Sea angels, also known as cliones, and previously known as one kind of pteropod, are a group of small swimming sea slugs.
In this suborder 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 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. Other species of sea angel feed mostly on zooplankton.
Another large polar species of sea angel, Clione antarctica, defends itself from predators by synthesizing a previously unknown molecule, named pteroenone. As predators will not eat the sea angel some animals, such as amphipods, take up home inside them. Local population density of Clione antarctica may reach claustrophobic levels; up to 300 animals per cubic metre have been recorded.
The animals are simultaneous hermaphrodites, and fertilization occurs internally. A gelatinous egg mass is released during spawning, and the eggs float freely until hatching. Their embryonic shells are lost within the first few days after hatching.