|A female of the Pterosoma planum species|
A female of the Pterosoma planum species, photographed in the Pacific Ocean between Honolulu, Hawaii, USA, and Sydney, Australia. Pterosoma planum belongs to a group of predatory snails that live in the open sea. Individuals can grow up to three or four centimeters long. The species is carnivorous and hunts fish and other snails and slugs. It has a shell, but its body cannot be completely withdrawn. Pterosoma planum is planktonic—it floats passively, depending on currents for its movement—and pelagic, spending its entire lifecycle independent of either the bottom of the ocean or the coast.