Gymnosomata
| Sea angel | |
|---|---|
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| Clione limacina | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Mollusca |
| Class: | Gastropoda |
| (unranked): | clade Heterobranchia clade Euthyneura clade Euopisthobranchia clade Gymnosomata |
| Families | |
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Sea angels (clade Gymnosomata) are a large group of extremely small, swimming sea slugs, not to be confused with Hydrozoans (Jellies and other similar creatures), classified into six different families. They are pelagic opisthobranchs in the clade Gymnosomata within the larger clade Heterobranchia. Sea angels were previously referred to as a type of pteropod. The appear as though they are tiny little angels flying through the sea, hence the name sea angels.
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 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.
In this clade, the foot of the gastropod has developed into wing-like flapping appendages (parapodia) and their shells have been lost. Both adaptations suit their free-swimming oceanic lives. The adaptations also explain the common name sea angel and the scientific name of the order; from Greek gymnos meaning "naked" and soma meaning "body".
The other suborder of pteropods, Thecosomata, is 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. C. limacina is a polar species; those found in warmer waters are far smaller. Some species of sea angels 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. By rowing their "wings" back and forth at 1–3 Hz, the sea angels swim at speeds up to 100 mm/s. This is faster than their prey, the sea butterfly, which swims at speeds up to approximately 50 mm/s. It is not yet clear whether the sea angel uses its swimming appendages as 'rowing paddles' or as 'wings'.
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