On 2 December 2012, I was walking Salmon Creek Beach and came upon this beautiful blue animal washed ashore. Although it had been windy, I was a little surprised to see it at this time of year. Spring is a more typical season to encounter them.
Did you guess that this would be a By-the-wind Sailor (Velella velella)?
This individual was small, ~2 cm long. It was the only one I noticed after walking on the beach for about an hour. Here's a view from above.
Velella is an unusual animal. (It might take a few posts to fully describe them!) They're cnidarians, related to jellyfish, sea anemones, and hydroids. Of these groups, they're most closely related to hydroids. So think about a hydroid colony floating upside down on the surface of the ocean.
The major features include a tall, triangular sail; a flattened, oval float (with concentric air chambers); and numerous tentacle-like structures below.
In the next picture, taken under a microscope (in 2009), you can see some of the tentacle-like structures below and extending beyond the edges of the float.
Their life cycle is very complicated. The stage that most people are familiar with is the floating colony. Some of the tentacle-like structures hanging from the underside of the float are specialized for reproduction and release tiny medusae (only a few mm tall). The medusae release eggs or sperm. A fertilized egg develops into a planula larva. The planula continues to develop into a conaria larva, which eventually undergoes metamorphosis into the floating colony stage.
To make it easier, here's an illustration of Velella's life cycle:
Modified from an illustration in The Living Bay: The Underwater World of Monterey Bay by Langstroth and Langstroth (2000)
This video from the Plankton Chronicles offers some nice views of Velella below the surface, as well as some swimming medusae (not easily observed!) and a larva undergoing metamorphosis.
Impressive numbers of Velella occasionally strand on beaches in the spring. Here's a photo from May 2006 when there were windrows of Velella on Salmon Creek Beach and phalaropes resting and feeding among them.
P.S. The terminology describing Velella has changed over the years. For this post, I referred to this recent paper: Schuchert, P. 2010. The European athecate hydroids and their medusae (Hydrozoa, Cnidaria): Capitata part 2. Revue suisse de Zoologie 117: 337-555.
P.P.S. Eric and I used to imagine having a small sailboat. We'd joke that it would be fun to have a blue hull with a white sail and that we'd call it Velella. Well, thanks to Crawford Boat Building in Humarock, MA, that boat arrived in California and has been sailing by-the-wind in Tomales Bay since 2009!