Pteropod

Pteropods, also known as sea butterflies, are marine gastropods adapted to pelagic life.[…]When pteropods constitute a high percentage of the ooze the deposit is called pteropod ooze.

For fans of the undersea sci-fi classic The Abyss, a pteropod seems a likely inspiration for the ethereal alien creatures that were the saviors at the film's end. The muscular foot that most snails use for locomotion has been modified in the pteropods into delicate fins for swimming.

Pteropods, or sea butterflies, have a foot that is modified to form a pair of winglike structures that animals use to propel themselves through the water column.