Giant sea spiders emit eerie glow deep below the ocean waves

Deep in the dark Australian Abyss, what’s visible can be scarier than the unseen. There, giant sea spiders scuttle along like glowing ghosts.

The critters are not true spiders but relatives that split off from a common ancestor on the arthropod family tree some 500 million years ago. Researchers have just described the sea spiders’ glow in the October Deep Sea Research Part I: Oceanographic Research Papers.

“It’s quite impressive for people scared of spiders,” says Jérôme Mallefet. The marine biologist works for the Catholic University of Louvain in Belgium.

His team’s photos confirm that these creatures can emit light. Some 76 percent of deep-sea creatures produce a clear glow. Known as bioluminescence, this trait dates back 540 million years.

Shiny sea spidey

Mallefet and his colleagues discovered the sea spiders on a 2017 expedition. They had been surveying part of a deep-sea trench off southeastern Australia. The team hauled up 25,000 specimens in nets. The critters had been retrieved from nearly 5,000 meters (more than 3 miles) below the surface.

Aboard the RV Investigator, Mallefet tested these deep-sea creatures for bioluminescence. “I spent most of my time in a dark cold room,” Mallefet recalls. “I believe I was a little bit crazy, but I like that.”

Poking the biggest of the sea spiders (Colossendeis tasmanica) raised a dim light. But adding potassium chloride, a type of salt, can excite bioluminescent cells. The chemical triggered them to glow brightly for up to 10 minutes. Another giant sea spider, C. minor, also shone on its own.

All living creatures emit a weak glow. That’s different from the bright light coming from these stimulated sea spiders.

The images Mallefet’s team took support past reports of glowing sea spiders. One report from more than a century ago was especially poetic: “The creature, as it lay on its back, shone like a star, all its legs being lit along their surface with a strange blue radiance.” 

Giant sea spiders’ legs contain the digestive tract, Mallefet says. That means these animals might get their glow from the creatures they eat — such as sea anemones, his team suggests.

Three sea-spider species glowed green when Mallefet shone ultraviolet and blue light on them. Glowing in response to certain wavelengths of light is called fluorescence. Several amphibians and even platypuses gleam the same way, too.

Mallefet isn’t sure what value sea spiders get from fluorescing. He suspects their glow might attract mates. Or, since the glow is in their legs, maybe these critters use them as a searchlight for seafloor prey.

Once the critters spot something tasty, the sea spiders might collapse their legs down, grabbing their victim. Then a needlelike part of their proboscis — a tube-like mouth structure — might stab into their prey, liquefying its tissues before sucking it up. Confirming that would take deep-sea video with next-level imaging.