Sea angel

Sea angels (cladeGymnosomata) are a large group of small free-swimming sea slugs, classified into six separate families. They are pelagicopisthobranchs in the clade Gymnosomata within the larger mollusc clade Heterobranchia. Sea angels were previously considered to be pteropods.

Sea angels are also sometimes known as "sea butterflies" but this is potentially misleading because the family Clionidae is just one of the families within this clade, and the term "sea butterfly" is also applied to the shelled Thecosomata.

Recent molecular data suggest the Gymnosomata form a sister group to the Thecosomata (other planktonic, weakly or nonmineralized gastropods), but this long-standing hypothesis has also had some recent detractors.