The velella (Velella velella) is a siphonophore that can be found floating on the surface of the water. The velella bears a striking resemblance to the Portuguese Man o’ War having the same rudimentary shape and colors. Specialized predatory gastropod mollusks prey on these creatures such as predators like the blue glaucus.
Velella are carnivorous. They catch their prey, generally plankton, by means of tentacles that hang down in the water and bear nematocysts. 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. Velella‘s nematocysts are relatively harmless to humans, although each person may respond differently to contact with the nematocyst toxin. It is wise to avoid touching one’s face or eyes after handling velella, and itching may develop on parts of the skin that have been exposed to velella’s nematocysts.
Each apparent individual is a hydroid colony, and most are less than about 7cm long. They are usually deep blue in color, but their most obvious feature is 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 thousand on beaches. Having no means of locomotion other than its sail, velella’s are at the mercy of prevailing winds moving around the seas, and are thereby also subject to mass-strandings on beaches throughout the world. For example, most years in the spring, there is a mass stranding that occurs along the West Coast of North America, from British Columbia to California, beginning in the north and moving south over several weeks’ time. In some years, so many animals are left at the tide line by receding waves, that the line of dying animals may be many centimeters deep, along hundreds of kilometers of beaches. Mass strandings have been reported also on the west coast of Ireland. The small rigid sail projects into the air and catches the wind. However, velella sails always align along the direction of the wind where the sail may act as an aerofoil so that the animals tend to sail downwind at a small angle to the wind.