Blue Zoo: Velella
Featuring one amazing marine animal per week.
Though it has common names, such as the “by-the-wind sailor” and “purple sail,” this is one of the rare species that is also often referred to by its scientific name: Velella velella.
Velella is a species you can recognize right away. It has a flat blue disc with concentric rings only a few centimeters long that holds air and floats on the surface of the water. Upon the disc stands a little sail to catch wind in order to travel. Below the disc hang many little stinging tentacles for catching plankton, as well as other tentacles for reproducing and feeding.
Photo by Doug Beckers via Flickr, Creative Commons License.
The sail of velella stands diagonally across the disc. Different individuals have sails oriented different ways – either from right to left or from left to right. This means that the wind and wave movement will send them in different directions, helping dispersal in the open ocean. They can be seen in large masses in tropical and temperate waters, and often wash up on a beach, dying in huge groups. It has been found that the two different sail directions disperse to opposite sides of the ocean – while one sail orientation is only found on beaches on the west side of the ocean, the other form washes up on the east side.
Photo by newslighter via Flickr, Creative Commons License.
Velella has a lifecycle with two main parts, the floating polyp (what you see on beaches), and the medusa (it's like a tiny jellyfish). To reproduce, polyps on the underside of velella’s disc create tiny medusae asexually that bud off and sink to deeper water. The medusae will mature and then produce eggs and sperm, which will combine to grow into new polyps. Both the reproductive polyps and the medusae contain zooxanthellae, just like corals do, symbiotic algae that provide energy through photosynthesis.
Photo by Jymm via Wikimedia Commons, Public Domain.
Taxonomists have long debated how to categorize within its class, Hydrozoa. For a long time, it was considered a member of the order Siphonophora – this meant that the structure of the organism wasn’t just made up of one animal, but a colony of many individuals. It was thought that the stinging tentacles were all individual polyps, and so were the many little reproductive tentacles. Now, however, it is believed to be more likely that the whole thing is, after all, just a single large polyp so it has been relocated into the order Chondrophora, more recently also known as the Porpitidae.