„By the wind sailor“, Velella velella
On our visits to Piha and Karekare we came upon a lot of miniature sailboats washed upon the beach. Some of them lay drying in the sun, others turned in circles on the edges of tidepools. All of them belong to the species Velella velella, a hydroid that occurs in subtropical to tropical waters. They are also called „by the wind sailors“, and there are left and righthanded individuals that the wind drifts into different directions as they ride on the oceans currents. The polyps are kept afloat by a chitin raft with airchambers, and feed upon plankton as well as being supported by zooxanthellae. With them one of their biggest enemies got washed upon shore, the beautifully blue colored snail Janthina. It feeds on Vellella with the help of a float of mucus that it secretes. Janthina even make a bubble nest for their eggs, that otherwise would swim like a rock, and male snails turn into females when they reach a certain size. In this case the predator was in the same boat with it’s prey though, and they both perished as they hit the New Zeland coast.
The blue Janthina exigua and the bottomside of Velella velella, showing the different polyps.
