Rafting snails (family Janthinidae) are a niche group of commonly purple-shelled molluscs, which have taken to the open ocean: they live suspended under rafts of bubbles, preying on coelenterates (jellyfish-like organisms). Snails hardly seem well-adapted to life in the plankton, however, as they typically exist as grazers on the sea floor, and possess no real swimming apparatus. The jump from sea bed to sea surface is a significant one; only recently has light been shed upon the way the group evolved, by C. Churchill and associates of the University of Michigan. Genetic analysis indicates a close relationship between the Janthinid snails and the Epitoniids, bottom-dwelling predators, but the apparent key to the matter is in the breeding strategies of the two groups. Both deposit their eggs in frothy masses of capsules, which can become buoyant on their own, and with the addition of mucous bubbles would make a fine raft; the photo below shows a pair of Janthinids, one with several rows of suspended egg capsules attached to its bubble float. Transitioning to the open ocean gave the snails access to an abundance of coelenterate prey, which few other predators can feed upon.
-TJT
Further detail on the study, and the snails, can be found here, along with the photo below showing the species Janthina exigua (credit to Denis Riek) and pictures of other species: http://bit.ly/1PYKfmU
