The sea angel, a graceful and seemingly delicate organism, is a predatory mollusk that drifts through the cold, open ocean. Classified within the genus Clione and the suborder Gymnosomata, this creature is a fascinating example of a highly specialized diet in the marine environment. Despite its ethereal appearance, the sea angel is a formidable hunter whose existence is intertwined with a single, unique prey item.
Defining the Sea Angel
The sea angel is a type of pelagic sea slug, a gastropod mollusk that lacks a shell in its adult form. Its body is mostly translucent and gelatinous, allowing views of its reddish-brown internal organs. The most distinctive feature is a pair of wing-like appendages, known as parapodia, which are modified from the mollusk’s foot. These parapodia flap rhythmically, propelling the creature through the water column in a motion that inspired its common name. Sea angels are found in cold, polar, and subpolar waters worldwide, inhabiting the epipelagic and mesopelagic zones, from the surface down to over 500 meters.
The Exclusive Prey: Sea Butterflies
Sea angels are obligate predators, meaning their survival depends almost exclusively on other planktonic mollusks known as sea butterflies. Sea butterflies belong to the suborder Thecosomata, and Clione limacina feeds almost entirely on species within the genus Limacina. This predator-prey relationship is an example of coevolution between the predatory Gymnosomata (“naked body”) and the shelled Thecosomata. Both creatures are pteropods, or “wing-footed” sea snails, but the sea butterfly retains a delicate, coiled calcium carbonate shell.
The sea angel’s metabolic rate is closely linked to the availability and size of its prey. Larval stages begin feeding on similarly sized sea butterflies, and adults continue this selective predation. This dependence means that fluctuations in the sea butterfly population directly impact the sea angel’s ability to thrive. A single sea angel can consume hundreds of sea butterflies in a season, accumulating fat deposits that allow it to survive for months without food during periods of scarcity.
Specialized Hunting Tactics
The sea angel employs a specialized hunting mechanism to capture its shelled prey. When a sea angel detects a sea butterfly, often by touch or chemical receptors, it actively pursues the creature. To secure the prey, the sea angel rapidly everts a complex feeding apparatus hidden within its head, which includes three pairs of finger-like projections called buccal cones.
The buccal cones inflate and extend, grasping the sea butterfly’s shell, sometimes forming a basket-like structure around it. Once secured, the sea angel manipulates the shell until the prey’s opening faces its mouth. The sea angel then uses chitinous hooks and a toothed radula—a tongue-like structure common to mollusks—to extract the soft body of the sea butterfly completely. This extraction process does not involve cracking the shell but rather pulling the prey out whole, a quick and efficient meal that can take as little as two minutes.
Ecological Significance of the Relationship
The relationship between the sea angel and the sea butterfly positions both organisms as foundational elements in the polar food web. They represent a significant portion of zooplankton biomass in Arctic and Antarctic waters, serving as a food source for larger animals like fish, baleen whales, and seabirds. The specialized predator-prey link also makes them sensitive bioindicators of environmental change.
The sea butterfly’s delicate calcium carbonate shell is highly vulnerable to ocean acidification, caused by the absorption of increasing atmospheric carbon dioxide. As ocean waters become more acidic, shell formation is inhibited, and existing shells can dissolve. A decline in the sea butterfly population due to shell degradation directly threatens the obligate predatory sea angel, potentially leading to cascading effects throughout the polar marine ecosystem.