Two mating sea angels flutter through the deep waters of the Arctic Ocean off the coast of Novaya Zemlya, an archipelago near northern Russia, in recently captured footage.
In the video, which was filmed by marine biologist Alexander Semenov, a single clearly visible sea angel is joined by a second one. The pair of sea slugs then swims through the water side-by-side in a flowing mating ritual that resembles a dance.
When two sea angels find each other, they turn out their reproductive organs and attach themselves to their partner’s body with a sucker to stay together during the mating process. This attachment leaves scars on their bodies, and some adult sea angels have up to four scars, which can indicate frequent mating rituals.
The fertilization process can last up to four hours, and while it happens, the sea angels stay connected to each other, swimming gracefully through the water with the help of all four of their wings. Semenov says their mating ritual doesn’t affect their appetite, and sea angels can hunt for prey while they are attached to each other.
Once the mating ritual is complete, the sea angels move in a spiral shape in order to disconnect.
“This miniature creature is an incredibly graceful swimmer; watching it is a complete pleasure,” says Semenov. "They seem to float in the air, slowly waving their wings.”
Sea angels, so named because their shape resembles a snow angel, have translucent white bodies that are long, with a wing-like structure on both sides of their bodies. Because they are semi-transparent, it is easy to see the coral-pink and yellow coloring of their internal structures.
Sea angels’ lovely outward appearance and name belie their status as a kind of sea slug, related to other forms of snails in the gastropod class. They inhabit the frigid waters of the Arctic, subarctic Atlantic, and Pacific oceans, and prey on sea butterflies—specifically a small type of sea snail called Limacina helicina. Some sea snails have even developed small tentacles with which they can catch their prey and hold it while they eat.
Scientifically named Clione limacina, they are protandrous hermaphrodites, which means they are both male and female during their life cycles, according to Semenov. Young sea angels start out as males, developing eggs as they grow into adults. Mature sea angels have both eggs and spermatozoa in their bodies.