Adaptations facilitating feeding
The gulper catfish is a strict carnivore and swallows its prey, mostly other fish, whole. This prey can be exceptionally large for its size, sometimes even larger than the gulper catfish itself (although such feeding attempts can be unsuccessful). This is made possible by its ability to greatly expand the oropharyngeal opening, which is in turn facilitated by a variety of specialized features, including the loosely connected dentary symphysis, slight mobility of the premaxillae along the symphysis, and the highly flexible midpoint connection at the ventral side of the pectoral girdle.
Feeding techniques
Potential prey fish are apparently unable to recognize the gulper catfish as dangerous, as fish will not generally consider other fish of roughly the same size as a major threat; its slow, unobtrusive approach also helps convince potential prey of its harmlessness. A commonly observed attacking approach involves the gulper catfish quickly turning to the side and biting on the head of the targeted fish. Even if the first attempt fails, the gulper catfish will not try to pursue the prey: it will typically still not realize that the gulper catfish represents a threat, and the same approach can be reused. If the gulper catfish succeeds in engulfing the prey (head-first), it stimulates the prey to slide into its oropharyngeal chamber. Escape out of the mouth is generally prevented by the fine, backwards-pointing teeth, and the struggles of the prey cause it to be sent further down the digestive tract, until it eventually ends up folded (with both its head and tail pointing towards the head of the gulper catfish) in the greatly extendable stomach. The fully extended stomach may result in an abnormal appearance of the gulper catfish and impair swimming.
Another attacking approach was observed by a snorkeler in the Atabapo River: in this case, the gulper catfish in question hides in crevices between rocks and darts out to catch passing prey (such as freshwater angelfish). Hence, it may be said that the gulper catfish knows how to set up an ambush attack. It has also been observed to swallow large amounts of water only to expel it later along with the remains of earlier prey.
