Sea Angel

Photo courtesy of: Arctic Ocean Diversity

What is a sea angel, particularly species C. limacina?

This particular type of sea angel, Clione limacina, is a type of pelagic sea slug. Its taxonomic classification includes it in phylum Mollusca and class Gastropoda. It is a 3-5 cm long, shell-less sea slug that lives in the northern most part of the Earth’s oceans south to the latitude line 35°N. C. limacina has been studied extensively for its novel swimming behavior and orange coloration. It is an important part of the pelagic food chain and can indicate the “health status” of the polar ecosystem.

How is C. limacina’s body structured?

When observed swimming in the water, the sea angel is generally positioned vertically with its head pointing towards the water’s surface. C. limacina has small head with a long tail. It has two wing-like parapodia and multiple cilia along its body that allows it to propel itself through the water. The wings are just inferior to the head, and two anterior tentacles are positioned on the front of the head which are inflated by hemocoel. It has a small mouth shaped like a slit between the two tentacles. On the inside, the sea angel has a fully functional heart on the right side of the body along with an open circulatory system. This circulatory system acts as a hydrostatic skeleton which supports the wings and movements. Photo courtesy of: Encyclopedia of Life

How does C. limacina move?

C. limacina’s swimming behavior has been extensively studied. C. limacina uses its wing-like parapodia and cilia to propel it through the water by making rhythmic oscillatory movements. Its wings are its main locomotory organ. The sea angel utilizes either fast or slow swimming depending on its environment and needs. Slow swimming is utilized to maintain constant position in the water. If C. limacina does not actively swim, it can drop up to 20-30 cm. This means that the sea angel actively swims its whole life in order to maintain adequate body position in the water. Fast swimming is used in order to catch prey, when feeding, and when escaping danger. Fast swimming is controlled by serotonin in neurons stemming from the cerebral ganglia. The hydrostatic pressure inside the wings increases causing the wings to stiffen. This gives the wings increased strength in order to push through the water more quickly.

What are C. limacina’s feeding behaviors?

The pelagic sea angel feeds mainly on sea snails, notably Spiratella (Limacina) retroversa and Limacina helicina. These are essentially the only two organisms that C. limacina eats. You could say it is a very picky eater! C. limacina utilizes fast swimming when catching sea snails and while feeding. When catching its prey it increases its wing oscillations to induce fast swimming and releases its tentacles from the top of its head. It catches the prey with the tentacles and keeps swimming quickly while holding its prey. This holds the prey against the sea angel's mouth so it does not escape. A study was done in the waters of Nova Scotia in order to more closely observe feeding behaviors and growth of the sea angel. It was observed that at 17°C and below, C. limacina fed and grew substantial amounts. At temperatures above 17°C, C. limacina slowed its feeding and growth slowed as well. Not only does C. limacina have a very particular diet, but it has a specific environment in which it likes to feed in. In addition to feeding, C. limacina is also a source of food for many whales and other fish such as salmon. C. limacina can be used to determine how healthy a particular body of water is. When C. limacina is abundant, waters are healthy. C. limacina has been recently disturbed by melting ice and increasing water temperatures.