Cephalopyge trematoides is a pelagic species of nudibranch. A free-swimming marine gastropod in the family Phylliroidae. Cephalopyge trematoides is the only known species in its genus (i.e. the genus is monotypic).
Cephalopyge is a contraction of cephalus (Greek: Κέφαλος Kephalos, "head") and pyge (πῦγή, "behind") referring to the position of the anus close to the head. The species epithet trematoides expresses a likeness to flukes.
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. 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 C. 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):
- Lalli, Carol M.; Gilmer, Ronald W. (1989). Pelagic Snails: The Biology of Holoplanktonic Gastropod Mollusks. Stanford University Press.
- World Register of Marine Species
- Marine species identification portal: Cephalopyge and Cephalopyge trematoides.
- Sea Slugs of Hawaii
- J.E. Steinberg, The pelagic nudibranch, Cephalopyge trematoides (Chun, 1889), in New South Wales with a note on other species in this genus, Proceedings of The Linnean Society of New South Wales 81:184-192 (1956) 
- G.M. Mapstone & M.N. Arai, Siphonophora (Cnidaria, Hydrozoa) of Canadian Pacific Waters, p.33. "The best documented predators of pelagic cnidarians from the phylum Mollusca are the neustonic nudibranchs and snails [...and] the pelagic nudibranch [...]"
- Orso Angulo-Campillo, Gerardo Aceves-Medina and Raymundo Avedaño-Ibarra, Holoplanktonic mollusks (Mollusca: Gastropoda) from the Gulf of California, México