Sea angels… Small, yet graceful. It has no shell, but is still a snail.  Sea angels (Gymnosomata) are a group of pelagic, free-swimming molluscs, present all the way from polar regions under the sea ice, to warm tropical seas. There are many species of sea angels, and the largest ones can only reach a size of 5 cm.

A sea angel swimming in the deep sea.

Most gastropods (snails) have a developed foot used for attaching itself to surfaces. However, the sea angels have developed their foot into two wing-like appendages instead, which they flap back and forth to swim with. As such they resemble small angels, floating around in the world’s ocean.  Another feature common of snails, is that they bear shells. Sea angels only have an embryonic shell for a few days after hatching, discarding it as an adaptation which makes them more suited for their free-swimming lifestyle.

Sea angels can be highly specialized predators, with only one item on their menu. They may only eat “sea butterflies” (Thecosomata) – which also are free-swimming snails belonging to the same order “pteropoda”, but with shells! Sea angels feeding strategies vary according to which prey species they target – some sit and wait for their prey to swim by, while others actively swim to pursue sea butterflies. Once a sea angel has caught a sea butterfly, it uses hooks and a toothed radula (tounge-like structure) to extract it from its shell, and digest it efficiently.

Close-up of an Arctic sea angel. (Photo from Adobe Stock, by Animanish)

As sea butterflies perform diel vertical migration, actively migrating from the mesopelagic zone during night-time to feed at surface waters, the sea angels follow – participating in the worlds largest animal migration. However, sea angels also have to watch out for predators such as fish and whales, which like to eat them! This might also be a reason for why the sea angels like to dive deep into the mesopelagic zone during daytime, to hide itself from potential predators.

Despite their small size, sea angels are graceful, gelatinous and translucent predators of the ocean twilight zone.

Would you like to explore more of the deep sea ecosystem, the mesopelagic zone? Dive deep with our new video, and discover the incredible twilight zone for yourself!