Cephalopyge is a genus of pelagic nudibranchs comprising the single species Cephalopyge trematoides, a free-swimming marine gastropod in the family Phylliroidae.
Cephalopyge is a contraction of cephalus (Greek: úõÃÂñûî , "head") and (ÃÂà «Ã³Ã®, "behind") referring to the position of the anus close to the head. The species epithet trematoides expresses a likeness to flukes.
Cephalopyge trematoides grows to in length. It swims at approximately , by passing several undulatory waves down its body each second. It is flattened and transparent; its internal organs are visible.
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. It is estimated to be one of only five planktonic nudibranch species (another epipelagic example is Phylliroe bucephala).
Further information (including photos):