Velella velella (aka By-the-Wind Sailor)
These bright blue, free-floating hydrozoans can be found in the open ocean in most warm and temperate waters. They are what is known as "neuston": animals that live at the interface between the water and the air, partly in and partly out of the water. Velella are much smaller relatives of the Portuguese man o’war, and are also carnivorous colonial organisms. However, their toxin is more benign to humans (although it could still provoke an allergic response), and instead of a gas-filled float they have a stiff sail-like structure that helps propel them through the water.
Each colony is made up of all-male or all-female polyps, and they have a bipartite life cycle. The feeding and reproductive polyps (gonozooids) reproduce via asexual budding that produces thousands of tiny medusae (jellyfish), which, when mature, release eggs and sperm that then develop into a new free-floating Velella colony. The feeding polyps sting and feed on small planktonic organisms, and they are in turn preyed on by bubble snails and nudibranchs that are also specialized to live at the air-water interface. Because the Velellas are at the mercy of the winds, they can cluster into enormous drifts, and occasionally wash ashore in mass stranding events.
References:
https://en.wikipedia.org/wiki/Velella
