Tuesday, May 5, 2015

TOO MUCH WIND FOR BY-THE-WIND SAILORS


For more than a month now, Pacific Northwest outer ocean beaches have been the scene of a massive stranding of by-the-wind sailors, Velella velella. These little blue critters are washing up on our beaches by the billions. Each time a new high tide comes in, they are deposited at the upper end of the waves in windrows on the beach.

     Velella wrecks are commonplace on the Pacific coast, usually happening in the spring of most years. But this year has produced a bumper crop of them, many more than usual, from Vancouver Island to California. First noticed in August of 2014, they came in in stupendous numbers in the spring of 2015 with unusually strong onshore winds. They are everywhere in tropical and temperate oceans, but their strandings seem to be most common on the American Pacific coast.


     Velella is a colonial animal, each sailor made up of hydroids attached together under a chitinous float, the individuals thought to be specialized for prey capture, digestion and reproduction. A stiff semicircular sail projects upward from the float, so any breeze blows the sailor across the surface. The sail is set at a bit of a diagonal, so the Velella is actually tacking off the downwind direction. It is speculated that the direction of tacking is such that the animals are kept offshore for the most part, but changing winds can undo that safety factor.

     One of the most characteristic things about Velella is its beautiful blue color. The blue pigment is apparently a protection against harmful ultraviolet radiation from the sun that streams down through the day.

     Velella are typical cnidarians, possessing nematocysts in their tentacles that kill tiny planktonic prey, including crustacean larvae and fish eggs, and digest them in a central stomach area. This nutrition is shared among the individuals and is augmented by algae (zooxanthellae) in the colony that photosynthesize and produce organic carbon and nitrogen. In the class Hydrozoa, they are more closely related to hydras than to jellyfish, although not as closely related to Portuguese man-o’-war as previously believed.

     They feature alternation of generations, reproducing like typical members of the phylum Cnidaria. The sailors that we see bud off tiny medusae (“jellyfish”) that then produce male and female gametes that fuse and form larvae that at some point presumably assemble into the animals that we see. Exactly how this happens is still quite unknown, and the smallest ones that are usually seen are fully developed.


     You might guess that Velella, just because of its abundance, would be attractive as prey, yet much of the animal is inedible chitin, the same protein that makes up the exoskeleton of insects. Their best-known predators are spectacular pelagic nudibranchs (Glaucus) and a group of unusual snails (Janthina) called violet snails that produce mucus with their foot that forms into bubbles that support them. They float on the surface darkside up, countershaded like oceanic fish!


     Two jellyfish predators, ocean sunfish (Mola mola) and leatherback turtle (Dermochelys coriacea), are said also to consume Velella, but there seems little evidence for this. Many sandpipers fed among the Velella at Grays Harbor recently, and some poked into the tentacles, perhaps finding small prey among them.



     These Velella velella are not to be mistaken for the Seattle musical group of the same name!

Dennis Paulson

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naresh nari said...
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