Sea Angels: a large group of small swimming sea slugs in six different families. These are pelagic marine opisthobranch gastropod molluscs [Pteropods - ‘Winged foot’] in the clade Gymnosomata within the larger clade Heterobranchia.
They discard their shell at metamorphosis, with the retractor muscles being severed and the shell lost. The group does not truly, therefore, lack a shell. Few larval shells have been described (and consequently an understanding of their fossil record it as yet outwith our grasp).
Their distribution ranges from sub-sea ice polar regions to equatorial seas.
The IPCC reports that increasing atmospheric carbon dioxide is causing acidification of the oceans, which could eliminate pteropods from the Southern Ocean and cause serious repercussions throughout the food chain.