Using a pair of winglike structures, the sea angel propels itself gracefully through the deep waters of the ocean. Sea angels look quite ethereal, with translucent bodies and internal organs of pink and orange. However, despite its celestially inspired name, the sea angel is not so angelic in disposition as they in fact are fierce predators of the deep.
Development and Habitat
Sea angels, Clione limacina, are invertebrates within the phylum Mollusca. Despite their shell-less appearance, these organisms are classified with other snails! Though bare in adulthood, these organisms were not always in this state. Representatives of C. limacina are born with shells that are shed upon adulthood, leaving them with soft gelatinous bodies for later in life. The visible ‘wing’ structures, also known as parapodia, are homologous (or similar due to common ancestry) with the muscular foot used by land snails for locomotion.
Sea angels have a wide distribution in the earth’s oceans, ranging from temperate to arctic zones. Regardless of temperature, these organisms are known to dwell within the mesopelagic zone of the ocean, 200-1,000 meters, occupying only as deep as 600 meters.
Diet and Reproduction
Individuals of C. limacina are sequentially hermaphroditic, meaning they have both male and female reproductive organs with the ability to swap sexes if needed. When the mating pool becomes limited, this is an incredibly useful adaptation! These creatures are also very small, only growing to be about 5 cm at the most, but are fierce predators nonetheless.
Clione limacina has a preferred diet of one its own close relatives, its sister species: the sea butterfly! Sea butterflies, like sea angels, are born with shells. Differing from the sea angels, however, sea butterflies retain their shells throughout their lifetime. Unfortunately, the presence of a shell doesn’t offer much protection against their ravenous cousins. To deal with the pesky shells of sea butterflies, C. limacina has an adaptation of tentacle-like structures, called buccal cones, that originate from their heads and latch onto prey. These buccal cones have a radula, a mouth with teeth-like structures, and hooks to scoop the sea butterfly out of its shell like a kiwi from its skin! The sea angel then devours its prey whole and flaps away to hunt down another sea butterfly delicacy. Altogether, the process of locating prey and feeding can take anywhere from two to 45 minutes.
A favorite for now, unfortunately C. limacina might need to find a more sustainable favorite as sea butterflies are becoming increasingly endangered by ocean acidification. As the acidity of the ocean increases (or pH decreases), the calcium carbonate making up the shell of the sea butterfly disintegrates, leaving the organism vulnerable to predation and environmental variables.
Fear not, however, as the sea angel has other nearby food sources. Some of these are phytoplankton, or floating photosynthetic organisms. In fact, consuming these is the mechanism behind the sea angels’ vibrant colorations!
Sea angels are an incredible example of the diverse life dwelling in the depths of our oceans, and a great reminder that size is no indication for how well-adapted an ocean predator can be.
Check back soon for more of the Diving Deep series!
References
Monterey Bay Aquarium | Animals A to Z | Meet the Sea Angel
Smithsonian | Ocean, Find Your Blue | Angels of the Sea
Compassion Crossing 