Angels of the Sea

A small, transparent, shell-less sea snail in the open ocean.

This is a tree topper unlike any other! Reminiscent of a freshly made snow angel, these pteropods are actually shell-less sea snails (Clione limacina). Unlike the typical snail, they flap their adapted foot ‘wings’ to get around in the water column. They are extremely small, with the largest species reaching only 5 centimeters long. Sea angels' mostly eat their relatives, the sea butterfies, which are threatened by ocean acidification.

(Copyright © Alexander Semenov)

Tis the season when angels grace mantelpieces, treetops, and front stoops. Perhaps then, the timing also warrants an introduction to the fluttering angel that lives in the sea. Sea angels are a group of gelatinous sea snails within the larger mollusk division that have earned a heavenly distinction despite their modest existence as a snail. Scientists refer to them collectively as Gymnosomes and the most common species are Clione limacina and Clione antarctica. Even though they grow to be only one and a half to about three inches long (4-7 cm), ocean explorers delight at their sight because of their beauty and cool anatomy. For those not as familiar with the angels of the sea, take a look below to find out what makes sea angels a snail worthy of distinction.

Graceful swimmers, sea angels are beautiful to watch.  Their wing-like fins propel them in a manner reminiscent of flying, which is partially why they are commonly known as sea angels. Sea angels are also fairly translucent, creating an almost ethereal illusion. The translucent skin allows for view of their internal organs, like their gonads, which are a pretty pink and orange hue. 

A translucent pteropod swims freely in the open ocean with a black background.
(©Brian Skerry)

Ravenous Eaters
Though their name implies an angelic nature, sea angels are, in fact, predators with quite a bite. In a sick twist, their preferred food is one of their close relatives, the shelled sea snails commonly known as sea butterflies. Some species are ambush predators, lying in wait until prey passes by, while other species actively attack. Once the sea angel gets hold of its prey, it uses two specialized eating appendages known as buccal cones to extract sea butterflies completely out of their shells. The buccal cones have numerous hooks and a toothed radula that enable a quick and efficient meal. From capture to consumption, it can take as little as two minutes!

A pair of sea butterflies float in the Arctic ocean.
This pair of sea butterflies (Limacina helicina) flutter not far from the ocean's surface in the Arctic. (Courtesy of Alexander Semenov, Flickr)

A Sour Taste 
Even predators need tactics to avoid becoming lunch, and sea angels in Antarctic waters have evolved to produce a chemical compound that deters fish. This is a pretty extraordinary feat for a mollusk—it is the only example in which a chemical defense is produced by the mollusk and not absorbed from a food source. The tiny crustacean called an amphipod even seeks out sea angels to benefit from their chemical protection by hitching rides on their backs. 

Gender Switch
Sea angels have the remarkable ability to switch genders. Specifically, they are protandrous hermaphrodites, which means they start out male and turn female throughout the course of their lives. Catching the mating between two sea angels is quite the sight. The two swirl, locked together for up to four hours, almost as if they were partners in dance.

a sea angel spawns a cloud of eggs

A female sea angel releases her eggs.

(Richard Collins)

As adults, sea angels are soft bodies creatures, but when they are initially born, they reside within a shell. As they age, they undergo a metamorphosis, much like a caterpillar transforms into a butterfly. During the transformation the shell is shed.  The closely related sea butterfly, however, keeps its shell throughout life. Built of calcium carbonite, both the sea butterfly’s shell and the sea angel’s larval shell may have trouble forming as ocean acidification due to climate change becomes a greater threat.


Editor's note: Special thanks to Dr. Tricia Thibodeau for her consultation

December 2020