Billions of small, jellyfish-like creatures known as “by-the-wind sailors” have washed ashore all along the west coast of North America this summer, from southern California to British Columbia.
Images of vast swarms of electric-blue sails covering the ocean’s surface and littering the sand are indeed spectacular, but people might well wonder what exactly these strange-looking beings actually are. And this of course leads to the next question – should we be afraid of them?
Velella velella (to give them their scientific name) are often assumed to be a type of jellyfish but, while biology does lump them in with jellyfish, sea anenomes, and corals in a group known as Cnidaria, Velellas are not all that closely related to the common or moon jellyfish, Aurelia aurita.
Cnidarians have two body forms: the umbrella-shaped, tentacle-trailing “medusa”, your classic jellyfish; and “polyps” such as seas anemonies that typically live attached to the seabed.Velella is a colony of specialised individual polyps, much like their fellow sailors the Portuguese Man o’ War. Instead of living attached to rocks on the seabed, the water surface has become its substrate.
The by-the-wind sailor’s body is a flat oval disk 6-7 cm in diameter containing a series of air-filled chambers that provide buoyancy. Below hangs a central mouth surrounded by specialised reproductive bodies that produce tiny medusae, little “jellyfish”, and stinging tentacles – which are harmless to humans.
Projecting vertically up is a stiff translucent triangular vane made of chitin, a substance derived from glucose that is also used in crab and insect skeletons or squid beaks. This vane acts like a small sail. Interestingly, the sail runs diagonally across the top of the float, so that the individual sails at a 45 degree angle to the prevailing wind, just like a sailing boat.
Another striking feature is the bright blue colour, which is thought to serve as camouflage and/or protection from the sun’s rays. Animals that wash up on the beach dry up and become bleached white within a day or two.
Velella velella use their stinging tentacles to capture and feed on small fish larvae and zooplankton – microscopic animals that drift in the sea. But this is not their only source of food. If you look closely, you will also see a golden-brown colour inside the tissues which are zooxanthspellae – symbiotic photosynthetic microalgae – that provide the host animal an additional source of nutrition.
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