Genus synonymy Species synonymy Cephalopyge is a genus of pelagic nudibranchs comprising the single species Cephalopyge trematoides, a free-swimming marine gastropod in the family Phylliroidae.

The species epithet trematoides expresses a likeness to flukes.

It swims at approximately 12 cm/s (4.7 in/s), by passing several undulatory waves down its body each second.

[5] Of the approximately 3000 species of nudibranch, the vast majority are benthic, only a couple are neustonic, and Cephalopyge trematoides is very unusual in that it is pelagic.

[6][7] It is estimated to be one of only five planktonic nudibranch species (another epipelagic example is Phylliroe bucephala).