Hydrothermal vents in the deep sea have an interesting fauna specially adapted for living not only in the deep sea but for living in a special part of the deep sea where cracks in the earth's crust have created an often hot, sulphurous anoxic environment. Only one opisthobranch, Dendronotus comteti, has been positively identified from this environment although as Angel Valdes notes, large unidentified nudibranchs have been reported from an active hydrothermal vent in the Bering Sea (Sagalevich et al.,1992).
Valdés, A. & Bouchet, P. 1998. Naked in toxic fluids: a nudibranch mollusc from hydrothermal vents. Deep Sea Research Part II: Tropical Studies in Oceanography, 45: 319-327.
Rudman, W.B., 2001 (October 4) Hydrothermal Vents. [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/factsheet/vents