| Limacina | |
|---|---|
| Limacina helicina | |
| Scientific classification | |
| Kingdom: | Animalia |
| Subkingdom: | Metazoa |
| Phylum: | Mollusca |
| Class: | Gastropoda |
| (unranked): | clade Heterobranchia clade Euthyneura |
| Superfamily: | Cavolinioidea |
| Family: | Limacinidae |
| Genus: | Limacina Bosc, 1817[1] |
| Species | |
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See text. |
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Limacina is a genus of swimming predatory sea snails commonly known as sea butterflies in the family Limacinidae. This genus contains some of the world's most abundant gastropod species.[2]
Etymological meaning of the generic name Limacina is "snail-like".[3]
These little snails are pelagic, and they are marine gastropod mollusks. They swim by flapping their parapodia, and that flapping action is the reason why they were given the common name sea butterflies.
The sea butterflies are part of the order Thecosomata. The sea angels, which look a little similar and are also pelagic swimming snails, are in a different order, the Gymnosomata. Both of these orders are still sometimes called "pteropods"; the sea butterflies of the order Thecosomata have a shell, while the sea angels in the order Gymnosomata lack a shell.
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The shells of these sea butterflies are well developed, sinistrally coiled, turret-like and unpigmented. Shell sizes and thicknesses vary within Limacina, but they are still large enough for the animal to fit into. There is also an operculum. In Arctic and temperate waters, the diameter of the shell does not exceed 15 mm. In warmer waters, the diameter varies from 1 to 3 mm.
There are two large winglike parapodia, derived from foot tissue. The sea butterflies are continually flapping these wings to prevent sinking, because the shell gives them some negative buoyancy. During the daytime, they tend to move to deeper waters, but no lower than 100 m.
The sea butterflies have a peculiar way of feeding. They used to be regarded as passive feeders, but in reality they are active hunters, feeding mostly on plankton but also bacteria, small crustaceans, gastropod larvae, dinoflagellates and diatoms. They entangle their planktonic food through a mucous web that can be up to 5 cm wide, many times larger than themselves. This web is eaten as soon as there is enough food entangled and then a new one is soon deployed. This net also provides them additional buoyancy. If disturbed, they dump the net and flap away. There is a posterior footlobe with cilia, and a pair of lateral footlobes. They transport food, collected by the mucous web, to the mouth.
When they migrate to the surface, they may do so in unbelievably huge numbers. These aggregations usually attract their predators, the sea angels of the genus Clione (family Clionidae, suborder Gymnosomata). They are also on the menu of baleen whales, chunk salmon, pink salmon, herring [4] and certain seabirds.
Species within the genus Limacina include: