By the Wind Sailors

Though it may well be a single animal (a highly modified upside down hydroid polyp), here we will call it a colony, mainly because that is the way it is still described in recent text books. Each Velella is composed of highly specialized and modified zooids, which are individual polyps and medusae that suspend downward and form the "organs" of this super-animal. There is a gastrozooid that ingests food, dactylozooids that capture food and protect the colony, and gonozooids that are involved in reproduction. There is an upper, many-chambered, chitinous float that bears a clear sail set at an angle of about 30 degrees. Around the float is a beautiful Prussian blue mantle, called a raft, with fore and aft lobes that are about 15 degrees oblique to the float, so that the sail is 45 degrees to the long axis of the colony or organism. The direction of the sail's angle is different depending on which part of the world Velella occurs. On our Pacific Coast, the sail angle is such to take advantage of winds prevailing from the north, and that serves to keep them offshore. Velella’s sail is fixed – it can not shift its sail like a sailboat to change direction. It is, in sailing parlance, "running before the wind" or "by the wind", hence its poetic name. When the winds shift, Velella may be driven ashore to litter the beaches with their blue bodies which decompose in a few days, leaving just the cellophane sail and floats. In other parts of the world, the direction of the sail is actively selected to take advantage of prevailing winds there. In yet other locations, the sail may be in both directions. Some authors say that all Velella populations are born with sails angled in both directions, so that unfavorable winds will blow only half of them to the beach, allowing the other half to live.

It is interesting to speculate on the growth rate of Velella based on the sizes of those washed on to the beaches during different parts of the year. For several years, including 2005, I’ve seen windrows of tiny Velella, most less than a millimeter, washed ashore during winter and early spring. In March of 2005, many Velella were driven ashore at Oceanside by the winds, most less than a centimeter in length. As the year progressed, others came ashore. In early May there were both small and larger ones, and by late June most were large, six, seven, even ten centimeters long. If all the Velella that were stranded during 2005 were from the same population, then there differences in length during the different months may indicate growth from young to older Velella. But these sizes may well have been from different younger and older populations that mixed during May and do not necessarily represent growth in a single group of animals. It is hard to say which is true.

Velella is a carnivore, feeding on fish and invertebrate eggs and small zooplankton that live near the surface because its tentacles (dactylozooids), which contain nematocysts (cells that sting on contact), are short and do not reach far into the water. Some of the zooids contain photosynthetic, symbiotic algae (zooanthellae) in their tissues. It spends most of its life floating on the ocean, only the reproductive medusae descending into deep waters to produce eggs and sperm and the resulting young embryos that then rise back to the surface.

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