Sea Butterfly -

Tiny Cavolinia pteropod, less than 1 cm floating in the water column, seen during a night dive.

This planktonic mollusc is commonly known as a pteropod, or flapping snail, a name derived from its characteristic 'wing foot'. It possesses a globose, bilaterally symmetrical calcareous shell that houses and protects the soft parts of the organism. The large mantle lobes extend beyond the shell margin and, by a flapping motion, enable movement. Feeding is performed passively whilst drifting in the plankton community by casting a large, mucous web in order to catch a variety of phytoplankton (diatoms and dinoflagellates) and zooplankton (copepods and crustacea larvae).

This picture is a Sea Butterfly from the genus Cavolina, possibly Cavolina tridentata.