View source for Amphipoda

{{Short description|Order of malacostracan crustaceans}}
{{Redirect|Sea flea|the Jacques Cousteau diving saucers|SP-350 Denise#SP-500 Sea Fleas}}
{{Lead rewrite|date=July 2025}}
{{Automatic taxobox
| fossil_range = {{Geological range|Hauterivian|Recent}}
| image = Gammarus roeselii.jpg
| image_caption = ''[[Gammarus roeselii]]''
| display_parents = 2
| taxon = Amphipoda
| authority = [[Pierre André Latreille|Latreille]], 1816
| subdivision_ranks = Suborders
| subdivision = [[#Taxonomy|See text]]
}}

'''Amphipoda''' ({{IPAc-en|æ|m|ˈ|f|ɪ|p|ə|d|ə}}) is an [[order (biology)|order]] of [[malacostraca]]n [[crustacean]]s with no [[carapace]] and generally with laterally compressed bodies. Amphipods ({{IPAc-en|ˈ|æ|m|f|ɪ|p|ɒ|d|z}}) range in size from 1 to 340 millimetres (0.039 to 13&nbsp;in) and are mostly [[detritivore]]s or [[scavenger]]s. More than 10,700 amphipod species are currently recognized. They are mostly marine animals but are found in almost all aquatic environments. Some 2,250 species live in fresh water, and the order also includes the terrestrial [[Talitridae|sandhoppers]], such as ''[[Talitrus saltator]]'' and ''[[Arcitalitrus sylvaticus]]''.

==Description==
===Anatomy===
[[File:Scheme amphipod anatomy-en.svg|thumb|Anatomical diagram of the [[gammaridea]]n amphipod ''[[Leucothoe incisa]]'']]
Amphipods are [[Autapomorphy|characterized]] by sessile eyes (without [[eyestalk]]s), no [[carapace]], and the presence of [[gill]]s on the [[Arthropod coxa|coxae]].<ref name=Lowry2017/> Their bodies are divided into 13 segments, which can be [[tagmosis|grouped into]] the head, [[thorax]] and [[Arthropod abdomen|abdomen]].<ref name="Wade">{{cite book |first1=Sam |last1=Wade |first2=Tracy |last2=Corbin |first3=Linda-Marie |last3=McDowell |isbn=1-876562-67-6 |year=2004 |publisher=Waterwatch South Australia |title=Critter Catalogue. A guide to the aquatic invertebrates of South Australian inland waters |chapter=Class Crustacea |url=http://www.sa.waterwatch.org.au/pdfs/critters_5_crustaceans.pdf  |archive-url=https://web.archive.org/web/20091017035859/http://sa.waterwatch.org.au/pdfs/critters_5_crustaceans.pdf |archive-date=2009-10-17 }}</ref>

The head is fused to the thorax, and bears two pairs of [[antenna (biology)|antennae]] and one pair of [[compound eye]]s.<ref name="TAFI">{{cite web|url=http://www.tafi.org.au/zooplankton/imagekey/malacostraca/peracarida/amphipoda/amphipoda.html |title=Order Amphipoda |work=Guide to the marine zooplankton of south eastern Australia |year=2008 |publisher=[[Tasmanian Aquaculture & Fisheries Institute]]  |archive-url=https://web.archive.org/web/20080720143838/http://www.tafi.org.au/zooplankton/imagekey/malacostraca/peracarida/amphipoda/amphipoda.html |archive-date=2008-07-20 }}</ref> It also carries the [[arthropod mouthparts|mouthparts]], but these are mostly concealed.<ref name="Holsinger"/> Amphipods are one of the few groups to possess [[gnathopod]]s, which are uniquely modified feeding legs.<ref>{{cite web |title=Definitions Gnathopods |url=https://research.nhm.org/glossary/define.html?term=Gnathopods |website=research.nhm.org |publisher=Natural History Museum of Los Angeles County |access-date=1 July 2025}}</ref>

The thorax and abdomen are usually quite distinct and bear different kinds of legs; they are typically laterally compressed.<ref name="TAFI"/> The thorax bears eight pairs of [[uniramous]] [[appendage]]s, the first of which are used as accessory mouthparts; the next four pairs are directed forwards, and the last three pairs are directed backwards.<ref name="TAFI"/> Gills are present on the thoracic segments, and there is an [[open circulatory system]] with a [[heart]], using [[Hemocyanin|haemocyanin]] to carry [[oxygen]] in the [[hemolymph|haemolymph]] to the tissues. The uptake and [[excretion]] of [[salt (chemistry)|salts]] is controlled by special [[gland]]s on the antennae.<ref name="Wade"/>

The abdomen is divided into two sections comprising three segments each:<ref name=Lowry2017/> the pleosome which bears [[pleopod|swimming legs]], and the urosome, which comprises a [[telson]] and three pairs of [[uropod]]s which do not form a tail fan as they do in animals such as [[true shrimp]].<ref name="TAFI"/> Furthermore, amphipods are the only malacostracan crustaceans that possess more than one pair of uropods.<ref name=wad/> Both of these limb types are usually well-developed. [[wikt:epimeron|Epimera]] are present on the abdomen.<ref name=Lowry2017/>

Some amphipods possess aberrant body plans, such as the elongate [[skeleton shrimp]] (Caprellidae) and ''[[Macrohectopus]]'' (Macrohectopidae).

===Size===
{{multiple image
| height1     = 200
| total_width = 420
| image1      = Americorchestia_sp.png
| class1      = bg-transparent
| alt1        = A clump of landhoppers in the palm of a hand
| caption1   = ''[[Americorchestia]] sp.''; [[Talitridae]]
| image2      = Alicella gigantea in hand.jpg
| class2      = bg-transparent
| alt2        = A giant amphipod bigger than a hand
| caption2   = ''[[Alicella gigantea]]''; [[Alicellidae]]
| footer      = Amphipods in hands
| footer_align = center
}}
Amphipods are typically less than {{convert|10|mm|1}} long, but the largest known amphipods are much larger; [[Zoological specimen|specimens]] of ''[[Alicella gigantea]]'' collected from {{convert|5300|m}} deep are {{convert|27|-|28|cm|1|abbr=on}} long,<ref name="Morris">{{cite book |first1=J. Laurens |last1=Barnard |first2=Darl E. |last2=Bowers |first3=Eugene C. |last3=Haderlie |chapter=Amphipoda: The Amphipods and Allies |title=Intertidal Invertebrates of California |editor1-first = Robert H. | editor1-last = Morris | editor2-first = Robert Hugh | editor2-last = Morris | editor3-first = Donald Putnam | editor3-last = Abbott | editor4-first = Eugene Clinton | editor4-last = Haderlie  |pages=559–566 |year=1980 |publisher=[[Stanford University Press]] |isbn=0-8047-1045-7}}</ref> reconstructed gut contents from a [[black-footed albatross]] indicates {{convert|34|cm}},<ref>{{cite journal |title=The supergiant amphipod ''Alicella gigantea'' Chevreux from the North Pacific Gyre |first1=J. Laurens |last1=Barnard |first2=Camilla L. |last2=Ingram |journal=[[Journal of Crustacean Biology]] |volume=6 |issue=4 |year=1986 |pages=825–839 |jstor=1548395 |bibcode=1986JCBio...6..825B }}</ref> and photographed animals from the [[Kermadec Trench]] indicate lengths of up to {{convert|34.9|cm|abbr=out}}.<ref>{{Cite journal |last1=Jamieson |first1=A. J. |last2=Lacey |first2=N. C. |last3=Lörz |first3=A. -N. |last4=Rowden |first4=A. A. |last5=Piertney |first5=S. B. |date=2013-08-01 |title=The supergiant amphipod Alicella gigantea (Crustacea: Alicellidae) from hadal depths in the Kermadec Trench, SW Pacific Ocean |url=https://www.sciencedirect.com/science/article/pii/S0967064512001932 |journal=Deep Sea Research Part II: Topical Studies in Oceanography |series=Deep-Sea Biodiversity and Life History Processes |language=en |volume=92 |pages=107–113 |doi=10.1016/j.dsr2.2012.12.002 |bibcode=2013DSRII..92..107J |issn=0967-0645|url-access=subscription }}</ref> The smallest known amphipods are less than {{convert|1|mm|2}} long.<ref>{{cite journal |doi=10.1098/rstb.1969.0014 |title=The fauna of Rennell and Bellona, Solomon Islands |first=T. |last=Wolff |journal=Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences |volume=255 |issue=800 |year=1969 |pages=321–343 |jstor=2416857|bibcode=1969RSPTB.255..321W |doi-access= }}</ref> The size of amphipods is limited by the availability of [[dissolved oxygen]], such that the amphipods in [[Lake Titicaca]] at an altitude of {{convert|3800|m}} can only grow up to {{convert|22|mm}}, compared to lengths of {{convert|90|mm}} in [[Lake Baikal]] at {{convert|455|m|-2}}.<ref>{{cite journal |first1=L.&nbsp;S. |last1=Peck |first2=G. |last2=Chapelle |title=Reduced oxygen at high altitude limits maximum size |doi=10.1098/rsbl.2003.0054 |journal=[[Proceedings of the Royal Society B]] |year=2003 |volume=270 |issue=Suppl 2 |pages=S166–S167 |pmid=14667371 |pmc=1809933}}</ref>

Some amphipods exhibit [[sexual dimorphism]]. In dimorphic species, males are usually larger than females, although this is reversed in the genus ''[[Crangonyx]]''.<ref name=Glazier/>

===Reproduction and life cycle===
Amphipods engage in [[amplexus]], a precopulatory guarding behavior in which males will grasp a female with their gnathopods and carry the female held against their ventral surface. Amplexus can last from two to over fifteen days, depending on water temperature, and ends when the female molts, at which point her eggs are ready for fertilisation.<ref name=Glazier>{{cite encyclopedia |last=Glazier |first=Doug |editor-last=Likens |editor-first=Gene |encyclopedia=Encyclopedia of Inland Waters |title=Amphipoda |year=2009 |publisher=Academic Press |isbn=978-0-12-088462-9 |doi=10.1016/B978-012370626-3.00154-X |pages=89–115 }}</ref>

Mature females bear a ''marsupium'', or [[brood pouch (Peracarida)|brood pouch]], which holds her [[egg (biology)|eggs]] while they are [[fertilisation|fertilised]],<ref name="Wade"/> and until the young are ready to hatch.<ref name="TAFI"/> As a female ages, she produces more eggs in each brood. Mortality is around 25–50% for the eggs. There are no [[larva]]l stages; the eggs hatch directly into a [[Juvenile (organism)|juvenile]] form, and [[sexual maturity]] is generally reached after six [[ecdysis|moults]]. Some species have been known to eat their own [[exuvia]]e after moulting.<ref name="Wade"/>

==Ecology==
{{Expand section|with text and images from the monograph<ref name="Monograph"/>|date=July 2025}}
[[File:Talitrus saltator 2c.jpg|thumb|right|''[[Talitrus saltator]]'' is an abundant animal of sandy beaches around [[Europe]].]]
[[File:Expl1152 - Flickr - NOAA Photo Library.jpg|thumb|right|Dorsal (top) view of a newly{{Update inline|date=July 2025|reason=Not newly discovered anymore. Has the species been described by now?}} discovered amphipod living in a [[commensal]] relationship with a bamboo coral (''[[Keratoisis flexibilis]]'')]]

Amphipods are found in almost all aquatic environments, from [[fresh water]] to water with twice the [[salinity]] of [[Seawater|sea water]]<ref name="Wade"/> and even in the [[Challenger Deep]], the deepest known point in the ocean.<ref name="James Cameron on Earth's Deepest Spot: Desolate, Lunar-Like">{{cite web |author=National Geographic |title=James Cameron on Earth's Deepest Spot: Desolate, Lunar-Like |url=http://news.nationalgeographic.com/news/2012/03/120326-james-cameron-mariana-trench-challenger-deepest-lunar-sub-science/ |archive-url=https://web.archive.org/web/20120328064958/http://news.nationalgeographic.com/news/2012/03/120326-james-cameron-mariana-trench-challenger-deepest-lunar-sub-science/ |url-status=dead |archive-date=March 28, 2012 |date=27 March 2012 |publisher=National Geographic Society |access-date=27 March 2012 }}</ref> They are almost always an important component of aquatic ecosystems,<ref name="Lowry">{{cite web |url=http://www.crustacea.net/crustace/amphipoda/index.htm |title=Introduction |work=Amphipoda: Families |first1=J.&nbsp;K. |last1=Lowry |first2=R.&nbsp;T. |last2=Springthorpe |publisher=[[Australian Museum]] |access-date=April 5, 2010 |archive-url=https://web.archive.org/web/20060221225649/http://crustacea.net/crustace/amphipoda/index.htm |archive-date=February 21, 2006  }}</ref> often acting as mesograzers;<ref name="Duffy">{{Cite journal |last1=Duffy |first1=J.&nbsp;E. |first2=Mark E. |last2=Hay |year=2000 |title=Strong impacts of grazing amphipods on the organization of a benthic community |journal=Ecological Monographs |volume=70 |issue=2 |pages=237–263 |doi=10.1890/0012-9615(2000)070[0237:SIOGAO]2.0.CO;2|citeseerx=10.1.1.473.4746 |s2cid=54598097 }}</ref> a monograph calls them ″integral members of global marine ecosystems″.<ref name="Monograph">{{cite journal |last1=Ritter |first1=Carmen J. |last2=Bourne |first2=David G. |title=Marine amphipods as integral members of global ocean ecosystems |journal=Experimental Marine Biology and Ecology |date=March 2024 |volume=572 |article-number=151985 |doi=10.1016/j.jembe.2023.151985 |bibcode=2024JEMBE.57251985R |url=https://www.sciencedirect.com/science/article/pii/S002209812300117X/pdfft?md5=db06176f1777adbfe62c48d840b9ae6f&pid=1-s2.0-S002209812300117X-main.pdf |access-date=23 July 2025}}</ref>

Most species in the suborder Gammaridea are [[epibenthos|epibenthic]], although they are often collected in [[plankton]] samples. Members of the Hyperiidea are all planktonic and marine;<ref name="TAFI"/> many are [[symbiosis|symbionts]] of gelatinous animals, including [[salp]]s, [[Medusa (biology)|medusae]], [[Siphonophorae|siphonophores]], colonial [[radiolaria]]ns and [[Ctenophora|ctenophores]], and most hyperiids are associated with gelatinous animals during some part of their life cycle.<ref>{{cite journal |journal=[[Deep-Sea Research]] |year=1977 |volume=24 |pages=465–488 |title=The associations of Amphipoda Hyperiidea with gelatinous zooplankton. II. Associations with Cnidaria, Cteuophora and Radiolaria |first1=G.&nbsp;R. |last1=Harbison |first2=D.&nbsp;C. |last2=Biggs |first3=L.&nbsp;P. |last3=Madin |doi=10.1016/0146-6291(77)90484-2 |issue=5|bibcode=1977DSR....24..465H }}</ref> 

Some 1,900 species, or 20% of the total amphipod diversity, live in fresh water or other non-marine waters. Notably rich [[Endemism|endemic]] amphipod faunas are found in the ancient [[Lake Baikal]] and waters of the [[Caspian Sea]] basin,<ref name=Fresh>{{cite journal |first1=R. |last1=Väinölä |first2=J.&nbsp;D.&nbsp;S. |last2=Witt |first3=M. |last3=Grabowski |first4=J.&nbsp;H. |last4=Bradbury |first5=K. |last5=Jazdzewski |first6=B. |last6=Sket |year=2008 |title=Global diversity of amphipods (Amphipoda, Crustacea) in freshwater |url=http://decapoda.nhm.org/pdfs/27701/27701.pdf |journal=[[Hydrobiologia]] |volume=595 |issue=1 |pages=241–255 |doi=10.1007/s10750-007-9020-6|bibcode=2008HyBio.595..241V |s2cid=4662681 }}</ref> with over 350 species inhabiting Lake Baikal.<ref>{{cite book |title=Thorp and Covich's Freshwater Invertebrates |date=2019 |publisher=Academic Press - Elsevier |isbn=978-0-12-385024-9 |edition=4 |url=https://www.sciencedirect.com/book/9780123850249/thorp-and-covichs-freshwater-invertebrates |access-date=23 July 2025}}</ref>

The [[landhopper]]s of the family [[Talitridae]] (which also includes semi-terrestrial and marine animals) are [[terrestrial animal|terrestrial]], living in damp environments such as [[leaf litter]].<ref name="Massey">{{cite web |url=http://soilbugs.massey.ac.nz/amphipoda.php |title=Amphipoda |work=Guide to New Zealand Soil Invertebrates |publisher=[[Massey University]] |access-date=April 7, 2010 |first1=M.&nbsp;A. |last1=Minor |first2=A.&nbsp;W. |last2=Robertson |date=March 5, 2010 |archive-url=https://web.archive.org/web/20100510121618/http://soilbugs.massey.ac.nz/amphipoda.php |archive-date=10 May 2010  |url-status=live}}</ref> Landhoppers have a wide distribution in areas that were formerly part of [[Gondwana]], but have colonised parts of [[Europe]] and [[North America]] in recent times.

Around 750 species in 160 genera and 30 families are [[troglobite|troglobitic]] and [[Troglobiont|live in caves]], and are found in almost all suitable habitats, but with their [[center of diversity|centres of diversity]] in the [[Mediterranean Basin]], southeastern [[North America]] and the [[Caribbean]].<ref>{{cite book |chapter=Crustacea |first=Horton H. III |last=Hobbs |editor-first = John | editor-last = Gunn |title=Encyclopedia of Caves and Karst Science |publisher=[[Routledge]] |year=2003 |url=http://www.routledge-ny.com/ref/caves/crustacea.pdf |isbn=978-1-57958-399-6 |pages=254–257}}</ref>

Amphipods are a potential means of mitigating [[eutrophication]] in [[aquaculture]] facilities.<ref>{{cite book |last1=Guerra-García |first1=José Manuel |last2=Martínez-Pita |first2=Inés |last3=Šegvić-Bubić |first3=Tanja |last4=Manchado |first4=Manuel |last5=Arechavala-Lopez |first5=Pablo |last6=Calado |first6=Ricardo |last7=Marchio |first7=Elizabeth |last8=Gentry |first8=Rebecca |last9=Tlusty |first9=Michael F. |last10=Rhyne |first10=Andrew |last11=Moreno |first11=Patricia |last12=Béjar |first12=Julia |last13=Alonso |first13=María del Carmen |last14=García-Rosado |first14=Esther |last15=Jiménez-Prada |first15=Pablo |title=New Perspectives and Sustainable Development of Biodiversity in the Anthropocene |chapter=5 - Aquaculture and conservation |date=2023 |publisher=Academic Press |pages=111–146 |doi=10.1016/B978-0-323-85613-3.00003-7 |isbn=978-0-323-85613-3 }}</ref>

Compared to other crustacean groups, such as the [[Isopoda]], [[Rhizocephala]] or [[Copepod]]a, relatively few amphipods are [[parasitism|parasitic]] on other animals. The most notable example of parasitic amphipods are the [[whale louse|whale lice]] (family Cyamidae). Unlike other amphipods, these are dorso-ventrally flattened, and have large, strong claws, with which they attach themselves to [[baleen whale]]s. They are the only parasitic crustaceans which cannot swim during any part of their [[Biological life cycle|life cycle]].<ref>{{cite web|url=http://web.viu.ca/goatert/PARASITE/PARAMPH.HTM |title=Parasitic Amphipoda |publisher=[[Vancouver Island University]] |access-date=April 7, 2010 |date=May 4, 1996 |first=Tim |last=Goater |work=Interactive Parasitology  |archive-url=https://web.archive.org/web/20100714173551/http://web.viu.ca/goatert/PARASITE/PARAMPH.HTM |archive-date=July 14, 2010 }}</ref>

===Foraging behaviour===
Most amphipods are [[detritivore]]s or [[scavenger]]s,<ref name="Wade" /> with some being [[Grazing|grazers]] of [[algae]], [[omnivore]]s or [[predator]]s<ref name="TAFI" /> of small [[insect]]s and [[crustacean]]s.<ref name="Wade" /> Food is grasped with the front two pairs of legs, which are armed with large claws.<ref name="Wade" /> More immobile species of amphipods eat higher quantities of less nutritious food rather than actively seeking more nutritious food.<ref name="Cruz_Rivera_2000"/> This is a type of compensatory feeding.<ref name="Cruz_Rivera_2000" /> This behaviour may have evolved to minimise predation risk when searching for other foods.<ref name="Cruz_Rivera_2000" /> ''[[Ampithoe longimana]]'', for example, is more sedentary than other species and have been observed to remain on host plants longer.<ref name="Cruz_Rivera_2000" /> In fact, when presented with both high- and low-nutrition food options, the sedentary species ''Ampithoe longimana'' does not distinguish between the two options.<ref name="Cruz_Rivera_2000"/> Other amphipod species, such as ''[[Gammarus mucronatus]]'' and ''Elasmopus levis'', which have superior [[Anti-predator adaptation|predator avoidance]] and are more mobile, are better able to pursue different food sources.<ref name="Cruz_Rivera_2000">{{Cite journal |last1=Cruz-Rivera |first1=Edwin |first2=Mark E. |last2=Hay |year=2000 |title=Can quantity replace quality? Food choice, compensatory feeding, and fitness of marine mesograzers |journal=[[Ecology (journal)|Ecology]] |volume=81 |pages=201–219 |doi=10.1890/0012-9658(2000)081[0201:CQRQFC]2.0.CO;2|hdl=1853/36755 |hdl-access=free }}</ref> In species without the compensatory feeding ability, survivorship, fertility, and growth can be strongly negatively affected in the absence of high-quality food.<ref name="Cruz_Rivera_2000"/> Compensatory feeding may also explain the year-round presence of ''A.&nbsp;longimana'' in certain waters.<ref name="Cruz_Rivera_2001">{{Cite journal |last1=Cruz-Rivera |first1=Edwin |first2=Mark E. |last2=Hay |year=2001 |title=Macroalgal traits and the feeding and fitness of an herbivorous amphipod: the roles of selectivity, mixing, and compensation |journal=[[Marine Ecology Progress Series]] |volume=218 |pages=249–266 |url=https://www.int-res.com/abstracts/meps/v218/p249-266/ |doi=10.3354/meps218249|bibcode=2001MEPS..218..249C |doi-access=free |hdl=1853/34241 |hdl-access=free }}</ref> Because algal presence changes throughout the year in certain communities, the evolution of flexible feeding techniques such as compensatory feeding may have been [[Fitness (biology)|beneficial to survival]].<ref name="Cruz_Rivera_2001"/>

''Ampithoe longimana'' has been observed to avoid certain compounds when foraging for food.<ref name="Schnitzler"/> In response to this avoidance, species of seaweed such as ''[[Dictyopteris membranacea]]'' or ''[[Dictyopteris hoytii]]'' have evolved to produce C<sub>11</sub> sulfur compounds and C-9 [[oxo-acid]]s in their bodies as defence mechanisms that specifically deter amphipods instead of deterrence to consumption by other predators.<ref name="Schnitzler">{{Cite journal |last1=Schnitzler |first1=Iris |first2=Georg |last2=Pohnert |first3=Mark |last3=Hay |first4=Wilhelm |last4=Boland |year=2001 |title=Chemical defense of brown algae (''Dictyopteris'' spp.) against the herbivorous amphipod ''Ampithoe longimana'' |journal=[[Oecologia]] |volume=126 |issue=4 |pages=515–521 |doi=10.1007/s004420000546|pmid=28547236 |bibcode=2001Oecol.126..515S |s2cid=12281845 }}</ref>

In populations found in [[benthic ecosystem]]s, amphipods play an essential role in controlling [[brown algae]] growth.<ref name="Duffy" /> The mesograzer behaviour of amphipods greatly contributes to the suppression of brown algal dominance in the absence of amphipod predators.<ref name="Duffy" /> Amphipods display a strong preference for brown algae in benthic ecosystems, but due to removal of mesograzers by predators such as fish, brown algae is able to dominate these communities over green and red algae species.<ref name="Duffy"/>

The incidence of [[cannibalism]] and [[intraguild predation]] is relatively high in some species,<ref name="Dick">{{cite journal |first=Jaimie T.&nbsp;A. |last=Dick |year=1995 |title=The cannibalistic behaviour of two ''Gammarus'' species (Crustacea: Amphipoda) |journal=[[Journal of Zoology]] |volume=236 |issue=4 |pages=697–706 |doi=10.1111/j.1469-7998.1995.tb02740.x}}</ref> although adults may decrease cannibalistic behaviour directed at juveniles when they are likely to encounter their own offspring.<ref>{{cite journal |first1=Susan E. |last1=Lewis |first2=Jaimie T.&nbsp;A. |last2=Dick |first3=Erin K. |last3=Lagerstrom |first4=Hazel C. |last4=Clarke |year=2010 |title=Avoidance of filial cannibalism in the amphipod ''Gammarus pulex'' |journal=[[Ethology (journal)|Ethology]] |volume=116 |issue=2 |pages=138–146 |doi=10.1111/j.1439-0310.2009.01726.x|bibcode=2010Ethol.116..138L }}</ref> In addition to age, sex may affect cannibalistic behaviour as males cannibalised newly moulted females less than males.<ref name="Dick"/>

They have, rarely, been identified as feeding on humans; in [[Melbourne]] in 2017 a boy who stood in the sea for about half an hour had severe bleeding from wounds on his legs that did not coagulate easily. This was found to have been caused by "sea fleas" identified as lysianassid amphipods, possibly in a feeding group. Their bites are not [[venom]]ous and do not cause lasting damage.<ref>{{Cite news |last=Zhou |first=Naaman |date=2017-08-08 |title=Australian teen just 'unfortunate' to be attacked by meat-loving sea fleas |url=https://www.theguardian.com/environment/2017/aug/08/australian-teen-just-unfortunate-to-be-attacked-by-meat-loving-sea-fleas |work=The Guardian |language=en-GB |issn=0261-3077 |access-date=2024-01-22}}</ref>

==Nomenclature==
The name ''Amphipoda'' comes, via [[Neo-Latin]] ''{{lang|la|amphipoda}}'', from the [[Greek language|Greek]] [[root (linguistics)|roots]] {{lang|el|[[:wiktionary:ἀμφί|ἀμφί]]}} 'on both/all sides' and {{lang|el|[[:wiktionary:πούς|πούς]]}} 'foot'. This contrasts with the related [[Isopoda]], which have a single kind of thoracic leg.<ref>{{OED|Amphipoda}}</ref> Particularly among [[Angling|anglers]], amphipods are known as ''freshwater shrimp'', ''scuds'', or ''sideswimmers''.<ref name="Wade"/><ref>{{cite web |url=http://www.flyfishersrepublic.com/entomology/freshwater-shrimp/ |title=Freshwater shrimp (scuds, sideswimmers) – Class: Crustacea, Order: Amphipoda |first=Brian |last=Chan |publisher=[[Fly Fishers' Republic]] |access-date=April 7, 2010 |archive-url=https://web.archive.org/web/20200117234302/http://www.flyfishersrepublic.com:80/entomology/freshwater-shrimp/ |archive-date=17 January 2020  |url-status=live}}</ref> The common names relate to their swimming habits, described as "[[wikt:scudding|scudding]]" and often having the animal tilt to one side.<ref>{{cite book |title=Encyclopedia of Inland Waters |date=2009 |publisher=Academic Press - Elsevier |isbn=978-0-12-370626-3 |url=https://www.sciencedirect.com/referencework/9780123706263/encyclopedia-of-inland-waters |access-date=23 July 2025}}</ref>
===Taxonomy===
Over 10,700 species of amphipods are currently recognised. The higher taxonomy of Amphipoda has been thoroughly rearranged in the 21st century, and currently comprises six suborders:<ref name=wad>{{Cite taxon|WoRMS|url=https://www.marinespecies.org/amphipoda/index.php |title=Introduction |db= World Amphipoda Database |access-date=9 January 2025}}</ref> 

* [[Amphilochidea]] (89 families; 700 genera; 4,308 species)
* [[Colomastigidea]] (2 families; 3 genera; 60 species)
* [[Hyperiidea]] (35 families; 76 genera; 284 species)
* [[Hyperiopsidea]] (3 families; 5 genera; 15 species)
* [[Pseudingolfiellidea]] (1 families; 1 genera; 4 species)
* [[Senticaudata]] (109 families; 983 genera; 6,007 species) 

<gallery widths=200 heights=160>
File:Lepidepecreum longicornis.jpg|''[[Lepidepecreum longicorne]]'' ([[Amphilochidea]]: [[Lysianassidae]])
File:Hyperia galba.jpg|''[[Hyperia galba]]'' ([[Hyperiidea]]: [[Hyperiidae]])
File:Pariambus typicus.jpg|''[[Pariambus typicus]]'' ([[Senticaudata]]: [[Caprellidae]])
</gallery>

A previous classification comprised the four suborders [[Gammaridea]], [[Caprellidea]], [[Hyperiidea]], and [[Ingolfiellidea]],<ref name="M&D">{{cite book |url=http://atiniui.nhm.org/pdfs/3839/3839.pdf |title=An Updated Classification of the Recent Crustacea |first1=Joel W. |last1=Martin |first2=George E. |last2=Davis |year=2001 |page=132 |publisher=[[Natural History Museum of Los Angeles County]] |access-date=2010-04-08 |archive-date=2013-05-12 |archive-url=https://web.archive.org/web/20130512091254/http://atiniui.nhm.org/pdfs/3839/3839.pdf  }}</ref> of which  Gammaridea contained the majority of taxa, including all the freshwater and [[terrestrial animal|terrestrial]] species.<ref name="Holsinger">{{cite web |last=Holsinger |first=John R. |title=What Are Amphipods? |publisher=[[Old Dominion University]] |access-date=April 7, 2010 |url=http://sci.odu.edu/biology/directory/Holsinger/jrh/whatis.htm  |archive-url=https://web.archive.org/web/20110720021054/http://sci.odu.edu/biology/directory/Holsinger/jrh/whatis.htm |archive-date=July 20, 2011 }}</ref> Gammaridea was recognised as a phylogenetically problematic group, and a new classification was developed by [[James K. Lowry]] and Alan Myers in a series of works over 2003–2017, using [[cladistic]] analysis of morphological characters.<ref name=Lowry2003>{{cite journal |last1=Myers |first1=A. A. |last2=Lowry |first2=J. K. |year=2003  |title=A Phylogeny and a New Classification of the Corophiidea Leach, 1814 (Amphipoda)  |journal=Journal of Crustacean Biology |volume=23 |issue=2 |pages=443–485  |doi=10.1651/0278-0372(2003)023[0443:APAANC]2.0.CO;2 |issn=0278-0372 |s2cid=85750244 }}</ref><ref name=Lowry2013>{{cite journal |first1=J.&nbsp;K. |last1=Lowry |first2=A.&nbsp;A. |last2=Myers |year=2013 |title=A Phylogeny and a Classification of the Senticaudata subord. nov. (Crustacea: Amphipoda) |journal=[[Zootaxa]] |volume=3610 |issue=1 |pages=1–80 |url=http://www.mapress.com/zootaxa/2013/f/zt03610p080.pdf |doi=10.11646/zootaxa.3610.1.1|pmid=24699701 }}</ref><ref name=Lowry2017>{{cite journal |url=http://www.mapress.com/j/zt/article/view/zootaxa.4265.1.1/11015 |doi=10.11646/zootaxa.4265.1.1 |title=A Phylogeny and Classification of the Amphipoda with the Establishment of the New Order Ingolfiellida (Crustacea: Peracarida) |first1=J.K. |last1=Lowry |first2=A.A. |last2=Myers |journal=[[Zootaxa]] |publisher=[[Magnolia Press]] |volume=4265 |issue=1 |year=2017 |pages=001–089 |pmid=28610392 |doi-access=free }}</ref> It started with breaking up and replacing Gammaridea. The largest of the new suborders, Senticaudata, comprises over half of the known amphipod species, including practically all freshwater taxa.<ref name = wad/>  At the same time, Ingolfiellidea was split from Amphipoda and reclassified as order [[Ingolfiellida]].<ref name=Lowry2017/> The more recent work  of Copilaş-Ciocianu et al. (2020) using analysis of molecular data found general support for three major groups corresponding to suborders Amphilochidea, Hyperiidea and Senticaudata, but suggests some groups need to move between Amphilochidea and Senticaudata in a taxonomic revision.<ref>{{Cite journal|last1=Copilaş-Ciocianu|first1=Denis|last2=Borko|first2=Špela|last3=Fišer|first3=Cene|year=2020|title=The Late Blooming Amphipods: Global Change Promoted Post-Jurassic Ecological Radiation Despite Palaeozoic Origin |journal=Molecular Phylogenetics and Evolution |volume=143 |language=en |article-number=106664 |doi=10.1016/j.ympev.2019.106664 |biorxiv=10.1101/675140 |pmid=31669816 |bibcode=2020MolPE.14306664C |s2cid=196649863 }}</ref>

{| class="collapsible <!--collapsed-->" style="width:100%; border:solid 1px #aaa"
|-
! style="background:#F0F2F5" | New Amphipoda classification of Lowry and Myers<ref name=Lowry2013 /><ref name=Lowry2017/>
|-
|
{{clade |style=font-size:100%;line-height:95%; 
|caption=______________Suborder________Infraorder_______Parvorder_______Superfamily____
|captionstyle=text-align:left;font-weight:bold;color:purple;padding:2px 10px 20px 1px;
|label1='''Amphipoda'''
|1={{clade
   |1={{clade |length=8.5em;
      |label1=[[Pseudingolfiellidea]]
      |1={{clade |length=9em
         |label1=[[Pseudingolfiellida]]<!--monophyletic parent-->
         |1={{clade |length=11em
            |label1=<!--[[Pseudingolfiellidira]] monophyletic parent-->
            |1=[[Pseudingolfiellidae|Pseudingolfielloidea]]
            }}
         }}
      }}
   |2={{clade
      |1={{clade |length=8em;
         |label1=[[Hyperiidea]]
         |1={{clade |length=9em;
            |label1=[[Physosomata]]
            |1={{clade  |length=9em;
               |label1=<!--[[Physosomatidira]] monophyletic parent-->
               |1={{clade |length=2em;
                  |1=[[Lanceoloidea]]
                  |2=[[Scinoidea]]
                  }}
               }}
            |label2=[[Physocephalata]]
            |2={{clade  |length=9em;
               |label1=<!--[[Physocephalatidira]] monophyletic parent-->
               |1={{clade  |length=2em;
                  |1=[[Vibilioidea]]
                  |2={{clade
                     |1=[[Phronimoidea]]
                     |2=[[Platysceloidea]]
                     }}
                  }}
               }}
            }}
         }}
      |2={{clade
         |1={{clade 
            |label1=[[Colomastigidea]]
            |1={{clade |length=9em
               |label1=[[Colomastigida]]<!--monophyletic parent-->
               |1={{clade |length=9em
                  |label1=<!--[[Colomastigidira]]monophyletic parent-->
                  |1={{clade |length=2em;
                     |1=[[Colomastigoidea]]
                     |2=[[Pagetinoidea]]
                     }}
                  }}
               }}
            |label2=[[Hyperiopsidea]]
            |2={{clade |length=9em
               |label1=[[Hyperiopsida]]<!--monophyletic parent-->
               |1={{clade |length=9em
                  |label1=<!--[[Hyperiopsidira]]monophyletic parent-->
                  |1={{clade |length=2em;
                     |1=[[Podosiroidea]]
                     |2=[[Hyperiopsoidea]]
                     }}
                  }}
               }}
            }}
         |2={{clade
            |label1=[[Senticaudata]]
            |1={{clade
               |1={{clade
                  |label1=[[Carangoliopsida]]
                  |1={{clade |length=10em;
                     |label1=<!--[[Carangoliopsidira]] monophyletic parent-->
                     |1={{clade |length=1em;
                        |1=[[Carangoliopsoidea]]
                        }}
                     }}
                  }}
               |2={{clade
                  |1={{clade |length=8em;
                     |label1=[[Talitrida]]
                     |1={{clade |length=8em;
                        |label1=<!--[[Talitridira]] monophyletic parent-->
                        |1={{clade |length=2em;
                           |1=[[Biancolinoidea]]
                           |2=[[Caspicoloidea]]
                           |3=[[Kurioidea]]
                           |4=[[Talitroidea]]
                           }}
                        }}
                     }}
                  |2={{clade
                     |1={{clade |length=7em
                        |label1=[[Hadziida]]
                        |1={{clade |length=8.5em
                           |label1=<!--[[Hadziidira]]monophyletic parent-->
                           |1={{clade |length=2em;
                              |1=[[Calliopioidea]]
                              |2=[[Hadzioidea]]
                              }}
                           }}
                        |label2=[[Corophiida]]
                        |2={{clade |length=8.5em
                           |label1=[[Corophiidira]]
                           |1={{clade |length=2em;
                              |1=[[Aoroidea]]
                              |2=[[Cheluroidea]]
                              |3=[[Chevalioidea]]
                              |4=[[Corophioidea]]
                              }}
                           |label2=[[Caprellidira]]
                           |2={{clade |length=2em;
                              |1=[[Aetiopedesoidea]]
                              |2=[[Isaeoidea]]
                              |3=[[Microprotopoidea]]
                              |4=[[Neomegamphoidea]]
                              |5=[[Photoidea]]
                              |6=[[Rakirooidea]]
                              |7=[[Caprelloidea]]
                              }}
                           }}
                        }}
                     |2={{clade |length=7em
                        |label1=[[Bogidiellida]] 
                        |1={{clade |length=8.5em
                           |label1=<!--[[Bogidiellidira]]-->
                           |1={{clade |length=2em;
                              |1=[[Bogidielloidea]]
                              }}
                           }}
                        |label2=[[Gammarida]]
                        |2={{clade |length=8.5em
                           |label1=[[Gammaridira]]
                           |1={{clade |length=2em;
                              |1=[[Gammaroidea]]
                              }}
                           |label2=[[Crangonyctidira]]
                           |2={{clade |length=2em;
                              |1=[[Allocrangonyctoidea]]
                              |2=[[Crangonyctoidea]]
                              }}
                           }}
                        }}
                     }}
                  }}
                }}
             |length=8em;
             |label2=[[Amphilochidea]]
             |2={{clade |length=9em;
                |label1=[[Amphilochida]]
                |1={{clade
                   |1={{clade |length=8em;
                      |label1= <!--[[Maxillipiidira]] -->
                      |1={{clade |length=2em;
                         |1=[[Maxillipioidea]]
                         }}
                      }}
                   |2={{clade
                      |1={{clade |length=7.5em;
                         |label1= <!--[[Oedicerotidira]] -->
                         |1={{clade |length=2em;
                            |1=[[Oedicerotoidea]]
                            }}
                         }}
                      |2={{clade |length=7.5em;
                         |label1=[[Eusiridira]]
                         |1={{clade |length=2em;
                            |1=[[Liljeborgioidea]]
                            |2=[[Eusiroidea]]
                            }}
                         |label2=[[Amphilochidira]]
                         |2={{clade |length=2em;
                            |1=[[Amphilochoidea]]
                            |2={{clade
                               |1=[[Leucothoidea]]
                               |2=[[Iphimedioidea]]
                               }}
                            }}
                         }}
                      }}
                   }}
                |label2=[[Lysianassida]]
                |grouplabel2={{clade labels|label1=[[Synopiidira]]<br/>(paraphyletic) |top1=10%}}
                |2={{clade
                   |1={{clade  |length=10em;
                      |1=[[Dexaminoidea]] |bar1=green
                      }}
                   |2={{clade
                      |1={{clade  |length=9.5em;
                         |1=[[Synopioidea]] |bar1=green
                         }}
                      |2={{clade |length=8em;
                         |label1=<!-- [[Haustoriidira]] -->
                         |1={{clade |length=2em;
                            |1=[[Haustorioidea]]
                            }}
                         |label2=[[Lysianassidira]]
                         |2={{clade |length=2em;
                            |1=[[Alicelloidea]]
                            |2={{clade
                               |1=[[Stegocephaloidea]]
                               |2={{clade
                                  |1=[[Lysianassoidea]]
                                  |2=[[Aristioidea]]
                                  }}
                               }}
                            }}
                         }}
                      }}
                   }}
                }}
             }}
          }}
       }}
   }}
}}
|}

===Fossil record===
Amphipods are thought to have originated in the [[Lower Carboniferous]]. Despite the group's age, however, the [[fossil record]] of the order Amphipoda is meagre, comprising specimens of one species from the [[Lower Cretaceous]] ([[Hauterivian]]) [[Weald Clay]] ([[United Kingdom]])<ref>{{Cite journal|author1=Edmund A. Jarzembowski |author2=Cédric Chény |author3=Yan Fang |author4=Bo Wang |year=2020 |title=First Mesozoic amphipod crustacean from the Lower Cretaceous of SE England |journal=Cretaceous Research |volume=112 |article-number=104429 |doi=10.1016/j.cretres.2020.104429 |bibcode=2020CrRes.11204429J |s2cid=213609157 |url=https://hal-insu.archives-ouvertes.fr/insu-02495739/document }}</ref> and 12 species dating back only as far as the [[Upper Eocene]], where they have been found in [[Baltic amber]].<ref>{{cite journal |title=A new terrestrial amphipod from tertiary amber deposits of Chiapas province, Southern Mexico |first1=E.&nbsp;L. |last1=Bousfield |first2=G.&nbsp;O. Jr. |last2=Poinar |doi=10.1080/10292389409380448 |journal=[[Historical Biology]] |volume=7 |issue=2 |year=1994 |pages=105–114|bibcode=1994HBio....7..105B }}</ref><ref>The species ''[[Rosagammarus]] minichiellus'' from the considerably older [[Late Triassic]] [[Luning Formation]] of Nevada was originally described as an amphipod, but subsequently reinterpreted as the right half of a [[Decapoda|decapod]] tail (Starr, Hegna & McMenamin 2015, ''The Geological Society of America North-Central Section 49th Annual Meeting'' [https://gsa.confex.com/gsa/2015NC/webprogram/Paper256121.html])</ref>

== See also ==
{{Portal bar|Crustaceans}}

==References==
{{Reflist|32em}}

==External links==
* {{Commons category-inline}}
* {{Wikispecies-inline}}

{{Malacostraca}}
{{Taxonbar|from=Q193418}}
{{Authority control}}

[[Category:Amphipoda| ]]
[[Category:Crustacean orders]]
[[Category:Extant Hauterivian first appearances]]
[[Category:Taxa named by Pierre André Latreille]]