The pteropods (winged foot) are holoplanktonic sea slugs. They have a holoplanktonic existence and have parts of their foot modified into “wings” for propulsion. There are two informal clades the thecosomes and the gymnosomes.
The thecosomes, are commonly referred to as “Sea Butterflies” because most possess two large wings for swimming that protrude through their shell. These paired wings are derived from modified portions of the foot so could be termed parapodia. Most are small in size and possess a fragile bilateral external shell of various shapes. Many feed by use of an expansive mucous web, the only sea slugs to employ such a method. They live their entire pelagic lifecycle in the plankton.
The gymnosomes, are commonly called “Sea Angels”. They too have paired “wings” for swimming modified from the foot, though these are shorter, broader and smaller in relation to body size to those of the thecosomes.The gymnosomes are mostly small and have lost the shell. The body is bilaterally symmetrical with a distinct head connected to the body proper by a constricted region or neck. Across the order there is prodigious assortment of feeding appendages ranging from sucker bearing tentacles, not unlike those of an octopus, to chitinous hooks to buccal cones with papillae for holding prey. They too live their entire pelagic lifecycle in the plankton.
Neither types are commonly encountered by the scuba diver. The practice of black-water diving is permitting the recording of these molluscs in their natural habitat.