Clione or “sea angels” swimming in a Tokyo aquarium (Photo by REUTERS/Kimimasa Mayama)

In terms of taxonomical diversity the gastropods are second most diverse class of animals on Earth (outnumbered only by the teeming class Insecta of the other great invertebrate phylum Arthropoda).  This means that there are some deeply strange arthropods out there. While we traditionally think of gastropods as snails and slugs there are odd subcategories of these creatures, like the subject of today’s post, sea angels (of the clade Gymnosomata).

A “Sea Angel” (Clione limacina)

Sea angels consist of six different families of pelagic marine opisthobranch gastropod molluscs.  Gastropods are named for their famous foot (the name means “stomach-foot”–a misnomer since gastropods all have true stomachs elsewhere) however the name is even more inappropriate for sea angels.  In these free-smimming predators, the gastropod foot, so familiar to us as seen on snails, has evolved into a pair of delicate wings for swimming through the water. Sea angels are very small: the largest species only reach 5 cm (2 inches) in length and most varieties are much more miniscule.  They prey on other tiny creatures swimming among the plankton—particularly other smaller slower species of gelatinous mollusks.

A hunting sea angel (photo by Alexander Semenov)

Adult sea angels lack any sort of shell—which they discard when they metamorphose into adulthood.  Their feeding apparatuses can be strangely complicated—pseudoarms and tentacles which recall their cousins the cephalopods. Sea angels are numerous in the oceans but some scientists are concerned that the acidification of the world’s oceans will cause substantial problems for the tiny translucent gastropods.

Sea Angel (Platybrachium antarcticum)