Hingemouth
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The hingemouth (Phractolaemus ansorgii) is a freshwater fish found only in western Central Africa.<ref name=":6">Template:Cite journal</ref> It is less commonly known as the “African Mudminnow”,<ref name=":7">Template:Cite book</ref> “African mudfish”, “snake mudfish”, and “snake mudhead”.<ref>Template:Cite web</ref> In Lontomba, it is called “mobili”.<ref name=":8">Template:Cite journal</ref> It is best known for its unique oral anatomy.<ref>Template:Cite web</ref><ref>Template:Cite web</ref>
Species description
The hingemouth is a small, olive-gray<ref name=":3">Template:Cite journal</ref> or brown fish with an elongated, semi-cylindrical body<ref name=":4">Template:Cite book</ref> measuring up to 18 cm.<ref name=":2">Template:Cite book</ref> Its namesake mouth points upwards,<ref name=":7"/> although this is a bit of a simplification, given its unique jaw anatomy. Unlike in most bony fishes, where the dentary bones comprise the front tip of the lower jaw with the articulation point farther back,<ref name=":0">Template:Cite book</ref> the hingemouth’s jaw is jointed between the dentary and quadrate bones at the anterior tip of the head. When its mouth is closed, the dentary and attached upper jaws (the maxillae and premaxillae) lay back against the quadrate. When the mouth opens, the dentary swings upwards and outwards, projecting the upper jaws forward.<ref name=":1">Template:Cite journal</ref> Protrusion of the premaxilla is a trait of teleost fishes in general, and many teleost taxa have developed variations of the anatomy that enables protrusion,<ref name=":0" /> but the hingemouth’s jaw anatomy and method of premaxillary protrusion have not been observed in any other fishes.<ref name=":1" />
The mouth itself, which resembles a proboscis, is not formed entirely around the skeletal jaws. The proboscis is composed of two fleshy “lips” made of cartilage and connective tissue. The upper jaws project into the sides of the upper lip; the lower lip has no skeletal support. When the hingemouth’s jaw is closed, the edges of the lips sit partly-open at the top of the head, with the rest of the mouth tissues tucked inside the head and fatty “chin”. When the jaw opens, premaxillary protrusion extends the entire proboscis forward, reaching as far as 30% of the hingemouth’s head length. The proboscis is extended, retracted, and otherwise manipulated through a complex arrangement of tendons, ligaments, cartilage, and muscles, including highly-modified adductor mandibulae.<ref name=":1" />
Although the hingemouth lacks pharyngeal teeth,<ref name=":3" /> it does have a small “tooth” on each dentary bone;<ref name=":5">Template:Cite book</ref> these are actually projections of the dentaries themselves. The proboscis is also lined with rows of keratinous projections.<ref name=":1" />
The rest of the head is bony, with large, overlapping interopercular bones covering the throat.<ref name=":3" /> The hingemouth has one barbel<ref name=":3" /> and two narial tubes.<ref name=":4" /> Its small eyes are positioned on the sides of the head.<ref name=":5" /> Its gill openings are narrow.<ref name=":3" /> [[File:|thumb|The skeletal jaws of the hingemouth (closed and open, respectively) imposed onto CT scans of a preserved specimen]]] The hingemouth’s pectoral fins are small, rounded, and low on the body. Its homocercal tail has a rounded fan shape. Its dorsal, anal, and pelvic fins are all pointed. The pelvic fins sit at the midpoint of the body, and the dorsal fin sits midway between the pelvic and anal fins.<ref name=":3" /> All its fins are generally colorless, although the unpaired fins may have a red tint on the edges.<ref name=":4" />
Its scales are cycloid.<ref name=":2" /> Male hingemouth have sharp, prominent tubercles on some of their scales and rays, likely used in conflicts with other males and to maintain contact during reproduction. Female hingemouth also have tubercles on some scales and fins, although they are less-developed and likely used only for reproduction. In both sexes, the tubercles are well-vascularized with keratin caps.<ref>Template:Cite journal</ref>
Systematics
The hingemouth is currently considered the sole member of the family Phractolaemidae.<ref name=":6" /> Bayesian analysis of mitochondrial DNA sequences has placed the hingemouth and its sister clade Kneriidae (another endemic freshwater African family) in the order Gonorynchiformes.<ref>Template:Cite journal</ref> Although no fossils directly related to the hingemouth have been found as of yet, the first kneriid fossil was discovered in 2002; comparisons of the fossil against modern hingemouth and kneriid specimens led some researchers to argue that the hingemouth should be subsumed into Kneriidae.<ref>Template:Cite book</ref>
Distribution
Hingemouth are found in freshwater swamps<ref name=":7" /> and slow-moving waters of the Congo and Niger River systems<ref name=":2" /> east of the Dahomey Gap.<ref>Template:Cite book</ref> In the dry season, they can be found in the benthos of floodplain pools and lagoons.<ref>Template:Cite journal</ref>
In a survey of the upper Ekole River in Nigeria that used local fishers to collect samples over several months, hingemouth comprised 0.77% of the catch. The study authors considered hingemouth relatively abundant, though not dominant, among the 70 different species observed.<ref name=":9">Template:Cite journal</ref> In another study, carried out in flooded forest swamps near Lake Tumba in the Democratic Republic of Congo, hingemouth represented 4.55% of the fish sampled. Their distribution across the Lake Tumba micro-basin was higher in areas with deeper, slower, warmer waters. Hingemouth also preferred areas with less plant debris on the bottom.<ref name=":8" />
Life history
The hingemouth is a facultative air-breather,<ref name=":2" /> supported in low-oxygen environments by a swim bladder that can function as a lung due to its alveoli,<ref name=":4" /> as well as the ability to project its mouth above the surface.<ref name=":1" /> This enables the hingemouth to survive in the warm, slow-moving, poorly-oxygenated swamps and floodplains it inhabits.<ref name=":0" />
It feeds primarily on organic detritus and phytoplankton,<ref name=":1" /> as well as other small benthic organisms.<ref name=":5" /> Like many teleost fishes,<ref name=":0" /> the hingemouth uses premaxillary protrusion to increase efficiency in suction-feeding and striking at food, but its unique jaws also allow fine motor control similar to an elephant’s trunk or the lips of mammals. This ability to probe, scrape, pinch, and otherwise extract small food particles with its mouth makes the hingemouth uniquely effective as a bottom-feeder, especially in waters with uneven substrates.<ref name=":1" />
Little information is available on the non-feeding behavior of hingemouths, although they are suspected to be fairly solitary.<ref name=":5" /> However, as a member of the superorder Ostariophysi, the hingemouth likely produces and responds to Schreckstoff. This is a chemical released when Ostariophysan fish are injured, serving as an alarm signal to nearby conspecifics.<ref name=":0" /> Because this trait is only useful in a species with some level of sociality, the hingemouth may be more social than suspected. Alternatively, it may have lost the trait, or never developed it at all (although the latter is unlikely, given its evolutionary relationship to the other Ostariophysi).
Like other gonorynchiform fishes, hingemouths are oviparous and reproduce via external fertilization.<ref name=":5" /> Their lifespan, growth rates, and life cycle are also poorly-described.<ref>Template:Cite web</ref>
Conservation status
Although the IUCN considers the hingemouth a species of least concern,<ref>Template:Citation</ref> it does experience anthropogenic threats, both directly via fishing and trade, and indirectly via damage to its habitat. For example, the upper Ekole River, where hingemouth are found in abundance, is subject to regular oil spills and other anthropogenic pollution.<ref name=":9" /> Similarly, the swampy forests near Lake Tumba are threatened by slash-and-burn agriculture and commercial deforestation, as well as pollution by humans.<ref name=":8" /> Hingemouth from the Anambra River system in Nigeria–another regularly-polluted river–bioaccumulate as much copper in their bodies as pollutes the sediment they feed from, and 1.6 times as much as the water they swim in.<ref>Template:Cite journal</ref>
Hingemouth populations may be disproportionately impacted by damage to their habitats because of their extensively-modified jaw anatomy, which could represent an evolutionary “dead end”.<ref name=":1" /> Anatomical modification alone does not keep a species from adapting to new circumstances–for example, the famously-diverse cichlids have a broad range of very specialized mouths, which has likely contributed to their success as a family.<ref name=":0" /> However, the hingemouth’s jaws are relatively inflexible compared to other specialized fish jaws. This may explain why the hingemouth is so isolated, phylogenetically speaking, when compared to fish like cichlids. If this is the case, the hingemouth will likely struggle to adapt to habitat changes over time.<ref name=":1" />
Although there are no formal records of hingemouth fisheries available in English,<ref>Template:Cite web</ref> personal and commercial fishing does occur. The hingemouth is used ethnomedically by Ifa practitioners in Southwest Nigeria as both an antidote to malicious poisoning and a treatment for erectile dysfunction. Practitioners source the hingemouth from fish traders, fishermen, traditional doctors and pharmacists, and even catch the fish themselves.<ref>Template:Cite journal</ref> The hingemouth may be caught for food as well; at the time of Western discovery, the hingemouth was already being consumed by locals.<ref name=":3" /> Given that it is still caught by local fishers and distributed by fish traders for ethnomedical use, it seems likely that it is also being fished for dietary purposes to some extent.
Outside its native range, the hingemouth is occasionally sold for fishkeeping in aquaria,<ref>Template:Cite web</ref> although hobbyists report that it is not a commonly-kept species and that information on its care is difficult to find.<ref>Template:Cite web</ref><ref>Template:Citation</ref>