This video was recently posted by Kevin Zelnio at Deep Sea News:
The animal shown in the video (swimming to music from Hayao Miyazaki's great ecofable Kaze no Tani no Naushikā [Nausicaa in the Valley of the Wind]) is the pelagic gastropod Clione limacina. Aquatic gastropods are usually members of the benthos, and relatively few groups have made the change to a pelagic lifestyle. Those that have are invariably freakishly bizarre:
Even the superficially more normal-looking (albeit attractively coloured) violet snails of the genus Janthina (also predators of planktic hydrozoans) make up for their unassuming appearance with their bohemian lifestyle, hanging upside down from a floating raft constructed from bubbles held together with mucus:
The sea angel Clione belongs to one of the larger groups of pelagic gastropods, the Gymnosomata. Together with another group, the Thecosomata, they have been classified into the Pteropoda, characterised by the adaptation of the foot into a pair of 'wings' for swimming. Recent authors have disagreed as to whether or not the pteropods form a monophyletic group (with probably the majority favouring "not"), or whether gymnosomates and thecosomates took to the seas independently, but pteropod monophyly was supported by Klussmann-Kolb & Dinapoli (2006). Pteropods have an interesting taxonomic history. When Cuvier first established the group in 1804, he regarded them not as gastropods but as a separate order in their own right. Pteropods seem to have been regarded as a link between gastropods and cephalopods, with some believing them closer to the latter than the former. Even after the modern pteropods were well-established to be gastropods, and 'Pteropoda' often abandoned as a formal category, the name maintained a strange shadow existence in palaeontology, with many Palaeozoic fossil problematica such as tentaculitoids, hyoliths or even conulariids (almost meaninglessly) regarded as or compared with "pteropods".
Within the pteropods, the most obvious difference between Gymnosomata and Thecosomata is that gymnosomates such as Clione completely lack a shell, while thecosomates retain one. And what incredible shells they are - the ultimate in lightweight, translucent construction, some thecosomates are ethereal constructions in blown glass:
Klussmann-Kolb, A., & A. Dinapoli. 2006. Systematic position of the pelagic Thecosomata and Gymnosomata within Opisthobranchia (Mollusca, Gastropoda) – revival of the Pteropoda. Journal of Zoological Systematics and Evolutionary Research 44 (2): 118-129.