Velella

Velella velella, commonly known as the by-the-wind sailor, is a small colonial hydrozoan that lives at the ocean surface. The colony consists of a single, rigid float that is blue and oval, typically 2 to 6 centimeters long, with a transparent, sail-like crest that projects above the water on its windward side. A column of feeding and reproductive polyps extends below the float; these polyps are specialized into gastrozooids for feeding, dactylozooids for defense, and gonozooids for reproduction.

Velella is pleustonic, meaning it resides at the air-water interface and is carried by winds across the sea surface. It has a worldwide distribution in temperate and tropical seas, and is often observed drifting in swarms. The organisms can be stranded on beaches after storms, where they are encountered by beachgoers.

Diet consists of small planktonic crustaceans and other tiny organisms captured by the tentacles of the polyps. The colony employs stinging cells (cnidocytes) to immobilize prey; contact with tentacles can cause mild skin irritation in humans.

Reproduction occurs both asexually, by budding within the colony, and sexually, with planula larvae that develop into new colonies. The life cycle remains tied to the colonial organization, and the free-living medusoid stage is not a prominent part of its biology. Velella is an important component of surface-dwelling plankton communities and is frequently noted for its striking blue coloration and distinctive sail.