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The pelican eel ( Eurypharynx pelecanoides) is a deep-sea eel. It is the only known member of the genus Eurypharynx and the family Eurypharyngidae. The species belongs to the "saccopharyngiforms", members of which were historically placed in their own order, but are now considered true eels in the order Anguilliformes. The pelican eel has been described by many synonyms, yet nobody has been able to demonstrate that more than one species of pelican eel exists. It is also referred to as the gulper eel (which can also refer to members of the related genus Saccopharynx), pelican gulper, and umbrella-mouth gulper. The specific epithet pelecanoides refers to the pelican, as the fish's large mouth is reminiscent of that of the pelican.
The pelican eel's most notable feature is its large mouth, which is much larger than its body. The mouth is loosely hinged, and can be opened wide enough to swallow a fish much larger than the eel itself. The lower jaw is hinged at the base of the head, with no body mass behind it, making the head look disproportionately large. Its jaw is so large that it is estimated to be about a quarter of the total length of the eel itself. (1998). 9780125476652, Academic Press. ISBN 9780125476652
While typically in a folded state, the pelican eel's mouth has the capacity to change to an inflated shape when hunting, giving the mouth a massive appearance. This transformation is possible due to the dual-mode biological morphing mechanism that takes place: geometric unfolding of the mouth followed by stretching. When the pelican eel is in pursuit of prey and opens its mouth, the head and jaw structure unfold and spread horizontally, followed by the inflation of the mouth. The inflation is made possible given the highly stretchable skin of the head, an additional characteristic that enables the eel to partake in this mechanism and engage in lunge feeding to consume large amounts of prey. When it feeds on prey, water that is ingested is expelled via the gills. Gulper Eel – Pelican Eel, Frightening Deep Sea Jaws | Animal Pictures and Facts. FactZoo.com. Retrieved 2016-11-14.
Pelican eels are smaller-sized eels. They grow to about in length, though lengths of are plausible.Bray, Dianne J. (2011), . Fishes of Australia, accessed 7 October 2014 Like most eels, E. pelecanoides lacks and scales. Otherwise, the pelican eel is very different in appearance from typical eels. Instead of having a swim bladder, the pelican eel has an aglomerular kidney that is thought to have a role in maintaining the gelatinous substance filling the "lymphatic spaces" that are found around the vertebrae. It has been hypothesized that these gelatinous substance filled "lymphatic spaces" function in a similar way to a swim bladder. Furthermore, the muscle segment shape of the pelican eel is different. Its muscle segments have a "V-shape", while other fish have "W-shaped" muscle segments. Pelican eels are also unusual because the ampullae of the lateral line system project from the body, rather than being contained in a narrow groove; this may increase its sensitivity.
Unlike many other deep sea creatures, the pelican eel has very small eyes. For reference, the horizontal eye size diameter of a male pelican eel specimen was measured to be . It is believed that the eyes evolved to detect faint traces of light rather than form images.
The pelican eel has a long, whip-like tail that it uses for movement and for communication via bioluminescence. Specimens that have been brought to the surface in fishing nets have been known to have their long tails tied into several knots. The end of the tail bears a complex organ with numerous tentacles, which Bioluminescence pink and gives off occasional red flashes. The colors on its tail are displayed through its light-emitting photophores. This is presumably a lure to attract prey, although its presence at the far end of the body from the mouth suggests the eel may have to adopt an unusual posture to use it effectively.
Pelican eels are black or olive and some subspecies may have a thin lateral white stripe. The coloration of E. pelecanoides is especially dark because the species exhibits ultra-black camouflage. This special pigmentation, which reflects less than 0.5% of light, allows these eels to be cloaked in darkness in their low light environments. The black camouflage allows bathypelagic eels to evade predators and hide from prey.
Pelican eels display sexual dimorphism, with the largest morphological difference in the structure of the nasal rosette. In female pelican eels, the nasal rosette is hardly noticeable whereas male pelican eels exhibit a larger nasal rosette. The male's nasal rosette is bulb-shaped and contains larger anterior and posterior nostrils. Sexual dimorphism is thought to aid with locating a potential mate in the bathypelagic zone.
Observations of gut contents and teeth morphology indicate that Eurypharynx pelecanoides larva, categorized as a type of leptocephali, feed on marine snow. Organisms, such as thraustochytrids and hydrozoan tissue, were consumed by these larva in a grouped manner such as they would be found in marine snow. Furthermore, the lesser number, larger size, and inwardly-pointing direction of leptocephali larval teeth point indicate that pelican eel larva rely on marine snow as a source of nutrients. As leptocephali develop into their mature form these distinct teeth were replaced by more, smaller teeth. This particular observation may explain a shift in the size of leptocephali heads, such as E. pelecanoides, in comparison to their food source as they mature.
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