Sea butterflies, also known as Thecosomata or flapping snails, are a taxonomic suborder of pelagic swimming sea snails. These are holoplanktonic opisthobranch gastropod mollusks in the order Opisthobranchia.
This group used to be known as pteropods. This term, however, is no longer scientifically precise, but is still used sometimes as a convenience. The word pteropod applies both to the sea butterflies in the order Thecosomata and also to the sea angels in the order Gymnosomata. Mollusks of the suborder Thecosomata have a shell, while the Gymnosomata lack a shell.
"Holoplanktonic" means that these snails spend their whole life in a planktonic form, rather than just being planktonic during the larval stage, as is more commonly the case in many marine gastropods.
Little is known about the behaviour of sea butterflies, but they are known to have a peculiar way of feeding. They are mostly passive plankton feeders, but at times they can be real hunters. They entangle planktonic food through a mucous web that can be up to 5 cm wide, many times larger than themselves. If disturbed, they abandon the net and flap slowly away. When descending to deeper water, they hold their wings up. Sometimes, they swarm in large numbers and can be found washed up in flotsam especially along the coast of eastern Australia.
Every day, they migrate vertically in the water column, following their planktonic prey. At night they hunt at the surface and return to deeper water in the morning.
These creatures, which are about the size of a lentil, are eaten by various marine species, including a wide variety of fishes that are, in turn, consumed by penguins and polar bears. Researcher Gretchen Hofmann, calls them the "potato chip" of the ocean. She says that as the ocean becomes more acidic and warmer, these creatures are not able to survive. "It's possible by 2050 they may not be able to make a shell anymore. If we lose these organisms, the impact on the food chain will be catastrophic."
Clade Thecosomata :
The superfamily Limacinoidea becomes redundant and the family Limacinidae becomes part of the superfamily Cavolinioidea. The families Creseidae and Cuvierinidae become the subfamilies Creseinae and Cuvierininae. The infraorder Pseudothecosomata becomes the superfamily Cymbulioidea. The superfamily Peraclidoidea becomes redundant and the family Peraclididae is included in the superfamily Cymbulioidea as the family Peraclidae.
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