Discovery and description of a remarkable bathypelagic nudibranch, Bathydevius caudactylus, gen. et. sp. nov.

We describe an exceptional nudibranch, new to science, from bathypelagic depths in the eastern North Pacific Ocean. More than 100 individuals of Bathydevius caudactylus gen. et. sp. nov. have been observed in the water column at depths between 1013 and 3272 m. Twenty spawning individuals were observed on the seafloor at depths between 2269 and 4009 m. Anatomy, diet, behavior, bioluminescence, and habitat distinguish this surprising nudibranch from all previously described species, and genetic evidence supports its placement in a new family. • We present a previously undescribed genus and species of deep-sea nudibranch. • It has unique morphology and is not closely related to any known families. • In a molecular phylogeny, it resolves as a distinct sister to the rest of known nudibranchs. • The species is bioluminescent and represents the third independent origin of bioluminescence among nudibranchs.