Velella are small hydrozoan, meaning they are relatives to jellyfish and coral. However, instead of living in solid colonies like the more well-known coral, these animals surf the waves, catching the wind with little sails called Velella, hence the animals’ name. They are relatively common, and if it is particularly windy, they can become stranded by the thousands on beaches along the coast of Ireland. These curious animals are carnivorous, and eat plankton using their tentacles that hang down in the water ready to stun any prey small enough to be digestible.
Although referred to in the singular, Velella are actually colonies of many small animals, each with their separate task of either catching food, reinforcing the sail, or other tasks in the colony. The tiny ecosystem is so complex it isn’t completely understood yet, but an excerpt from Wikipedia might explain better:
Like many Hydrozoa, Velella velella has a bipartite life cycle, with a sort of alternation of generations. The deep blue by-the-wind sailors that are recognized by many beach-goers are the polyp phase of the life cycle. Each “individual” with its sail is really a hydroid colony, with many polyps that feed on ocean plankton and are connected by a canal system that enables the colony to share whatever food is ingested by individual polyps. Each by-the-wind sailor is a colony of all-male or all-female polyps. The colony has several different kinds of polyps, some of which are both feeding and reproductive, called gonozooids, and others protective, called dactylozooids.
The gonozooids each produce numerous tiny jellyfish by an asexual budding process, so that each Velella colony produces thousands of tiny jellyfish (medusae), each about 1 mm high and wide, over several weeks. The tiny medusae are each provided with many zooxanthellae, single-celled endosymbiotic organisms typically also found in corals and some sea anemones, that can utilize sunlight to provide energy to the jellyfish. Curiously, although a healthy captive Velella will release many medusae under the microscope and thus must do the same in the sea, the medusae of Velella are rarely captured in the plankton and very little is known about their natural history. The medusae develop to sexual maturity within about three weeks in the laboratory and their free-spawned eggs and sperm develop into a planktonic larva called a conaria, which develops into a new floating Velella hydroid colony.