Velella

Velella is a genus of hydrozoa (a group of small, predatory marine cnidarians) in the family Porpitidae. Its only known species is Velella velella, a widely-distributed free-floating colonial animal that lives on the surface of the open ocean. It is commonly known by the names sea raft, by-the-wind sailor, purple sail, little sail, or simply Velella. It is a member of a specialised ocean surface community that includes the better-known Portuguese man o' war. Specialized predatory molluscs prey on these cnidarians. Such predators include nudibranchs (sea slugs) in the genus Glaucus and purple snails in the genus Janthina. Each apparent individual is a hydroid colony, and most are less than about 7 cm (2.8 in) long. They are usually deep blue in colour, with a small stiff sail that catches the wind and propels them over the surface of the sea. Under certain wind conditions, they may be stranded by the thousands on beaches. Like other cnidarians, V. velella are carnivorous. They catch their prey, generally plankton, by means of tentacles that hang down in the water and bear stinging nematocysts (also called cnidocysts). The toxins in their nematocysts are effective against their prey. While cnidarians all possess nematocysts, in some species the nematocysts and toxins therein are more powerful than other species. The nematocysts of V. velella are relatively benign to humans, although each person may respond differently to contact with the nematocyst toxin; irritation may occur to skin exposed to V. velella nematocysts.