Most species are neotenic, showing a number of features as adults that are more commonly associated with fish larvae.

The eyes, nasal organ and lateral line are also reduced, and they lack a swim bladder.

The males bundle their sperm into small sacs (spermatophores), so that they can be stored for extended periods.

This allows them to mate with immature females, which can then store the sperm inside the ovaries until they reach sexual maturity, and the eggs are ready to be fertilised.

This unusual adaptation is likely a response to the difficulty of finding a mate in their dark and sparsely inhabited deep-sea environment.