Sea Angel
Sea angels previously known as one kind of pteropod, are a large group of small swimming sea slugs in six different families. These are pelagic marine opisthobranch gastropod molluscs in the clade Gymnosomata within the larger clade Heterobranchia.
Sea angels are also sometimes known as "cliones", but this is potentially misleading because the family Clionidae is just one of the families within this clade.
Recent molecular data suggests that the gymnosomata form a sister group to the Thecosomata, other planktonic, weakly- or non-mineralized gastropods, although this long-standing hypothesis has had some recent detractors.
In this clade, the foot of the gastropod has developed into wing-like flapping appendages (parapodia) and their shells have been lost. These are both adaptations which suit their free-swimming oceanic lives. The adaptations also explain the common name sea angel and the New Latin name of the order; from Greek gymnos meaning "naked" and soma meaning "body."
The other suborder of pteropods, Thecosomata, are superficially similar to sea angels but are not closely related. They have larger, broader parapodia, and most species retain a shell; they are commonly known as sea butterflies.
Sea angels are gelatinous, mostly transparent and very small, with the largest species (Clione limacina) reaching 5 cm. Clione limacina is a polar species; those found in warmer waters are far smaller. Some species of sea angel feed exclusively on sea butterflies; the angels have terminal mouths with the radula common to mollusks, and tentacles to grasp their prey, sometimes with suckers similar to cephalopods. Their "wings" allow sea angels to swim much faster than the larger (usually fused) wings of sea butterflies.
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