Deep-Sea Giant Sea Spiders Confirmed to Glow in the Dark

Giant deep-sea creatures known as sea spiders glow with their own light, researchers report, confirming bioluminescence — light produced by a living organism — in the October issue of the journal Deep Sea Research Part I: Oceanographic Research Papers. Colossendeis tasmanica, among the largest sea spiders, has a finger-sized body but legs that stretch up to 25 centimeters long.

Marine biologist Jérôme Mallefet of the Catholic University of Louvain in Belgium and colleagues found the glow during a 2017 expedition to a deep-sea trench off southeastern Australia. Working from a makeshift darkroom aboard the research vessel RV Investigator, Mallefet tested specimens suspected of bioluminescence among 25,000 hauled up in nets from as deep as nearly 5,000 meters.

Poking the largest sea spiders, C. tasmanica, produced a dim light, and adding potassium chloride — a chemical that can excite bioluminescent cells — made them glow for up to 10 minutes. Another sea spider, C. minor, exhibited bioluminescence too. Most of the glow appears on the underside of the legs, where their digestive tract is located, leading the team to speculate the spiders might get their bioluminescent properties from prey such as sea anemones.

Three sea spider species — C. tasmanica, C. minor and a third without bioluminescence — also fluoresced green when Mallefet shone ultraviolet and blue light on them. Mallefet is not sure why sea spiders fluoresce, and speculates it could help them attract mates or spot prey on the seafloor before striking with a needlelike appendage in their proboscis to liquefy and suck up tissue.

The images confirm earlier descriptions of bioluminescence in sea spiders, including an account from over a century ago describing a creature that, as it lay on its back, "shone like a star, all its legs being lit along their ventral surface with a strange blue radiance." About 76 percent of deep-sea creatures produce their own light, a trait that dates back 540 million years.