Sea butterfly

The Thecosomata (collective/plural: thecosomes, from Ancient Greek (thḗkē), meaning "case", and (sôma), meaning "body"), or sea butterflies, are a taxonomicsuborder of small, pelagic, free-swimming sea snails known as holoplanktonicopisthobranchgastropodmollusks, in the orderPteropoda (also included within the informal group Opisthobranchia). Most pteropods have some form of calcified shell, although it is often very light, even translucent.

The sea butterflies include some of the world's most abundant gastropod species; as their large numbers are an essential part of the ocean food chain, they are a significant contributor to the oceanic carbon cycle.