Invertebrates
Sea Angel
Clione limacina · also called sea angel, naked sea butterfly
The sea angel is a shell-less sea snail barely a centimetre or two long that swims through cold ocean water by flapping two delicate wing-like flaps. It looks serene, but it is a ruthless specialist predator that hunts one prey almost exclusively, wrenching shelled sea butterflies out of their shells with tentacles that shoot from its head.
Overview
Sea angels drift and swim in the open water of cold and polar seas — the Arctic, North Atlantic, and North Pacific. Usually only 1–3 cm long, transparent with a small orange core of organs, they feed almost entirely on sea butterflies.
A snail that flies
It has lost its shell entirely and swims by rhythmically flapping wing-like extensions of its foot, moving like a slow-motion flying creature.
A hidden hunter
Spotting a sea butterfly, it everts six grasping tentacles and hooks from its head, seizes the prey, and extracts the body from its shell over several minutes.
Status
The sea angel is Not Evaluated but abundant, though its total dependence on shelled prey makes it potentially vulnerable to ocean acidification long-term.
Sea Angel: common questions
Is the sea angel a fish, a jellyfish, or something else?
It is none of those. The sea angel is a sea slug, specifically a type of free-swimming marine snail called a pteropod, or wing-foot, that has completely lost its shell as an adult. Its transparent, drifting body and gentle swimming motion can make it look superficially like a tiny jellyfish, but it has a defined head, internal organs, a nervous system, and a snail's basic body plan, and it is far more closely related to a garden snail or a sea slug than to any jellyfish. The wing-like flaps it swims with are modified parts of the muscular foot that a typical snail uses to crawl.
How does the sea angel catch its prey?
For most of the time the sea angel looks harmless, drifting slowly with its head appearing smooth and rounded, but when it encounters a sea butterfly, its main and often only prey, it transforms into an active predator in an instant. It rapidly turns its head structures inside out to deploy six finger-like grasping appendages called buccal cones, along with rows of tiny hooks and a rasping, tooth-covered tongue, and uses these to grip the sea butterfly firmly. It then works the soft-bodied snail out of its coiled shell over a period of several minutes and swallows the body whole, discarding the empty shell.
Why does the sea angel eat only one kind of prey?
The sea angel is an unusually extreme dietary specialist, and in many populations it feeds almost exclusively on sea butterflies of the genus Limacina, small shelled pteropods that share its open-water habitat. Its entire hunting apparatus, its hooks, grasping cones, and radula, is finely adapted to extracting a soft snail body from a small coiled shell, and it appears to have little ability to switch to other food. As a consequence, sea angel populations track their sea butterfly prey closely, and the sea angel's life cycle, including the timing of its reproduction, is often synchronised with that of the sea butterflies it depends on.
Could ocean acidification threaten the sea angel?
Potentially, though indirectly. The sea angel itself has no shell and so is not directly harmed by the way that more acidic seawater makes it harder for marine animals to build and maintain calcium carbonate shells. However, its prey, the shelled sea butterflies, are among the marine animals most vulnerable to ocean acidification, since their thin shells can begin to dissolve in more corrosive water. If sea butterfly populations decline significantly as the oceans continue to absorb carbon dioxide and acidify, the sea angel, as a near-total specialist on them, could lose its food supply, making it an example of how acidification could ripple up a food web.