Sea spiders are not true spiders. They are a separate group of sea animals with a small body and very long legs, and about 1,400 species have been described. Near the coast most span between 3 millimeters and 6 centimeters, but deep-sea kinds often span 20 to 50 centimeters, and one specimen at the Smithsonian measured about 76 centimeters across.
For more than a century, the idea that some of them glow rested on a few written notes. In 1891, the naturalists James Wood-Mason and Alfred Alcock described a giant sea spider dredged from the Indian Ocean by the survey steamer Investigator with a blue-green shine along the underside of its body and legs. Alcock later wrote that it shone like a star. Nobody photographed one.
That changed on a one-month survey along Australia's east coast in May and June 2017, aboard the research vessel Investigator, sampling between 1,006 and 4,977 meters deep. Researchers from UCLouvain in Belgium and Museums Victoria in Australia kept seven Colossendeis tasmanica and two Colossendeis minor in ice-cold seawater in a dark room. A gentle pinch gave a weak glow lasting from a few seconds to over two minutes. A bath of potassium chloride, a salt that sets off excitable cells, made all the legs glow deep blue for 5 to 10 minutes, with the light peaking at a wavelength of 449 nanometers.
The light came only from the legs. It was strongest in the last segment, near the claw, and faded toward the body. Stained slices of leg pointed to cells in the gut, which in sea spiders branches out into every leg. Lab tests suggest a photoprotein, a light-making protein that calcium switches on, like the one in some glowing jellyfish. Because the cells sit in the gut, the researchers think the spiders may get part of the recipe from their food.
With this study, four sea spider species are known to glow. Why they do it is still open. The authors suggest the blue legs could call a mate in total darkness or stun prey, but animals hauled up by a trawl are stressed, so they want deep-sea cameras to watch them where they live.
