Which tiny marine snail is commonly called the sea butterfly?

The tiny marine snail commonly called the sea butterfly is a pteropod.

Pteropods are pelagic gastropods that swim in the open ocean using wing-like lobes called parapodia. These appendages give them a fluttering motion that resembles a butterfly’s flight, inspiring the common name. Most are only a few millimetres long, although their transparent bodies can be striking under magnification.

Some pteropods retain a delicate shell, while others, known as sea angels, lack an external shell as adults and actively capture other pteropods. Their shells are commonly made of aragonite, a form of calcium carbonate that can dissolve more readily as seawater becomes more acidic. Pteropods are eaten by fish, seabirds, and whales, making them important links in pelagic food webs. They are molluscs, not insects or true butterflies.