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.
Another large polar species of sea angel, Clione antarctica, defends itself from predators by synthesizing a previously unknown molecule, named pteroenone. Because of this secretion, predators will not eat the sea angel and a species of amphipod has taken advantage of this fact: the amphipod will seize an individual of C. antarctica out of the water column, and carry it around for protection. Local population density of C. antarctica may reach extraordinary 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.