Phylliroe bucephala is a parasitic species of pelagic nudibranch, a marine gastropod mollusc in the family Phylliroidae. Biology This species of nudibranch is transparent with its guts visible through the laterally flattened body. It has a tail and two long smooth rhinophores. It feeds on jellyfish and plankton as an adult.

A sea slug that lives in open water, looks like a fish, and glows in the dark? That is Phylliroe bucephala, a transparent, three inches-long nudibranch mollusk that can be found swimming in the upper layers of the open ocean throughout the world’s tropical and subtropical ocean regions.

Phylliroe bucephala is highly bioluminescent possessing light emitting glandular cells scattered all over the surface of the body, but more abundant on the upper and lower margins. It appears to have the ability to bioluminesce at will – glow or flash – rather than just flashing as a reaction to being disturbed.