Saccopharyngiformes

Saccopharyngiformes is an order of unusual ray-finned fish, superficially similar to eels, but with multiple internal differences. Most of the fish in this order are deep-sea types known from only a handful of specimens, such as the pelican eel. Some species are bioluminescent. Some, such as the "swallowers", can live as deep as 3,000 m (10,000 feet) in the ocean, well into the aphotic zone. Saccopharyngiformes were found by deep sea fishermen's nets. Extensive research has not been conducted on them due to being indirectly observed.

Saccopharyngiforms lack several bones, such as the symplectic bone, the bones of the opercle, and ribs. They also have no scales, pelvic fins or swim bladder. The jaws are quite large, lined with small teeth, and several types are notable for being able to consume fish larger than themselves. Their myomeres (muscle segments) are V-shaped instead of W-shaped as in all other fishes, and their lateral lines have no pores, instead being modified to groups of elevated tubules.