What happened

A team including scientists from the Catholic University of Louvain, a Belgian university, studied two giant sea spiders. The animals were Colossendeis tasmanica and Colossendeis minor. Researchers collected them during a 2017 deep-sea survey off eastern Australia. They came from depths between 1,006 and 4,977 meters. The study appeared in 2026 in Deep-Sea Research Part I (research paper, reporting). It gives the first formal observations and photographs of bioluminescence in both species.

Why the glow was hard to prove

Sea spiders are marine animals with very long legs. They are not ordinary spiders from land. Deep oceans contain many glowing animals, but sea spider light was poorly understood. Reports from the late nineteenth century described blue light on collected animals from the Indian Ocean. Still, nobody knew whether the animals made the light themselves. They might have picked up glowing slime from another creature. Darkness also makes natural behavior hard to observe.

What the light looked like

The researchers studied Colossendeis tasmanica in greater detail. Seven animals of this species were available. Its light came only from the legs. The glow was deep blue, with a peak at 449 nanometers. A nanometer is a tiny length used to describe light. The leg tips shone brightest. Light faded toward the body. Chemical stimulation produced light for five to ten minutes. A gentle pinch produced weaker light. That glow lasted from several seconds to more than two minutes. Colossendeis minor also showed light, but researchers had only two animals.

How scientists tested the light

The team placed the animals in a dark room aboard the ship. They gently pinched the legs and used test chemicals. They photographed the glow and measured its brightness and color. They compared different parts of each leg. Back in the laboratory, they tested frozen leg tissue. They also examined thin tissue sections under a microscope. A standard test for the familiar luciferin-luciferase system produced almost no light. Hydrogen peroxide and potassium chloride produced stronger responses. Catalase and Fenton tests also produced light. More calcium strengthened one of the reactions.

What the clues mean—and what they do not prove

Microscopy suggested that cells near the gut produced the light. This raises the possibility that food supplies part of the needed material. The chemical results also point toward a photoprotein system, like those found in some jellyfish. A photoprotein is a body substance that may help produce light. However, the researchers have not identified the exact light-making molecule. They also do not know the glow’s purpose. It might help with defense, hunting, or reproduction. A response in a laboratory does not prove how the animals use light in the wild.

What comes next

Future work must identify the light-making molecule and study more animals. Researchers also need deep-sea video of living sea spiders. That would show whether they glow during normal behavior. More Colossendeis minor specimens would be especially useful. This was not a discovery of two new species. It was strong evidence that two known species can produce light. The finding turns an old, uncertain observation into a question scientists can test carefully.