Opisthobranch mollusks are almost exclusively marine with the exception of only a few groups that inhabit freshwater environments. Generally, opisthobranchs are benthic organisms, but there are a few pelagic species that live floating on the surface of the ocean or attached to floating objects or marine algae.

Seagrass	Coral reef

A relatively small number of species has been found in deep waters, more than 3,000 meters deep. Opisthobranchs are not normally found in extreme environments, but some species have been described from hydrothermal vents.