. Some are alarm [[pheromone]]s and escape-eliciting chemicals, the release of which trigger responses in starfish of the same species, but often stimulate flight in potential prey.<ref>{{cite book |chapter=8: Chemistry and Ecological Role of Starfish Secondary Metabolites |title=Starfish: Biology and Ecology of the Asteroidea |editor-last=Lawrence |editor-first=John M. |last1=McClintock |first1=James B. |last2=Amsler |first2=Charles D. |last3=Baker | first3=Bill J. |year=2013 |publisher=JHU Press |pages=81–89|isbn=978-1-4214-1045-6 |chapter-url=https://books.google.com/books?id=WQDvIQ5xR68C&q=%22+secondary+metabolite%22+starfish&pg=PT154 }}</ref> Research into the efficacy of these compounds for possible pharmacological or industrial use occurs worldwide.<ref>{{cite journal |author1=Zhang, Wen |author2=Guo, Yue-Wei |author3=Gu, Yucheng |year=2006 |title=Secondary metabolites from the South China Sea invertebrates: chemistry and biological activity |journal=Current Medicinal Chemistry |volume=13 |issue=17 |pages=2041–2090 |doi=10.2174/092986706777584960 |pmid=16842196 }}</ref>

==Life cycle==

===Sexual reproduction===

Most species of starfish are [[Gonochorism|gonochorous]], there being separate male and female individuals.<ref name="Ruppert888"/> Some species are [[hermaphrodites#Simultaneous hermaphrodites|simultaneous hermaphrodites]], producing eggs and sperm at the same time, and in a few of these the same gonad, called an [[ovotestis]], produces both eggs and sperm.<ref name="Byrne2005">{{Cite journal |doi=10.2307/3593116 |volume=208 |issue=2 |pages=81–91 |last=Byrne |first=Maria |title=Viviparity in the sea star ''Cryptasterina hystera'' (Asterinidae): conserved and modified features in reproduction and development |journal=Biological Bulletin |year=2005| pmid=15837957 |jstor=3593116 |citeseerx=10.1.1.334.314 |s2cid=16302535}}</ref> Other starfish are [[hermaphrodites#Sequential hermaphrodites|sequential hermaphrodites]]. [[Protandrous]] individuals of species like ''[[Asterina gibbosa]]'' start life as males before changing sex into females as they grow older.<ref name="Crump1983"/> In some species such as ''[[Nepanthia belcheri]]'', a large female can [[Fragmentation (reproduction)|split in half]] and the resulting offspring are males. When these grow large enough they change back into females.<ref name="Ottesen1982">{{Cite journal |doi = 10.1007/BF00397488 |volume=69 |issue=3 |pages=223–233 |author1=Ottesen, P. O. |author2=Lucas, J. S. | title = Divide or broadcast: interrelation of asexual and sexual reproduction in a population of the fissiparous hermaphroditic seastar ''Nepanthia belcheri'' (Asteroidea: Asterinidae) |journal=Marine Biology |year=1982 |bibcode=1982MarBi..69..223O |s2cid=84885523}}</ref>

Each starfish arm contains two gonads that release [[gametes]] through openings called gonoducts, located on the central disc between the arms. [[Fertilization]] is generally external<ref name="Ruppert888"/> but in a few species, internal fertilization takes place.<ref name="Byrne2005"/> In most species, the buoyant eggs and sperm are simply released into the water (free spawning) and the resulting [[embryo]]s and [[larva]]e live as part of the [[plankton]].<ref name="Ruppert888"/> In others, the eggs may be stuck to the undersides of rocks.<ref name="Crump1983">{{cite journal|author1=Crump, R. G. |author2=Emson, R. H. | title=The natural history, life history and ecology of the two British species of ''Asterina''| journal=Field Studies| year=1983| volume=5| issue=5 |pages=867–882| url=http://fsj.field-studies-council.org/media/342971/vol5.5_160.pdf |access-date=2011-07-27 }}</ref> In certain species of starfish, the females [[Egg incubation|brood]] their eggs – either by simply enveloping them or by holding them in specialised structures in different parts of the body, externally or internally.<ref name="McClary1989">{{Cite journal |doi = 10.1007/BF00399585 | volume=103 |issue=4 |pages=531–540 |author1=McClary, D. J. |author2=Mladenov, P. V. |title = Reproductive pattern in the brooding and broadcasting sea star ''Pteraster militaris'' |journal = Marine Biology |year = 1989 | bibcode=1989MarBi.103..531M |s2cid = 84867808}}</ref><ref name="Hendler1982">{{Cite journal| doi = 10.2307/1540983| volume = 162| issue = 3| pages = 273–289|author1=Hendler, Gordon |author2=Franz, David R. | title = The biology of a brooding seastar, ''Leptasterias tenera'', in Block Island |jstor=1540983 | journal = Biological Bulletin| year= 1982| url = https://www.biodiversitylibrary.org/part/6370}}</ref><ref name=Ruppert888/> Those starfish that brood their eggs by "sitting" on them usually assume a humped posture with their discs raised off the substrate.<ref name="Chia1966">{{Cite journal| volume = 130| issue = 3| pages = 304–315| author= Chia, Fu-Shiang| title = Brooding behavior of a six-rayed starfish, ''Leptasterias hexactis''| journal = Biological Bulletin| year = 1966 |jstor=1539738| doi = 10.2307/1539738| url = https://www.biodiversitylibrary.org/part/9408}}</ref><ref name="Crump1983"/> ''[[Pteraster militaris]]'' broods a few of its young and disperses the remaining eggs, that are too numerous to fit into its pouch.<ref name=McClary1989/> In these brooding species, the eggs are relatively large, and supplied with [[yolk]], and they generally develop directly into miniature starfish without an intervening larval stage,<ref name=Byrne2005/> called "lecithotrophic" .<ref>{{cite journal|last1=Lopes|first1=E. M.|last2=Ventura|first2=C. R. R.|year=2016|title=Development of the Sea Star ''Echinaster (Othilia) brasiliensis'', with Inference on the Evolution of Development and Skeletal Plates in Asteroidea|journal=Biological Bulletin|volume=230|issue=1|pages=25–34|doi=10.1086/BBLv230n1p25 |pmid=26896175 }}</ref>  In ''[[Parvulastra parvivipara]]'', an [[wikt:intragonadal|intragonadal]] brooder, the young starfish obtain nutrients by eating other eggs and embryos in the brood pouch.<ref name="Byrne1996">{{Cite journal | volume = 125 | issue = 3 |pages = 551–567| last=Byrne |first=M. | title = Viviparity and intragonadal cannibalism in the diminutive sea stars ''Patiriella vivipara'' and ''P. parvivipara'' (family Asterinidae) | journal = Marine Biology | year = 1996 | doi = 10.1007/BF00353268 | bibcode = 1996MarBi.125..551B | s2cid = 83110156 | url = https://link.springer.com/article/10.1007/BF00353268| url-access = subscription }}</ref> Brooding occurs in species that live in colder waters.<ref name="Ruppert888">Ruppert et al., 2004. pp. 887–888</ref> and in smaller species that produce just a few eggs.<ref name="Gaymer2013"/><ref name=reproduction>{{cite book |author1=Mercier, A. |author2=Hamel J-F. |contribution=Reproduction in Asteroidea |pages=37–38, 43, 45–48 |title=Starfish: Biology and Ecology of the Asteroidea|date=2014 |volume=10 |issue=1 |doi=10.1080/17451000.2013.820323 |bibcode=2014MBioR..10...93V }} in Lawrence (2013)</ref>

The timing of spawning may be influenced by lighting conditions, water temperature, food availability, and other factors. Individuals may gather together to release their gametes as once, using [[pheromone]]s to attract each other.<ref name="Miller1989">{{Cite journal | doi = 10.1016/0022-0981(89)90164-0 | volume = 130 | issue = 3 | pages = 205–221| author = Miller, Richard L. | title = Evidence for the presence of sexual pheromones in free-spawning starfish | journal = Journal of Experimental Marine Biology and Ecology | date = 12 October 1989| bibcode = 1989JEMBE.130..205M }}</ref><ref name=reproduction/> In some species, a male and female may come together and form a pair.<ref name="Bos2013">{{cite journal |author1=Bos A.R. |author2=G.S. Gumanao |author3=B. Mueller |author4=M.M. Saceda |year=2013 |title=Size at maturation, sex differences, and pair density during the mating season of the Indo-Pacific beach star ''Archaster typicus'' (Echinodermata: Asteroidea) in the Philippines |journal=Invertebrate Reproduction and Development |volume=57 |issue=2 |pages=113–119 |doi=10.1080/07924259.2012.689264 |bibcode=2013InvRD..57..113B |s2cid=84274160 |url=https://www.researchgate.net/publication/230851841 }}</ref><ref name="Run1988">{{Cite journal| doi = 10.1007/BF00391987| issn = 0025-3162| volume = 99| issue = 2| pages = 247–253|author1=Run, J. -Q. |author2=Chen, C. -P. |author3=Chang, K. -H. |author4=Chia, F. -S. | title = Mating behaviour and reproductive cycle of ''Archaster typicus'' (Echinodermata: Asteroidea)| journal = Marine Biology|year=1988| bibcode = 1988MarBi..99..247R| s2cid = 84566087}}</ref> They engage in [[pseudocopulation]] which involves the male crawling on the female and fertilising her gametes as she releases them.<ref name="Keesing2011">{{Cite journal| doi = 10.1007/s00227-011-1638-2 | volume = 158 | issue = 5 | pages = 1163–1173 |author1=Keesing, John K. |author2=Graham, Fiona |author3=Irvine, Tennille R. |author4=Crossing, Ryan | title = Synchronous aggregated pseudo-copulation of the sea star ''Archaster angulatus'' Müller & Troschel, 1842 (Echinodermata: Asteroidea) and its reproductive cycle in south-western Australia |journal = Marine Biology |year=2011 | bibcode = 2011MarBi.158.1163K | s2cid = 84926100}}</ref><ref name=reproduction/>

===Larval development===
[[File:Haeckel Asteridea Larvae.jpg|thumb|left|upright=1|alt=Starfish larvae|Three kinds of [[bilaterally symmetric]] starfish [[larvae]] (from left to right) scaphularia larva, [[bipinnaria]] larva, [[brachiolaria]] larva, all of ''[[Asterias]]'' sp. Painted by [[Ernst Haeckel]]]]

Starfish embryos typically hatch as [[blastula]]s. [[Invagination]]s take place, the first forming the anus is created from the [[blastopore]], while a second, taking place in the ectodermic layer, creates the mouth. The [[archenteron]] stretches towards the mouth and connects with it, forming the gut.<ref>Ruppert et al., 2004. pp. 875</ref> A band of [[Cilium|cilia]] develops on the exterior. This enlarges and extends around the surface and eventually onto two developing arm-like outgrowths. At this stage the larva is known as a [[bipinnaria]]. The cilia are used for locomotion and feeding, their rhythmic beat wafting [[phytoplankton]] towards the mouth.<ref name=Ruppert889/>

The next stage in development is a [[brachiolaria]] larva, and involves the growth of three short ventral-anterior arms with adhesive tips surrounding a sucker. Both bipinnaria and brachiolaria larvae are bilaterally symmetrical. When fully developed, the brachiolaria settles on the seabed and attaches itself with a short stalk made from its ventral arms and sucker. Metamorphosis now takes place with a radical rearrangement of tissues. The larvae develops an oral surface on the left and an aboral surface on the right. While the gut remains, the mouth and anus move to new positions. Some of the body cavities disappear while others become the water vascular system and the visceral coelom. The starfish is now pentaradially symmetrical. It casts off its stalk and becomes a free-living juvenile starfish up to {{convert|1|mm|2|abbr=on}} in diameter.<ref name="Ruppert889" />

===Asexual reproduction===
[[File:Tu - Linckia guildingi cropped.jpg|thumb|right|alt=Regeneration from an arm|"Comet" of ''[[Linckia guildingi]]'', showing starfish body regrowing from a single arm]]
{{Main|Asexual reproduction in starfish}}

Some species of starfish can reproduce [[Asexual reproduction|asexually]] as adults either by [[Fragmentation (reproduction)|fission]] of their central discs or by [[autotomy]] (self-amputation) of one or more of their arms.<ref>{{cite journal |author1=Achituv, Y. |author2=Sher, E. |year=1991 |title=Sexual reproduction and fission in the sea star ''Asterina burtoni'' from the Mediterranean coast of Israel |journal=Bulletin of Marine Science |volume=48 |issue=3 |pages=670–679  |url=http://www.ingentaconnect.com/content/umrsmas/bullmar/1991/00000048/00000003/art00007 }}</ref><ref>{{cite journal |last1=Rubilar |first1=Tamara |last2=Pastor |first2=Catalina |last3=Diaz de Vivar |first3=Enriqueta |title=Timing of fission in the starfish Allostichaster capensis (Echinodermata:Asteroidea) in laboratory |journal=Revista de Biología Tropical |issue=53 (Supplement 3) |pages=299–303 |date=30 January 2006 |url=https://tropicalstudies.org/rbt/attachments/suppls/sup53-3%20echinoderm/20-RUBILAR-Tim.pdf}}</ref> Single arms that regenerate a whole individual are called comet forms.<ref>{{cite book|last=Dipper|first=Frances|year=2016|title=The Marine World: A Natural History of Ocean Life|publisher=Princeton University Press|page=291|isbn=978-0-9573946-2-9}}</ref> The larvae of several species of starfish can reproduce asexually before they reach maturity.<ref name="Eaves2003">{{Cite journal |issn=0028-0836 |volume=425 |issue=6954|page=146|author1=Eaves, Alexandra A. |author2=Palmer, A. Richard |title=Reproduction: widespread cloning in echinoderm larvae |journal=Nature |year=2003 |doi=10.1038/425146a |pmid=12968170|bibcode=2003Natur.425..146E |s2cid=4430104 |doi-access=free }}</ref> They do this by autotomising some parts of their bodies or by [[budding]].<ref name="Jaeckle1994">{{Cite journal |doi=10.2307/1542036 |pmid=29283296 |volume=186 |issue=1 |pages=62–71 |last=Jaeckle |first=William B. |title=Multiple modes of asexual reproduction by tropical and subtropical sea star larvae: an unusual adaptation for genet dispersal and survival|journal=Biological Bulletin |year=1994 |jstor =1542036 |url=https://digitalcommons.iwu.edu/bio_scholarship/30 }}</ref> Larva increase asexual reproduction when they sense that food is plentiful.<ref name="Vickery200">{{Cite journal |volume=199 |issue=3 |pages=298–304 |author1=Vickery, M. S. |author2=McClintock, J. B. |title=Effects of food concentration and availability on the incidence of cloning in planktotrophic larvae of the sea star ''Pisaster ochraceus'' |journal=The Biological Bulletin |date=2000-12-01 |pmid=11147710 |doi=10.2307/1543186|jstor=1543186 |url=https://www.biodiversitylibrary.org/part/10580 }}</ref> Though this costs it time and energy and delays maturity, it allows a single larva to give rise to multiple adults when the conditions are appropriate.<ref name=Jaeckle1994/>

===Regeneration===
{{Main|Starfish regeneration}}
[[File:Sea star regenerating legs.jpg|thumb|right|alt=Arms being regenerated|[[Sunflower seastar]] regenerating missing arms]]

Some species of starfish have the ability to [[Regeneration (biology)|regenerate]] lost arms and can regrow an entire new limb given time.<ref name=Edmondson1935/> A few can regrow a complete new disc from a single arm, while others need at least part of the central disc to be attached to the detached part.<ref name=Ruppert887/> Regrowth can take several months, and starfish are vulnerable to infections during the early stages after the loss of an arm.<ref name="Edmondson1935">{{cite journal|last=Edmondson |first=C. H. |title=Autotomy and regeneration of Hawaiian starfishes |journal=Bishop Museum Occasional Papers |year=1935 |volume=11 |issue=8 |pages=3–20 |url=http://hbs.bishopmuseum.org/pubs-online/pdf/op11-8.pdf}}</ref> Other than fragmentation carried out for the purpose of reproduction, the division of the body may happen as a [[Autotomy|defense mechanism]].<ref name=Ruppert887/> The loss of body parts is achieved by the rapid softening of a special type of connective tissue in response to nervous signals. This type of tissue is called [[catch connective tissue]] and is found in most [[echinoderm]]s.<ref name="Hayashi1986">{{Cite journal| issn = 0022-0949| volume = 125| issue = 1| pages = 71–84|author1=Hayashi, Yutaka |author2=Motokawa, Tatsuo | title = Effects of ionic environment on viscosity of catch connective tissue in holothurian body wall| journal = Journal of Experimental Biology| year = 1986| doi = 10.1242/jeb.125.1.71| bibcode = 1986JExpB.125...71H| url =http://jeb.biologists.org/content/125/1/71.full.pdf+html | doi-access = free}}</ref> An autotomy-promoting factor has been identified which, when injected into another starfish, causes rapid shedding of arms.<ref name="Mladenov1989">{{Cite journal| issn = 0006-3185| volume = 176| issue = 2| pages = 169–175| author1 = Mladenov, Philip V.| author2 = Igdoura, Suleiman| author3 = Asotra, Satish| author4 = Burke, Robert D.| title = Purification and partial characterization of an autotomy-promoting factor from the sea star ''Pycnopodia helianthoides''| journal = Biological Bulletin| year = 1989| url = http://www.biolbull.org/content/176/2/169.full.pdf+html| doi = 10.2307/1541585| jstor = 1541585| access-date = 12 July 2013| archive-url = https://web.archive.org/web/20150923202150/http://www.biolbull.org/content/176/2/169.full.pdf+html| archive-date = 23 September 2015}}</ref>

===Lifespan===

The lifespan of a starfish varies considerably among species. For example, ''[[Leptasterias hexactis]]'' reaches sexual maturity at {{convert|20|g|1|abbr=on}} in two years and lives for about ten years. ''[[Pisaster ochraceus]]'' matures at {{convert|70|–|90|g|abbr=on}} in five years and has a maximum recorded lifespan of 34 years.<ref name="Ruppert889" />

==Ecology==

===Distribution and habitat===

Starfish live in marine waters around the world including both tropical and polar waters.<ref name=Ruppert876/><ref name=Echinaster/><ref>{{cite journal|last1=McClintock|first1=J. G.|last2=Pearse|first2=J. S.|last3=Bosch|first3=I|year=1988|title=Population structure and energetics of the shallow-water antarctic sea star ''Odontaster validus'' in contrasting habitats|journal=Marine Biology|volume=99|issue=2 |pages=235–246|doi=10.1007/BF00391986 |bibcode=1988MarBi..99..235M }}</ref> They are mainly [[benthic]] animals, living in sandy, muddy and rocky substrates.<ref name=Ruppert876/><ref name="Dorit"/> They range from shallow, [[intertidal zone|intertidal]] waters<ref name=Echinaster/> to the deep-sea floor down to at least {{convert|6000|m|abbr=on}}.<ref>{{cite journal |author1=Mah, Christopher |author2=Nizinski, Martha |author3=Lundsten, Lonny |year=2010 |title=Phylogenetic revision of the Hippasterinae (Goniasteridae; Asteroidea): systematics of deep sea corallivores, including one new genus and three new species |journal=Zoological Journal of the Linnean Society |volume=160 |issue=2 |pages=266–301 |doi=10.1111/j.1096-3642.2010.00638.x |doi-access=free }}</ref> Starfish are most common along the coast.<ref name=Ruppert876/>

===Diet===
[[File:Starfish eversion.jpg|thumb|alt=A starfish with its stomach turned outside its mouth to feed on coral|A ''[[Circeaster|Circeaster pullus]]'' starfish everting its stomach to feed on coral]]

Most species are generalist predators, eating [[Microphyte|microalgae]], [[sponge]]s, [[bivalve]]s, [[snail]]s and other small animals.<ref name=Carefootfeeding/><ref name=Lawrence124/> The [[crown-of-thorns starfish]] consumes [[coral]] polyps,<ref name=outbreak/> while other species are [[detritivore]]s, feeding on decomposing organic material and faecal matter.<ref name="Lawrence124">{{cite book |last=Pearse |first=J. S. |contribution=Odontaster validus |pages=124–25 |title=Starfish: Biology and Ecology of the Asteroidea|date=2014 |volume=10 |issue=1 |doi=10.1080/17451000.2013.820323 |bibcode=2014MBioR..10...93V }} in Lawrence (2013)</ref><ref name=Echinaster>{{cite book |last=Turner |first=R. L. |title=Starfish: Biology and Ecology of the Asteroidea |date=2014 |volume=10 |issue=1 |contribution=Echinaster |pages=206–207, 210|doi=10.1080/17451000.2013.820323 |bibcode=2014MBioR..10...93V }} in Lawrence (2012)</ref> A few are suspension feeders, gathering in [[phytoplankton]]; ''[[Henricia]]'' and ''[[Echinaster]]'' often feed with sponges, taking advantage of the water current they produce. Various species can absorb organic nutrients from the surrounding water, and this may form a significant portion of their diet.<ref name="Florkin">{{cite book|author=Florkin, Marcel|title=Chemical Zoology V3: Echinnodermata, Nematoda, and Acanthocephala |url=https://books.google.com/books?id=43zXixxPk74C&pg=PA75 |year=2012 |publisher=Elsevier |isbn=978-0-323-14311-0 |pages=75–77}}</ref>

The processes of feeding and capture may be aided by special parts; ''[[Pisaster brevispinus]]'', the short-spined pisaster from the [[Pacific Ocean|West Coast]] of America, can use a set of specialized tube feet to dig itself deep into the soft substrate to extract prey (usually [[clam]]s).<ref>{{cite book |title=Marine Biology: An Ecological Approach |last1=Nybakken |first1=James W. |last2=Bertness |first2=Mark D. |year=1997 |publisher=Addison-Wesley Educational Publishers |isbn=978-0-8053-4582-7 |page=174 }}</ref> Grasping the shellfish, the starfish slowly pries open the prey's shell, overcoming the clam's [[Adduction|adductor]] muscle, and inserts its everted stomach into the crack to digest the soft tissues. The gap between the clam's [[Bivalve shell|valves]] need only be a fraction of a millimetre wide for the stomach to gain entry.<ref name="Dorit">{{cite book |title=Zoology |url=https://archive.org/details/zoology0000dori |url-access=registration |last1=Dorit |first1=R. L. |last2=Walker | first2= W. F. |last3=Barnes |first3 = R. D. |year=1991 |publisher=Saunders College Publishing |isbn=978-0-03-030504-7 |page=778–[https://archive.org/details/zoology0000dori/page/782 782] }}</ref>

===Ecological impact===
[[File:Asteroidea bunch - 133 - 54001353478.jpg|thumb|Starfish on a wood pier]]
Starfish are [[keystone species]] in their respective marine [[Community (ecology)|communities]]. Their relatively large sizes, diverse diets, and ability to adapt to different environments makes them ecologically important.<ref>{{cite book |author1=Menage, B. A. |author2=Sanford, E. |title=Starfish: Biology and Ecology of the Asteroidea|date=2014 |volume=10 |issue=1 | contribution=Ecological Role of Sea Stars from Populations to Meta-ecosystems | page=67|doi=10.1080/17451000.2013.820323 |bibcode=2014MBioR..10...93V }} in Lawrence (2013)</ref> The term "keystone species" was in fact first used by [[Robert T. Paine (zoologist)|Robert Paine]] in 1966 to describe a starfish, ''Pisaster ochraceus''.<ref>{{cite web | author=Wagner, S. C. | year=2012 | title=Keystone Species | publisher=Nature Education Knowledge | access-date=2013-05-16 | url=http://www.nature.com/scitable/knowledge/library/keystone-species-15786127}}</ref> When studying the low intertidal coasts of [[Washington (state)|Washington state]], Paine found that predation by ''P.&nbsp;ochraceus'' was a major factor in the diversity of species. Experimental removals of this top predator from a stretch of shoreline resulted in lower species diversity and the eventual domination of ''[[Mytilus (bivalve)|Mytilus]]'' mussels, which were able to outcompete other organisms for space and resources.<ref>{{cite journal | author=Paine, R. T. | year=1966 | title=Food web complexity and species diversity | journal=American Naturalist | volume=100 | issue=190 | pages=65–75 | jstor=2459379 | doi=10.1086/282400| bibcode=1966ANat..100...65P | s2cid=85265656 }}</ref> Similar results were found in a 1971 study of ''[[Stichaster australis]]'' on the intertidal coast of the [[South Island]] of [[New Zealand]]. ''S.&nbsp;australis'' was found to have removed most of a batch of transplanted mussels within two or three months of their placement, while in an area from which ''S.&nbsp;australis'' had been removed, the mussels increased in number dramatically, overwhelming the area and threatening [[biodiversity]].<ref>{{cite journal | author=Paine, R. T. | year=1971 | title=A short-term experimental investigation of resource partitioning in a New Zealand rocky intertidal habitat | journal=Ecology | volume=52 | issue=6 | pages=1096–1106 | jstor=1933819 | doi=10.2307/1933819| bibcode=1971Ecol...52.1096P }}</ref>
[[File:Starfishmussel.jpg|thumb|left|alt=Starfish devouring mussel|''Pisaster ochraceus'' consuming a mussel in central [[California]]]]
The feeding activity of the [[Omnivore|omnivorous]] starfish ''[[Oreaster reticulatus]]'' on sandy and seagrass bottoms near the [[Virgin Islands]] effects the composition of communities of microorganisms. These starfish engulf piles of sediment removing the surface films and algae adhering to the particles. Organisms that dislike this disturbance are replaced by others better able to rapidly recolonise "clean" sediment. In addition, foraging by these starfish creates diverse patches of organic matter, which may attract larger organisms such as fish, crabs and sea urchins that feed on the sediment.<ref>{{cite journal|author=Scheibling, R. E. |year=1980 |title=Dynamics and feeding activity of high-density aggregations of ''Oreaster reticulatus'' (Echinodermata: Asteroidea) in a sand patch habitat |journal=Marine Ecology Progress Series |volume=2 |pages=321–27 |doi=10.3354/meps002321|bibcode=1980MEPS....2..321S |doi-access=free }}</ref><ref>{{cite journal|last1=Scheibling|first1=R. E.|year=1982|title=Habitat utilization and bioturbation by ''Oreaster reticulatus'' (Asteroidea) and ''Meoma ventricosa'' (Echinoidea) in a Subtidal Sand Patch|journal=Bulletin of Marine Science|volume=32|issue=2|pages=624–629|url=https://www.ingentaconnect.com/contentone/umrsmas/bullmar/1982/00000032/00000002/art00021?crawler=true}}</ref><ref name="Scheibling150"/>

Starfish sometimes have negative effects on ecosystems. Outbreaks of crown-of-thorns starfish have caused damage to coral reefs in Northeast Australia and [[French Polynesia]].<ref name="outbreak">{{cite journal |author1=Kayal, Mohsen |author2=Vercelloni, Julie |author3=Lison de Loma, Thierry |author4=Bosserelle, Pauline |author5=Chancerelle, Yannick |author6=Geoffroy, Sylvie |author7=Stievenart, Céline |author8=Michonneau, François |author9=Penin, Lucie |author10=Planes, Serge |author11=Adjeroud, Mehdi | year=2012 | title=Predator crown-of-thorns starfish (''Acanthaster planci'') outbreak, mass mortality of corals, and cascading effects on reef fish and benthic communities|journal=PLOS ONE| volume=7 | issue=10 | article-number=e47363 | doi=10.1371/journal.pone.0047363 | editor1-last=Fulton | editor1-first=Christopher | pmid=23056635 | pmc=3466260|bibcode=2012PLoSO...747363K |doi-access=free }}</ref><ref>{{cite journal |vauthors=Brodie J, Fabricius K, De'ath G, Okaji K |title=Are increased nutrient inputs responsible for more outbreaks of crown-of-thorns starfish? An appraisal of the evidence |journal=Marine Pollution Bulletin |volume=51 |issue=1–4 |pages=266–78 |year=2005 |pmid=15757727 |doi=10.1016/j.marpolbul.2004.10.035|bibcode=2005MarPB..51..266B }}</ref> A study in Polynesia found that coral cover declined drastically with the arrival of migratory starfish in 2006, dropping over 40% to under 5% in four years. This in turn had a [[cascade effect]] on both sessile bottom-dwelling animals and reef fish.<ref name=outbreak/> ''[[Asterias amurensis]]'' is a rare example of an [[invasive species|invasive]] echinoderm . Its larvae likely arrived in [[Tasmania]] from central Japan via water discharged from ships in the 1980s. The species has since grown in numbers to the point where they threaten important [[bivalve]] fisheries in Australia. As such, they are considered pests,<ref name="Byrne177">{{cite book |author1=Byrne, M. |author2=O'Hara, T. D. |author3=Lawrence, J. M. | contribution=Asterias amurensis|title=Starfish: Biology and Ecology of the Asteroidea |date=2014 |volume=10 |issue=1 | pages=177–179|doi=10.1080/17451000.2013.820323 |bibcode=2014MBioR..10...93V }} in Lawrence (2013)</ref> and are on the Invasive Species Specialist Group's [[list of the world's 100 worst invasive species]].<ref>{{cite web|url=http://www.issg.org/database/species/search.asp?st=100ss|title=100 of the World's Worst Invasive Alien Species|publisher=Global Invasive Species Database|access-date=2010-07-16|archive-date=29 April 2015|archive-url=https://web.archive.org/web/20150429045453/http://www.issg.org/database/species/search.asp?st=100ss}}</ref> Some species that prey on [[bivalve molluscs]] can transmit [[paralytic shellfish poisoning]].<ref>{{cite journal | title=Occurrence of paralytic shellfish poison in the starfish ''Asterias amurensis'' in Kure Bay, Hiroshima Prefecture, Japan | year=1997 |author1=Asakawa, M. |author2=Nishimura, F. |author3=Miyazawa, K. |author4=Noguchi, T. | journal=Toxicon | volume=35 | issue=7 | pages=1081–1087 | doi=10.1016/S0041-0101(96)00216-4 | pmid=9248006| bibcode=1997Txcn...35.1081A }}</ref>

===Threats===
[[File:Seagull eating starfish.jpg|thumb|alt=Gull feeding on starfish|[[American herring gull]] feeding on a starfish]]

Starfish may be preyed on by [[Biological specificity|conspecifics]], sea anemones,<ref>{{cite web |title=Fact Sheet: Sea Anemones |url=https://www.mba.ac.uk/fact-sheet-sea-anemones |publisher=Marine Biological Association |access-date=10 June 2019 |date=21 February 2017 |archive-url=https://web.archive.org/web/20191224194037/https://www.mba.ac.uk/fact-sheet-sea-anemones |archive-date=24 December 2019 }}</ref> other starfish species, [[Triton (gastropod)|tritons]], crabs, fish, [[gull]]s, and [[sea otter]]s.<ref name="Gaymer2013">{{cite book|title=Starfish: Biology and Ecology of the Asteroidea |author1=Gaymer, C. F. |author2=Himmelman, J. H. |date=2014 |volume=10 |issue=1 | contribution=Leptasterias polaris | pages=182–84|doi=10.1080/17451000.2013.820323 |bibcode=2014MBioR..10...93V }} in Lawrence (2013)</ref><ref name=Byrne177/><ref name="Robles166">{{cite book | author=Robles, C. | contribution=Pisaster ochraceus | pages=165–167|title=Starfish: Biology and Ecology of the Asteroidea| date=2014 | volume=10 | issue=1 | doi=10.1080/17451000.2013.820323 | bibcode=2014MBioR..10...93V }} in Lawrence (2013)</ref><ref name="Scheibling150">{{cite book| author=Scheibling, R. E. |contribution=Oreaster reticulatus|title=Starfish: Biology and Ecology of the Asteroidea |date=2014 |volume=10 |issue=1 |pages=150–151|doi=10.1080/17451000.2013.820323 |bibcode=2014MBioR..10...93V }} in Lawrence (2013)</ref> Their first lines of defence are the [[saponin]]s present in their body walls, which have unpleasant flavours.<ref name="AnderssonSaponins">{{cite journal |author=Andersson L, Bohlin L, Iorizzi M, Riccio R, Minale L, Moreno-López W |title=Biological activity of saponins and saponin-like compounds from starfish and brittle-stars |journal=Toxicon |volume=27 |issue=2 |pages=179–88 |year=1989 |pmid=2718189 |doi=10.1016/0041-0101(89)90131-1 |last2=Bohlin |last3=Iorizzi |last4=Riccio |last5=Minale |last6=Moreno-López |bibcode=1989Txcn...27..179A }}</ref> Some starfish such as ''[[Astropecten polyacanthus]]'' also include powerful toxins such as [[tetrodotoxin]],<ref name=Miyazawa>{{cite journal |author1=Miyazawa, K. |author2=Noguchi, T. |author3=Maruyama, J. |author4=Jeon, J. K. |author5=Otsuka, M. |author6=Hashimoto, K. |year=1985 |title=Occurrence of tetrodotoxin in the starfishes ''Astropecten polyacanthus'' and ''A. scoparius'' in the Seto Inland Sea  |journal=Marine Biology |volume=90 |issue=1 |pages=61–64 |doi=10.1007/BF00428215 |bibcode=1985MarBi..90...61M }}</ref> while the [[Pteraster tesselatus|slime star]] can ooze out large quantities of repellent mucus.<ref name=PNW>{{cite web |url=http://www.seastarsofthepacificnorthwest.info/species/slime_star.html |title=Slime star (''Pteraster tesselatus'') |date=2011 |publisher=Seastars of the Pacific Northwest |access-date=2012-09-24 |archive-url=https://web.archive.org/web/20120909031730/http://www.seastarsofthepacificnorthwest.info/species/slime_star.html |archive-date=2012-09-09 }}</ref> The [[crown-of-thorns starfish]] possesses sharp spines, toxins and bright [[aposematism|warning colours]].<ref>{{cite web|url=http://sea.sheddaquarium.org/sea/fact_sheets.asp?id=69 |title=Crown of Thorns Sea Star |publisher=Shedd Aquarium |year=2006 |access-date=2013-05-22 |author=Shedd, John G. |archive-url=https://web.archive.org/web/20140222070509/http://sea.sheddaquarium.org/sea/fact_sheets.asp?id=69 |archive-date=22 February 2014 }}</ref>

[[File:Crown of Thorns-jonhanson.jpg|thumb|right|[[Warning coloration]]<!--yes, this is the Brit. spelling--> in the [[crown-of-thorns starfish]]]]

Several species sometimes suffer from a [[wasting]] condition caused by ''[[Vibrio]]'' bacteria.<ref name="Robles166" /> A more widespread [[sea star wasting disease]] sporadically causes [[mass mortalities]].<ref>Dawsoni, Solaster. "Sea Star Species Affected by Wasting Syndrome." Pacificrockyintertidal.org Seastarwasting.org (n.d.): n. pag. ''Ecology and Evolutionary Biology''. Web.</ref> The results of a 2025 study of starfish off the coast of central [[British Columbia]] suggest that those living in the [[fjord]]s can better survive outbreaks of the disease due to the lower temperatures and higher salinity of their environment.<ref>{{Cite web |author=Nono Shen|agency=The Canadian Press |date=2025-04-02 |title=Critically endangered sunflower sea stars are seeking refuge in B.C. fiords |url=https://www.thecanadianpressnews.ca/national/critically-endangered-sunflower-sea-stars-are-seeking-refuge-in-b-c-fiords/article_2d1b3c80-3dbe-56f8-b955-d49b6fc8e458.html |access-date=2025-04-09 |website=thecanadianpressnews.ca |language=en}}</ref><ref>{{Cite journal |last1=Gehman |first1=Alyssa-Lois Madden |last2=Pontier |first2=Ondine |last3=Froese |first3=Tyrel |last4=VanMaanen |first4=Derek |last5=Blaine |first5=Tristan |last6=Sadlier-Brown |first6=Gillian |last7=Olson |first7=Angeleen M. |last8=Monteith |first8=Zachary L. |last9=Bachen |first9=Krystal |last10=Prentice |first10=Carolyn |last11=Hessing-Lewis |first11=Margot |last12=Jackson |first12=Jennifer M. |date=2025-04-02 |title=Fjord oceanographic dynamics provide refuge for critically endangered Pycnopodia helianthoides |journal=Proceedings of the Royal Society B: Biological Sciences |volume=292 |issue=2044 |article-number=20242770 |doi=10.1098/rspb.2024.2770 |pmc=11961252 |pmid=40169020}}</ref>

The protozoan ''[[Orchitophrya stellarum]]'' is known to infect and damage the gonads of starfish.<ref name="Robles166" /> Starfish are vulnerable to high temperatures. Experiments have shown that the feeding and growth rates of ''P.&nbsp;ochraceus'' reduce greatly when their body temperatures rise above {{convert|23|C|F|0|abbr=on}} and that they die when their temperature rises to {{convert|30|C|F|0|abbr=on}}.<ref>{{cite journal |author1=Peters, L. E. |author2=Mouchka M. E. |author3=Milston-Clements, R. H. |author4=Momoda, T. S. |author5=Menge, B. A. |year=2008 |title=Effects of environmental stress on intertidal mussels and their sea star predators |journal=Oecologia |volume=156 |issue=3 |pages=671–680 |doi=10.1007/s00442-008-1018-x |pmid=18347815|bibcode=2008Oecol.156..671P |s2cid=19557104 }}</ref><ref>{{cite journal|author1=Pincebourde, S. |author2=Sanford, E. |author3=Helmuth, B. |year=2008 |title=Body temperature during low tide alters the feeding performance of a top intertidal predator |journal=Limnology and Oceanography |volume=53 |issue=4 |pages=1562–1573|doi= 10.4319/lo.2008.53.4.1562|bibcode=2008LimOc..53.1562P |s2cid=1043536 |doi-access=free }}</ref><ref name="Robles166" /> This species has a unique ability to absorb seawater to keep itself cool when it is exposed to sunlight by a receding tide.<ref>{{cite journal|author1=Pincebourde, S. |author2=Sanford, E. |author3=Helmuth, B. |year=2009 |title=An intertidal sea star adjusts thermal inertia to avoid extreme body temperatures |journal=The American Naturalist |volume=174 |issue=6 |pages=890–897 |jstor=10.1086/648065|doi=10.1086/648065|pmid=19827942|bibcode=2009ANat..174..890P |s2cid=13862880 |url=https://scholarcommons.sc.edu/biol_facpub/54 |url-access=subscription }}</ref> It also appears to rely on its arms to absorb heat, so as to protect the central disc and vital organs.<ref>{{cite journal|author1=Pincebourde, S. |author2=Sanford, E. |author3=Helmuth, B. |year=2013|title=Survival and arm abscission are linked to regional heterothermy in an intertidal sea star|journal=Journal of Experimental Biology|volume=216|issue=12|pages=2183–2191|doi=10.1242/jeb.083881 |pmid=23720798|bibcode=2013JExpB.216.2183P |s2cid=4514808 |doi-access=free}}</ref>

Starfish and other echinoderms can be vulnerable to [[marine pollution]].<ref>{{cite journal|author1=Newton, L. C. |author2=McKenzie, J. D. |year=1995|title=Echinoderms and oil pollution: A potential stress assay using bacterial symbionts |journal=Marine Pollution Bulletin|volume=31 |issue=4–12|pages=453–456 |doi=10.1016/0025-326X(95)00168-M|bibcode=1995MarPB..31..453N }}</ref> The [[common starfish]] is considered to be a [[bioindicator|bioindicator species]] for marine ecosystems.<ref>{{cite journal|author1=Temara, A. |author2=Skei, J.M. |author3=Gillan, D. |author4=Warnau, M. |author5=Jangoux, M. |author6=Dubois, Ph. |year=1998|title=Validation of the asteroid ''Asterias rubens'' (Echinodermata) as a bioindicator of spatial and temporal trends of Pb, Cd, and Zn contamination in the field|journal=Marine Environmental Research|volume=45|issue=4–5 |pages=341–56 |doi=10.1016/S0141-1136(98)00026-9|bibcode=1998MarER..45..341T }}</ref> A 2009 study found that ''P. ochraceus'' is unlikely to be affected by [[ocean acidification]] as severely as other marine animals with [[calcareous]] skeletons. In other groups, structures made of calcium carbonate are vulnerable to dissolution when the [[pH]] is lowered. Researchers found that when ''P. ochraceus'' were exposed to {{convert|21|C|F}} and 770&nbsp;[[parts per million|ppm]] [[carbon dioxide]] (beyond rises expected in the next century), they were relatively unaffected. Their survival is likely due to the nodular nature of their skeletons, which are able to compensate for a shortage of carbonate by growing more fleshy tissue.<ref>{{cite journal |last1=Gooding|last2= Harley|last3= Tang|year=2009 |title=Elevated water temperature and carbon dioxide concentration increase the growth of a keystone echinoderm|journal=Proceedings of the National Academy of Sciences |volume=106 |issue=23 |pages=9316–9321 |doi=10.1073/pnas.0811143106 |pmid=19470464 |pmc=2695056 |first1=Rebecca A. |first2= Christopher D. G. |first3=Emily |bibcode= 2009PNAS..106.9316G|doi-access= free}}</ref>

==Evolution==

===Fossil record===
[[File:Riedaster reicheli.JPG|thumb|alt=Starfish fossil|Starfish fossil, ''[[Riedaster reicheli]]'', from the ''[[Plattenkalk]]'' [[Upper Jurassic]] limestone, [[Solnhofen]]]]
The earliest fossil echinoderms date to the [[Cambrian]], with the first asterozoans (a group that includes starfish and brittle stars) being the [[Asterozoa#Somasteroidea|Somasteroidea]], which exhibit traits of both groups.<ref name="Waggonerfossil">{{cite web |url=http://www.ucmp.berkeley.edu/echinodermata/echinofr.html |title=Echinodermata: Fossil Record |author=Wagonner, Ben |year=1994 |work=Echinodermata |publisher=The Museum of Paleontology of The University of California at Berkeley |access-date=2013-05-31}}</ref> Starfish are infrequently found as fossils, possibly because their hard skeletal components separate as the animal decays. Despite this, there are a few places where accumulations of complete skeletal structures occur, fossilized in place in [[Lagerstätte]]n – so-called "starfish beds".<ref>{{cite book |title=Introduction to Paleobiology and the Fossil Record |last1=Benton |first1=Michael J. |author-link = Michael Benton|last2=Harper |first2=David A. T. |author-link2 = David Harper (palaeontologist)|year=2013 |publisher=Wiley |chapter=15. Echinoderms |isbn=978-1-118-68540-2 |chapter-url=https://books.google.com/books?id=ZQ1U9wmoensC&q=%22Starfish+bed%22+fossil&pg=PT691 }}</ref>

By the late [[Paleozoic]], the [[Crinoidea|crinoids]] and [[blastoid]]s were the predominant echinoderms, fragments of which are almost the only fossil found in some limestones. In the two major [[extinction event]]s that occurred during the late [[Devonian]] and late [[Permian]], the blastoids were wiped out and only a few species of crinoids survived.<ref name=Waggonerfossil/> Many starfish species also became extinct in these events, but afterwards the surviving few species quickly diversified rapidly over sixty million years between the beginning and middle of the [[Middle Jurassic]].<ref name=treeoflife/><ref name="plosoneMah">{{cite journal |author1=Mah, Christopher L. |author2=Blake, Daniel B. |year=2012 |title=Global diversity and phylogeny of the Asteroidea (Echinodermata) |journal=PLOS ONE|volume=7 |issue=4 |article-number=e35644 |doi=10.1371/journal.pone.0035644 |pmid=22563389 |pmc=3338738 |bibcode=2012PLoSO...735644M |editor1-last=Badger |editor1-first=Jonathan H|doi-access=free }}</ref> A 2012 study found that [[speciation]] in starfish can occur rapidly. During the last 6,000 years, divergence in the larval development of ''[[Cryptasterina hystera]]'' and ''[[Cryptasterina pentagona]]'' has taken place, the former adopting internal fertilization and brooding and the latter remaining a broadcast spawner.<ref>{{cite journal|author1=Purit, J. B. |author2=Keever, C. C. |author3=Addison, J. A. |author4=Byrne, M. |author5=Hart, M. W. |author6=Grosberg, R. K. |author7=Toonen, R. J. |year=2012|title=Extraordinarily rapid life-history divergence between ''Cryptasterina'' sea star species|journal=Proceedings of the Royal Society B: Biological Sciences |volume=279 |issue=1744 |pages=3914–3922 |doi=10.1098/rspb.2012.1343 |pmid=22810427 |pmc=3427584}}</ref>

===Diversity===
{{see also|List of echinodermata orders}}
The scientific name Asteroidea was given to starfish by the French zoologist [[Henri Marie Ducrotay de Blainville|de Blainville]] in 1830.<ref>{{cite WoRMS |author=Hansson, Hans |year=2013 |title=Asteroidea  |id=123080 |access-date=2013-07-19 }}</ref> It is derived from the Greek ''aster'', ἀστήρ (a star) and the Greek ''eidos'', εἶδος (form, likeness, appearance).<ref>{{cite web |url=http://www.myetymology.com/latin/Asteroidea.html |archive-url=https://web.archive.org/web/20131021164652/http://www.myetymology.com/latin/Asteroidea.html |url-status=usurped |archive-date=21 October 2013 |title=Etymology of the Latin word Asteroidea |year=2008 |work=MyEtymology |access-date=2013-07-19}}</ref> Starfish are included in the subphylum [[Asterozoa]] along with brittle and basket stars (order [[Ophiuroidea]]), the characteristics of which include a star-shaped body as adults, with multiple arms surrounding central disc. The arms of asteroids are skeletally connected to the disc by ossicles in the body wall<ref name="treeoflife">{{cite web |url=http://tolweb.org/Asteroidea |title=Asteroidea: Sea stars and starfishes |author=Knott, Emily |year=2004 |work=Tree of Life web project |access-date=2012-10-19}}</ref> while ophiuroids have clearly demarcated arms.<ref>{{cite web |url=http://www.marinespecies.org/ophiuroidea/ |title=World Ophiuroidea Database |author1=Stöhr, S. |author2=O'Hara, T. |access-date=2012-10-19}}</ref>

The starfish are a large and diverse class with over 1,900 living species. There are seven [[Extant taxon|extant]] orders: [[Brisingida]], [[Forcipulatida]], [[Notomyotida]], [[Paxillosida]], [[Spinulosida]], [[Valvatida]] and [[Velatida]].<ref name=Knott/><ref name="Asterozoa"/> Living asteroids, the Neoasteroidea, are distinct from their forerunners in the Paleozoic. Their classification has changed little but there is debate in regards to Paxillosida. The deep-water [[Sea daisy|sea daisies]], though clearly Asteroidea and currently included in [[Velatida]], do not fit easily in any accepted lineage. [[Phylogenetics|Phylogenetic data]] suggests that they may be a [[sister group]], the Concentricycloidea, to the Neoasteroidea, or that the Velatida themselves may be a sister group.<ref name=plosoneMah/>

[[File:Manningstar 600.jpg|thumb|right|alt=18-arm starfish|A large 18-armed member of [[Brisingida]]]]

====Living groups====

; [[Brisingida]] (2 families, 17 genera, 111 species)<ref>{{cite WoRMS |author=Mah, Christopher |year=2012 |title=Brisingida |id=123085 |access-date=2012-09-15 |db=World Asteroidea}}</ref>
:Species in this order have a small, rigid disc and 6–20 long, thin arms, which they use for suspension feeding. They have one series of marginal plates, disc plates merged in a ring, fewer numbers of aboral plates, crossed pedicellariae, and several series of long spines on the arms. They mostly live in deep-sea habitats, although a few live in shallow waters in the Antarctic.<ref>{{cite journal |url=http://si-pddr.si.edu/dspace/bitstream/10088/5505/2/SCtZ-0435-Lo_res.pdf |title=Revision of the Atlantic Brisingida (Echinodermata: Asteroidea), with description of a new genus and family |author=Downey, Maureen E. |year=1986 |journal=Smithsonian Contributions to Zoology |issue=435 |pages=1–57 |publisher=Smithsonian Institution Press |doi=10.5479/si.00810282.435 |access-date=2012-10-18 |archive-date=27 February 2012 |archive-url=https://web.archive.org/web/20120227010606/http://si-pddr.si.edu/dspace/bitstream/10088/5505/2/SCtZ-0435-Lo_res.pdf }}</ref><ref>{{cite encyclopedia |url=http://accessscience.com/abstract.aspx?id=900559 |title=Brisingida |encyclopedia=Access Science: Encyclopedia |publisher=McGraw-Hill |author=Mah, Christopher |access-date=2012-09-15 |archive-url=https://web.archive.org/web/20121030063248/http://accessscience.com/abstract.aspx?id=900559 |archive-date=30 October 2012}}</ref> In some species, the tube feet have rounded tips and lack suckers.<ref name="Vickery">{{cite journal |author1=Vickery, Minako S. |author2=McClintock, James B. |year=2000 |title=Comparative morphology of tube feet among the Asteroidea: phylogenetic implications |journal=Integrative and Comparative Biology |volume=40 |issue=3 |pages=355–364 |doi=10.1093/icb/40.3.355 |doi-access=free }}</ref>
[[File:Starfish, Caswell Bay - geograph.org.uk - 409413.jpg|thumb|alt=Common starfish|[[Common starfish]], a member of [[Forcipulatida]]]]
; [[Forcipulatida]] (6 families, 63 genera, 269 species)<ref>{{cite WoRMS |author=Mah, Christopher |year=2012 |title=Forcipulatida |id=123086 |access-date=2012-09-15 |db=World Asteroidea}}</ref>
:Species in this order have distinctive [[pedicellaria]]e, consisting of a short stalk with forceps-like tips. and tube feet with flat-tipped suckers usually arranged in four rows.<ref name=Vickery/><ref name="Ruppert889" /><ref name="treeoflife"/> The order includes well-known species from temperate and cold-water regions, ranging from intertidal to abyssal zones.<ref>{{cite encyclopedia |url=http://accessscience.com/abstract.aspx?id=900558&referURL=http%3a%2f%2faccessscience.com%2fcontent.aspx%3fsearchStr%3dForcipulatida%26id%3d900558 |title=Forcipulatida |author=Mah, Christopher |encyclopedia=Access Science: Encyclopedia |publisher=McGraw-Hill |access-date=2012-09-15 |archive-url=https://web.archive.org/web/20121030063255/http://accessscience.com/abstract.aspx?id=900558&referURL=http%3A%2F%2Faccessscience.com%2Fcontent.aspx%3FsearchStr%3DForcipulatida%26id%3D900558 |archive-date=30 October 2012}}</ref>
; [[Notomyotida]] (1 family, 8 genera, 75 species)<ref>{{cite WoRMS |author=Mah, Christopher |year=2012 |title=Notomyotida |id=123087 |access-date=2012-09-15 |db=World Asteroidea}}</ref>
:These starfish are deep-sea dwelling and have particularly flexible arms with distinctive lines of musculature along the sides of the dorsal region.<ref name="Asterozoa"/> In some species, the tube feet lack suckers.<ref name=Vickery/>
[[File:Luidia magnifica mouth.jpg|thumb|alt=Magnificent star|[[Luidia magnifica|Magnificent star]], a member of [[Paxillosida]]]]
; [[Paxillosida]] (7 families, 48 genera, 372 species)<ref>{{cite WoRMS |author=Mah, Christopher |year=2012 |title=Paxillosida  |id=123088 |access-date=2012-09-15 |db=World Asteroidea}}</ref>
:This is a primitive order whose members do not extrude their stomach<ref name="Ruppert885"/> when feeding and both anus and tube feet suckers are absent.<ref name=Matsubara/> Papulae are present on their aboral surface, and they possess marginal plates and paxillae. They mostly inhabit soft substrates.<ref name="Ruppert889">Ruppert et al, 2004. pp. 887–889</ref> There is no brachiolaria stage in their larval development.<ref name=Matsubara>{{cite journal |author1=Matsubara, M. |author2=Komatsu, M. |author3=Araki, T. |author4=Asakawa, S. |author5=Yokobori, S.-I. |author6=Watanabe, K. |author7=Wada, H. |year=2005 |title=The phylogenetic status of Paxillosida (Asteroidea) based on complete mitochondrial DNA sequences |journal=Molecular Genetics and Evolution |volume=36 |issue=3 |pages=598–605 |doi=10.1016/j.ympev.2005.03.018 |pmid=15878829 |bibcode=2005MolPE..36..598M }}</ref> The comb starfish (''[[Astropecten polyacanthus]]'') is a member of this order.<ref>{{cite WoRMS |author=Mah, Christopher |year=2012 |title=''Astropecten polyacanthus'' Müller & Troschel, 1842  |id=213245 |access-date=2013-07-06 |db=World Asteroidea}}</ref>
[[File:Estrella de mar de espinas rojas (Protoreaster linckii), Zanzíbar, Tanzania, 2024-06-01, DD 06.jpg|thumb|alt=Red-knobbed starfish|[[Red-knobbed starfish]], a member of [[Valvatida]]]]
; [[Spinulosida]] (1 family, 8 genera, 121 species)<ref>{{cite WoRMS |author=Mah, Christopher |year=2012 |title=Spinulosida |id=123089 |access-date=2012-09-15 |db=World Asteroidea}}</ref>
:Most species in this order lack pedicellariae; all have a delicate skeletal arrangement with small or no marginal plates on the disc or arms. They have numerous groups of short spines on the aboral surface.<ref>{{cite encyclopedia |url=http://accessscience.com/overflow.aspx?SearchInputText=Spinulosida+&ContentTypeSelect=4 |title=Spinulosida |encyclopedia=Access Science: Encyclopedia |publisher=McGraw-Hill |access-date=2012-09-15 |archive-url=https://web.archive.org/web/20121030063309/http://accessscience.com/overflow.aspx?SearchInputText=Spinulosida+&ContentTypeSelect=4 |archive-date=30 October 2012}}</ref><ref name=Blake/>
; [[Valvatida]] (16 families, 172 genera, 695 species)<ref>{{cite WoRMS |author=Mah, Christopher |year=2012 |title=Valvatida |id=123090 |access-date=2012-09-15 |db=World Asteroidea}}</ref>
:Most species in this order have five arms and two rows of tube feet with suckers. There are conspicuous marginal plates on the arms and disc. Some species have [[Paxilla (ossicle)|paxillae]] and in some, the main pedicellariae are clamp-like and recessed into the skeletal plates.<ref name="Blake">{{cite journal |author=Blake, Daniel B. |year=1981 |title=A reassessment of the sea-star orders Valvatida and Spinulosida |journal=Journal of Natural History |volume=15 |issue=3 |pages=375–394 |doi=10.1080/00222938100770291 |bibcode=1981JNatH..15..375B }}</ref>
; [[Velatida]] (4 families, 16 genera, 138 species)<ref>{{cite WoRMS |author=Mah, Christopher |year=2012 |title=Velatida |id=123091 |access-date=2012-09-15 |db=World Asteroidea}}</ref>
:This order of starfish consists mostly of deep-sea and other cold-water starfish often with a global distribution. The shape is pentagonal or star-shaped with five to fifteen arms. The skeleton is underdeveloped.<ref>{{cite encyclopedia |url=http://accessscience.com/abstract.aspx?id=900662&referURL=http%3a%2f%2faccessscience.com%2fcontent.aspx%3fid%3d900662 |title=Velatida |author=Mah, Christopher |encyclopedia=Access Science: Encyclopedia |publisher=McGraw-Hill |access-date=2012-09-15 |archive-url=https://web.archive.org/web/20121030063303/http://accessscience.com/abstract.aspx?id=900662&referURL=http%3A%2F%2Faccessscience.com%2Fcontent.aspx%3Fid%3D900662 |archive-date=30 October 2012}}</ref>

====Extinct groups====
Extinct groups within the Asteroidea include:<ref name="Knott">{{cite web | url=http://tolweb.org/Asteroidea | title=Asteroidea. Sea stars and starfishes | publisher=Tree of Life web project| date=7 October 2004| access-date=2013-05-10 | author=Knott, Emily}}</ref>
* † [[Calliasterellidae]], with the type genus ''[[Calliasterella]]'' from the Devonian and Carboniferous<ref>{{cite web | url=https://paleobiodb.org/classic/checkTaxonInfo?taxon_no=101372 | title=Family Calliasterellidae | publisher=Paleobiology Database | access-date=2013-05-10 | archive-date=9 March 2016 | archive-url=https://web.archive.org/web/20160309101759/http://paleodb.org/?a=basictaxoninfo&taxon_no=101372 | url-status=live }}</ref>
* † [[Palasteriscus]], a Devonian genus<ref>[[Cyril Walker (paleontologist)|Walker, Cyril]], [[David Ward (paleontologist)|Ward, David]]''[[Fossil]]s : [[Smithsonian]] Handbook'', {{ISBN|0-7894-8984-8}} (2002, paperback, revisited), {{ISBN|1-56458-074-1}} (1992, 1st edition). Page 186</ref>
* † [[Trichasteropsida]], with the [[Triassic]] genus ''[[Trichasteropsis]]'' (at least 2 species)<ref name=Knott/>

===Phylogeny===

====External====

Starfish are [[deuterostome]] animals, like the [[chordate]]s. A 2014 analysis of 219 genes from all classes of echinoderms gives the following [[phylogenetic tree]].<ref name="TelfordLowe2014">{{cite journal |last1=Telford |first1=M. J. |last2=Lowe|first2=C. J.|last3=Cameron |first3=C. B. |last4=Ortega-Martinez |first4=O. |last5=Aronowicz |first5=J. |last6=Oliveri|first6=P. |last7=Copley |first7=R. R. |title=Phylogenomic analysis of echinoderm class relationships supports Asterozoa |journal=Proceedings of the Royal Society B: Biological Sciences |volume=281 |issue=1786 |year=2014 |article-number=20140479 |doi=10.1098/rspb.2014.0479|pmid=24850925 |pmc=4046411 }}</ref> The times at which the [[clade]]s diverged are shown under the labels in millions of years ago (mya).

{{Clade 
|label1=[[Bilateria]]
|1={{Clade
   |1=[[Xenacoelomorpha]] [[File:Proporus sp.png|80 px]]
   |label2=[[Nephrozoa]]
   |sublabel2=650 mya
   |2={{clade
      |label1 =[[Protostomia]]
      |sublabel1=610 mya
      |1={{Clade
         |1=[[Ecdysozoa]] [[File:Long nosed weevil edit.jpg|80 px]]
         |2=[[Spiralia]] [[File:Grapevinesnail 01.jpg|80 px]]
         }}
      |label2=[[Deuterostomia]]
      |sublabel2=>540 mya
      |2={{clade
         <!--|sublabel1=525 mya --><!--Chordates-->
         |1=[[Chordata]] [[File:Common carp (white background).jpg|80 px]]
         |label2=[[Echinodermata]]
         |sublabel2=c. 500 mya
         |2={{clade
            |1=[[Crinoidea]] [[File:Crinoid on the reef of Batu Moncho Island (cropped).JPG|50 px]]
            |label2=[[Eleutherozoa]]
            |2={{clade
               |label1=[[Asterozoa]]
               |1={{clade
                  |1='''Asteroidea''' [[File:Portugal 20140812-DSC01434 (21371237591).jpg|100 px]]
                  |2=[[Ophiuroidea]] [[File:Ophiura ophiura.jpg|80 px]]
                  }}
               |label2=[[Echinozoa]]
               |2={{clade
                  |1=[[Echinoidea]] [[File:Sea urchin (217110954).jpg|80 px]]
                  |2=[[Holothuroidea]] [[File:Holothuroidea.JPG|80 px]]
                  }}
               }}
            }}
         }}
      }}
   }}
}}

====Internal====

The phylogeny of the Asteroidea has been difficult to resolve, with visible (morphological) features proving inadequate, and the question of whether traditional [[taxon|taxa]] are [[clade]]s apply.<ref name=Knott/> The phylogeny proposed by Gale in 1987 is:<ref name=Knott/><ref>{{cite journal | title=Phylogeny and classification of the Asteroidea (Echinodermata) | author=Gale, A. S. | journal=Zoological Journal of the Linnean Society | year=1987 | volume=89 | pages=107–132 | doi=10.1111/j.1096-3642.1987.tb00652.x | issue=2}}</ref>

{{clade | style = font-size: 90%; line-height:75%
|1={{clade
   |1=† Palaeozoic Asteroids
   |2={{clade
      |1={{clade
         |1=Paxillosida
         |2={{clade
            |1=Forcipulatida, including Brisingida
            |2=Valvatida, including Velatida, Spinulosida ''(not a clade)''<ref name=Knott/>
            }}
         }}
      }}
   }}
}}

The phylogeny proposed by Blake in 1987 is:<ref name=Knott/><ref>{{cite journal | title=A classification and phylogeny of post-Paleozoic sea stars (Asteroidea: Echinodermata) | author=Blake, D. B. | journal=Journal of Natural History | year=1987 | volume=21 | pages=481–528 | doi=10.1080/00222938700771141 | issue=2| bibcode=1987JNatH..21..481B }}</ref>

{{clade | style = font-size: 90%; line-height:75%
|1={{clade
   |1=† Palaeozoic Asteroids
   |2={{clade
      |1={{clade
         |1=† Calliasterellidae
         |2={{clade
            |1=† Compasteridae
            |2={{clade
               |1=† Trichasteropsida
               |2={{clade
                  |1={{clade
                     |1=Brisingida
                     |2=Forcipulatida
                     }}
                  |2={{clade
                     |1={{clade
                        |1=Spinulosida
                        |2=Velatida
                        }}
                     |2={{clade
                        |1=Paxillosida
                        |2={{clade
                           |1=Notomyotida
                           |2=Valvatida
                           }}
                        }}
                     }}
                  }}
               }}
            }}
         }}
      }}
   }}
}}

Later work making use of [[Molecular clock|molecular evidence]], with or without the use of morphological evidence, had by 2000 failed to resolve the argument.<ref name=Knott/> In 2011, on further molecular evidence, Janies and colleagues noted that the phylogeny of the echinoderms "has proven difficult", and that "the overall phylogeny of extant echinoderms remains sensitive to the choice of analytical methods". They presented a phylogenetic tree for the living Asteroidea only; using the traditional names of starfish orders where possible, and indicating "part of" otherwise, the phylogeny is shown below. The Solasteridae are split from the Velatida, and the old Spinulosida is broken up.<ref name="Janies">{{cite journal | title=Echinoderm phylogeny including ''Xyloplax'', a progenetic asteroid |author1=Janies, Daniel A. |author2=Voight, Janet R. |author3=Daly, Marymegan | journal=Syst. Biol. | year=2011 | volume=60 | issue=4 | pages=420–438 | doi=10.1093/sysbio/syr044 | pmid=21525529| doi-access=free }}</ref>

{{clade | style = font-size: 85%; line-height:75%
|1={{clade
   |1= Velatida except for Solasteridae
   |2={{clade
      |1={{clade
         |1=Brisingida with part of Velatida, e.g. ''Caymanostella'' and part of Forcipulatida, e.g. ''Stichaster''
         |2=Forcipulatida
         }}
      |2={{clade
         |1=part of Spinulosida, e.g. ''[[Echinaster]]'', part of Valvatida, e.g. ''Archaster''
         |2={{clade
            |1=Paxillosida
            |2={{clade
               |1=Odontasteridae, which was a part of Valvatida
               |2=Solasteridae and part of Spinulosida, e.g. ''[[Stegnaster]]'' and part of Valvatida, e.g. ''[[Asterina (echinoderm)|Asterina]]''
               }}
            }}
         }}
      }}
   }}
}}

{{clade | style = font-size: 85%; line-height:75%
|1=Notomyotida (not analysed)
}}

==Human relations==

===In research===
[[File:CalcifiedStarfish.jpg|thumb|A calcified starfish from [[Los Roques Archipelago]]]]
Starfish have been used in reproductive and developmental studies. Female starfish produce large numbers of [[oocyte]]s that are easily isolated; these can be stored in a [[Meiosis|pre-meiosis]] phase and stimulated to complete division by the use of [[Adenine|1-methyladenine]].<ref>{{cite journal |author1=Wessel, G. M. |author2=Reich, A. M. |author3=Klatsky, P. C. |year=2010 |title=Use of sea stars to study basic reproductive processes |journal=Systems Biology in Reproductive Medicine |volume=56 |issue=3 |pages=236–245 |doi=10.3109/19396361003674879 |pmid=20536323 |pmc=3983664}}</ref> Starfish oocytes are well suited for this research as they are easy to handle, can be maintained in sea water at room temperature, are transparent, and develop quickly.<ref>{{cite web |url=http://www.embl.de/research/units/cbb/lenart/ |title=Cytoskeletal dynamics and function in oocytes |author=Lenart Group |publisher=European Molecular Biology Laboratory |access-date=2013-07-22 |archive-url=https://web.archive.org/web/20140801190144/http://www.embl.de/research/units/cbb/lenart/ |archive-date=1 August 2014 }}</ref> ''[[Asterina pectinifera]]'', used as a [[model organism]] for this purpose, is resilient and easy to breed and maintain in the laboratory.<ref name="Davydov">{{cite book |title=Animal Species for Developmental Studies: The Starfish ''Asterina pectinifera'' |last1=Davydov |first1=P. V. |last2=Shubravyi |first2=O. I. |last3=Vassetzky |first3=S. G. |year=1990 |publisher=Springer US |isbn=978-1-4612-7839-9 |doi=10.1007/978-1-4613-0503-3 |pages=287–311  |s2cid=42046815 }}</ref>

Another area of research is the ability of starfish to regenerate lost body parts. The [[stem cell]]s of adult humans are incapable of much differentiation and understanding the regrowth, repair and cloning processes in starfish may have implications for human medicine.<ref>{{cite journal |author1=Friedman, Rachel S. C. |author2=Krause, Diane S. |year=2009 |title=Regeneration and repair: new findings in stem cell research and ageing |journal=Annals of the New York Academy of Sciences |volume=1172 |issue=1 |pages=88–94 |doi=10.1111/j.1749-6632.2009.04411.x  |pmid=19735242 |s2cid=755324 }}</ref>

Starfish have an unusual ability to displace foreign objects from their bodies, which makes them difficult to tag for research tracking purposes.<ref>{{cite journal|last1=Olsen|first1=T. B.|last2=Christensen|first2=F. E. G.|last3=Lundgreen|first3=K.|last4=Dunn|first4=P. H.|last5=Levitis|first5=D. A.|year=2015|title=Coelomic Transport and Clearance of Durable Foreign Bodies by Starfish (Asterias rubens)|journal=The Biological Bulletin|volume=228|issue=2|pages=156–162|doi=10.1086/BBLv228n2p156 |pmid=25920718 |url=https://www.journals.uchicago.edu/doi/abs/10.1086/BBLv228n2p156|url-access=subscription}}</ref>

===In legend and culture===
[[File:Poems of the Sea, 1850 - Hope in God.png|thumb|200px|right|A starfish with five legs. Used as an illustration of "Hope in God", a poem by Lydia Sigourney which appeared in ''Poems for the Sea'', 1850]]

An [[Australian Aborigine|aboriginal Australian]] fable, retold by the Welsh school headmaster [[William Jenkyn Thomas]] (1870–1959),<ref>{{cite web | url=http://www.abgs.org.uk/teachers/headmasters/wjt/wjt.htm | title=William Jenkyn Thomas, M.A | publisher=The Aberdare Boys' Grammar School | access-date=2013-05-12 | archive-date=6 June 2014 | archive-url=https://web.archive.org/web/20140606204726/http://www.abgs.org.uk/teachers/headmasters/wjt/wjt.htm }}</ref> tells how some animals needed a canoe to cross the ocean. Whale had one but refused to lend it, so Starfish kept him busy, telling him stories and grooming him to remove parasites, while the others stole the canoe. When Whale realized the trick he beat Starfish ragged, which is how Starfish still is today.<ref>{{cite book | url=http://www.sacred-texts.com/aus/mla/mla08.htm | title=Some Myths and Legends of the Australian Aborigines | publisher=Whitcombe & Tombs | author=Thomas, William Jenkyn | year=1943 | pages=21–28}}</ref>

In 1900, the scholar [[Edward Tregear]] documented ''The Creation Song'', which he describes as "an ancient prayer for the dedication of a high chief" of [[Hawaii]]. Among the "uncreated gods" described early in the song is the starfish.<ref name="Tregear">{{cite journal | url=http://www.jps.auckland.ac.nz/document/Volume_9_1900/Volume_9,_No._1,_March_1900/%26%2339%3BThe_creation_song%26%2339%3B_of_Hawaii,_by_Edward_Tregear,_p_38-46/p1 | title="The Creation Song" of Hawaii | author=Tregear, Edward | journal=The Journal of the Polynesian Society | year=1900 | volume=9 | issue=1 | pages=38–46 | author-link=Edward Robert Tregear | access-date=13 May 2013 | archive-date=12 February 2016 | archive-url=https://web.archive.org/web/20160212145154/http://www.jps.auckland.ac.nz/document//Volume_9_1900/Volume_9%2C_No._1%2C_March_1900/%26%2339%3BThe_creation_song%26%2339%3B_of_Hawaii%2C_by_Edward_Tregear%2C_p_38-46/p1 }}</ref>

[[Georg Eberhard Rumpf]]'s 1705 ''The Ambonese Curiosity Cabinet'' describes the tropical varieties of ''Stella Marina'' or ''Bintang Laut'', "Sea Star" in Latin and [[Malay language|Malay]] respectively, known in the waters around [[Ambon Island|Ambon]]. He writes that the ''Histoire des Antilles'' reports that when the sea stars "see thunder storms approaching, [they] grab hold of many small stones with their little legs, looking to&nbsp;... hold themselves down as if with anchors".<ref name="Rumpf">{{cite book | title=The Ambonese Curiosity Cabinet (original title: Amboinsche Rariteitkamer) | publisher=Yale University Press | author=Rumphius, Georgious Everhardus (= Georg Eberhard Rumpf); Beekman, E.M. (trans.) | year=1999|orig-date=1705 | isbn=978-0-300-07534-2 | page=68}}</ref>

===As food===
[[File:What to eat - strange food in China.jpg|thumb|right|Fried starfish skewers in China]]

Starfish are sometimes eaten in China,<ref>{{cite web | url=http://www.thechinaguide.com/food/exotic.html | title=Indulging in Exotic Cuisine in Beijing | publisher=The China Guide | date=2011 | access-date=28 February 2014 | archive-url=https://web.archive.org/web/20140303173324/http://www.thechinaguide.com/food/exotic.html | archive-date=3 March 2014}}</ref> Japan<ref>{{cite web | url=http://www.amakusa.tv/enewshito.html#ibook | title=Cooking Starfish in Japan | publisher=Amakusa TV | work=ebook10005 | date=2011-08-07 | access-date=2013-05-18 | author=Amakusa TV Co. Ltd.}}</ref><ref>{{cite web | url=http://search.kenko.com/product/%E5%B0%8F%E8%A2%8B | title=Pouch A | language=ja | publisher=Kenko.com | access-date=2013-05-18 | archive-url=https://web.archive.org/web/20140803035529/http://search.kenko.com/product/%E5%B0%8F%E8%A2%8B | archive-date=3 August 2014}}</ref> and in Micronesia.<ref>{{cite book | title=Words of the Lagoon: Fishing and Marine Lore in the Palau District of Micronesia | url=https://archive.org/details/bub_gb_TloVDfV7QLoC | publisher=University of California Press | author=Johannes, Robert Earle | year=1981 | pages=[https://archive.org/details/bub_gb_TloVDfV7QLoC/page/n100 87]}}</ref> Georg Eberhard Rumpf found few starfish being used for food in the [[Indonesia]]n archipelago, other than as bait in fish traps, but on the island of "Huamobel" {{sic}} the people cut them up, squeeze out the "black blood" and cook them with sour [[tamarind]] leaves; after resting the pieces for a day or two, they remove the outer skin and cook them in [[coconut milk]].<ref name=Rumpf/>

===As collectables===
Starfish are in some cases taken from their habitat and sold to tourists as [[souvenir]]s, [[Ornament (art)|ornaments]], curios or for display in aquariums. In particular, ''Oreaster reticulatus'', with its easily accessed habitat and bright coloration, is widely collected in the Caribbean. In the early to mid 20th century, this species was numerous along the West Indian coasts, but collection and trade have severely diminished its numbers. In the [[Florida|State of Florida]], ''O. reticulatus'' is listed as [[endangered]] and its collection is illegal. Nevertheless, it is still sold both in and outside its range.<ref name=Scheibling150/> A similar phenomenon exists in the Indo-Pacific for species such as ''[[Protoreaster nodosus]]''.<ref>{{cite journal |author1=Bos, A. R. |author2=Gumanao, G. S. |author3=Alipoyo, J. C. E. |author4=Cardona, L. T. |year=2008 |title=Population dynamics, reproduction and growth of the Indo-Pacific horned sea star, ''Protoreaster nodosus'' (Echinodermata; Asteroidea)|journal=Marine Biology |volume=156 |issue=1 |pages=55–63 |url=https://www.researchgate.net/publication/230887109 |doi=10.1007/s00227-008-1064-2|s2cid=84521816 |doi-access=free |bibcode=2008MarBi.156...55B |hdl=2066/72067 |hdl-access=free }}</ref>

[[File:Starfish in Cyprus.JPG|thumb|center|Starfish on sale as souvenirs in Cyprus]]

==References==
{{reflist}}

==Bibliography==
{{Refbegin}}
* {{cite book | editor=Lawrence, J. M. | year=2013 | title=Starfish: Biology and Ecology of the Asteroidea | publisher=Johns Hopkins University Press | isbn=978-1-4214-0787-6}}
* {{cite book | last1=Ruppert | first1=Edward E. | last2=Fox | first2=Richard S. | last3=Barnes | first3=Robert D. | year=2004 | title=Invertebrate Zoology |edition=7th | publisher=Cengage Learning | isbn=978-81-315-0104-7}}
{{Refend}}

== External links ==
{{Commons|Starfish}}
* {{cite web |last1=Mah |first1=Christopher L. |date=24 January 2012 |title=The Echinoblog |url=http://echinoblog.blogspot.fr/2012/01/how-to-tell-apart-knobby-stars.html}} A blog about sea stars by a passionate and professional specialist.

{{Taxonbar|from=Q25349}}
{{Authority control}}

[[Category:Asteroidea| ]]
[[Category:Articles containing video clips]]
[[Category:Late Ordovician first appearances]]
[[Category:Extant Ordovician first appearances]]
[[Category:Taxa named by Henri Marie Ducrotay de Blainville]]