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I think it is a species of the genus Cavolinia, but I am not sure, so I use the family and call it Cavoliniidae sp. 01, which is placed in the clade
Thecosomata de Blainville, 1824.
The family Cavoliniidae Gray, 1850 is circumglobal, carried by the sea currents to all the seas of the world. They prefer deep waters, from 100 m up to 2,000 m. They do best in warm oceanic water. They are most common (in terms of diversity, species richness, and abundance) in the top 25 metres of the ocean, and become rarer the deeper one samples.
I quote Wikipedia on Thecosomata:
"Thecosomata beat their wing-like parapodia to "fly" through the water. They are holoplanktonic; that is, they 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, whose veliger larvae are part of the meroplankton.
Little is known about the behaviour of sea butterflies, but they are known to have a peculiar way of feeding. At times, they just float along, ventral-side up, with the currents. They are mostly passive plankton feeders, but at times they can be real hunters. They are generally herbivorous, entangling 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.
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."
Originally posted at these website as Cavoliniidae sp. A Family: Gray, 1850 (1815).
For some genera, such as Trinchesia is the alphabet almost through. I had to change the distinguishing alphabetic character into a number. I have chosen 2-digit numbers, even though '01' looks a little bit stupid, and now is only the genus italic written.