Upper: Dorsal view. Lower: lateral view.
Monterey Bay Aquarium 3, Monterey County, California, USA. 1996. Photo: Daniel Geiger.
The gymnosome pteropod Cliopsis is a highly specialised carnivore which feeds on thecosome pteropods of the genus Corolla sp. It can grow to about 40mm in length. Thecosome pteropods feed on phytoplankton which they catch with the aid of an enormous mucuous web or net they cast out in front of them. When Cliopsis krohni comes into contact with one of these feeding webs, which in Corolla can be up to 2 meters in diameter, it begins dragging it in until Corolla is close enough for Cliopsis's long proboscis to reach out and cut the columellar muscle, which attaches Corolla to its 'shell'. Cliopsis can eat animals three times its own size. Its proboscis can extend to two times its body length.
Gymnosome pteropods are highly specialised sea slugs which spend their whole lives in the plankton. They have no shell, and their foot is reduced to three small median lobes, two of which are visible between the wings in the upper photo. They also have a pair of wing-like paddles which are used for swimming and by a 'sculling' movement are able to propel the animal quite quickly. As well as being built for swimming, gymnosomes are also built for hunting. They all have very elaborate foreguts with a proboscis, entrapping tentacles and cutting radular teeth.
Planktonic sea slugs are known collectively as pteropods (= winged feet). Although this name persists in the popular literature, (and I have used it here), there is no close evolutionary relationship between the phytoplankton-feeding, shelled Thecosomata and the carnivorous, unshelled Gymnosomata.
• Lalli, C.M. & Gilmer, R.W. (1989) Pelagic Snails. The biology of holoplanktonic gastropod molluscs. Stanford University Press: Stanford, California.
Rudman, W.B., 2000 (July 4) Cliopsis krohni [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/factsheet/cliokroh