Cephalopyge trematoides is a pelagic species of nudibranch. A free-swimming marine gastropod in the family Phylliroidae.[2] Cephalopyge trematoides is the only known species in its genus (i.e. the genus is monotypic).[3]
Cephalopyge is a contraction of cephalus (Greek: κεφαλή kephale, "head") and pyge (πūγή, "behind") referring to the position of the anus close to the head. The species epithet trematoides expresses a likeness to flukes.[1]
C. trematoides grows to 2.5 cm in length. It swims at approximately 12 cm/s, by passing several undulatory waves down its body each second.[4] It is flattened and transparent; its internal organs are visible.[3]
Of the approximately 3000 species of nudibranch, the vast majority are benthic, only a couple are neustonic, and C. trematoides is very unusual in that it is pelagic.[5][6] It is estimated to be one of only five planktonic nudibranch species (another epipelagic example is Phylliroe bucephala).[7]