Chemically triggered luminescence persisted for five to ten minutes. Manually triggered light was weaker and lasted from a few seconds to more than two minutes.

Why was this uncertain for so long?

Earlier accounts described a blue glow along the undersides of sea spiders collected in the Indian Ocean during the late 1800s. Researchers could not determine whether the animals produced the light themselves or had merely brushed against glowing deep-sea corals and carried the mucus away.

Dr. Jerome Mallefet of UCLouvain in Belgium led the team that resolved the question. The findings appear in the journal Deep-Sea Research Part I under the title "Abyssal glowing sea spider, first insight into pycnogonid luminescence."

What produces the glow inside the spider?

Laboratory work on frozen leg extracts and thin tissue sections revealed that the light originated in gut-associated cells within the legs. The pattern suggested the spiders may obtain a key light-emitting ingredient from their diet.

A standard test for luciferase (the enzyme most commonly responsible for animal bioluminescence) produced almost no signal. Instead, hydrogen peroxide and other chemical reactions triggered the tissue to glow. The response intensified as calcium levels rose.

The chemistry resembles a photoprotein system, a light-emitting protein that activates when calcium binds to it, similar to the mechanism found in certain jellyfish. "Further research is needed to validate and characterize at the molecular level this putative photoprotein," the scientists wrote.

What does the team still not know?

The function of the light remains a mystery. "The precise ecological roles of pycnogonid bioluminescence remain elusive and warrant further study," the authors noted. Possible explanations include predator deterrence, prey attraction, or signaling between individuals, though no evidence yet supports any one hypothesis.

The team has also not identified the exact protein or molecule producing the glow. The animals inhabit a region of perpetual darkness that spans roughly 80 percent of the ocean's volume, where producing one's own light can determine whether an animal finds its next meal or becomes one.