There are about 30–40 species in this unusual group of deep-sea fish, divided among 4-5 families.

These fish are known for their long, slender bodies and enormous mouths. In some species, the mouth is larger than the rest of the body. This feature helps them survive in the deep ocean, where food is scarce—they can grab and eat almost anything they come across. Many also have expandable stomachs, allowing them to swallow prey bigger than themselves.

One of the best-known members of this order is the pelican eel (Eurypharynx pelecanoides), which uses its huge mouth like a net to scoop up prey. Some species in this group also have bioluminescent (glowing) organs on their bodies or tails. These lights may help them attract prey or communicate in the dark depths of the ocean.

Unlike many other fish, they have tiny fins, weak muscles, and soft bones. That’s because they don’t need to swim fast—in the deep sea, conserving energy can be a key to survival.