Glowing Sea Spiders: Giant Deep-Sea Spiders That Light Up the Abyss

Thousands of metres below the surface, where no sunlight reaches, scientists have photographed something that sounds like science fiction: giant sea spiders with legs that glow blue in the dark. The discovery, made off the coast of Australia, confirms a mystery that sailors and scientists have hinted at for more than a century. Here is what the researchers found, why the spiders glow, and what we still do not know.

The Short Answer

Yes, some giant sea spiders glow. A team of scientists documented bioluminescence in two species, Colossendeis tasmanica and Colossendeis minor, collected from the deep sea off southeastern Australia. The light comes mostly from their long legs. It is the first clear photographic confirmation of the effect, and it doubles the number of sea spider species known to glow, from two to four.

Key Facts at a Glance

• Species: Colossendeis tasmanica (one of the largest sea spiders) and Colossendeis minor
• Where: A deep-sea trench off the east and southeast coast of Australia
• Depth: Between about 1,006 and 4,977 metres
• When collected: May and June 2017, aboard the research vessel Investigator during the Sampling the Abyss voyage led by Museums Victoria
• Lead scientist: Jérôme Mallefet, a marine biologist at the Catholic University of Louvain in Belgium
• Published in: Deep Sea Research Part I: Oceanographic Research Papers (October 2026 issue)
• Leg length: About 25 centimetres on the biggest specimens
• Glow colour: Blue; some specimens glowed for up to 10 minutes

How Did Scientists Find Glowing Sea Spiders?

The discovery came from a month-long survey of the Australian deep sea. While examining animals brought up from the abyss, Mallefet tested them for bioluminescence. When the researchers poked the largest sea spider, it gave off a dim light. They also triggered a glow with a chemical, potassium chloride, which provoked a brighter and longer-lasting light. Photographs of the animals unambiguously confirmed the effect.

The team also shone ultraviolet and blue light on three sea spider species and found that they glowed green. That is a different effect called fluorescence, which we explain below.

What Are Sea Spiders?

Despite the name, sea spiders are not true spiders. They are marine arthropods with long, spindly legs, and they have lived on Earth for at least 425 million years. Most are small, but some deep-sea species grow remarkably large. Their legs are not only for walking. In giant sea spiders, the legs contain part of the digestive tract, which becomes important when we ask why they glow.

Bioluminescence vs Fluorescence: What Is the Difference?

• Bioluminescence is light made by a living thing through a chemical reaction. Fireflies and many deep-sea fish do it.
• Fluorescence is light that an object gives off after absorbing other light, such as ultraviolet light. It does not produce its own light in the dark.

The sea spiders showed both: a blue bioluminescent glow in the legs, and a green fluorescence under ultraviolet and blue light.

Why Do Sea Spiders Glow?

Nobody knows for sure. The researchers have offered some ideas, but none is confirmed:

• A dietary link: Because the glow comes mostly from the legs, which hold the digestive tract, scientists suspect the spiders may get a light-producing ingredient from their food. They might not make the light entirely on their own.
• Attracting mates: Mallefet speculated that the glow could help sea spiders find each other in the dark.
• Other purposes: Light is widely used in the deep sea. Roughly 76 percent of deep-sea creatures produce a clear glow, so the trait is common, even if its purpose here is unclear.

To answer the question, researchers would need to film the spiders in their natural habitat using advanced deep-sea cameras. The study’s authors say the ecological role of the glow remains unclear and needs further research.

A Mystery That Is Over 100 Years Old

The new study connects to old reports. In the late 19th century, scientists described a blue light along the undersides of sea spiders collected in the Indian Ocean. At the time, it was unclear whether the spiders made their own light or had picked up glowing mucus from other creatures, such as deep-sea corals. The new photographs provide the first unambiguous evidence that sea spiders really can glow.

One caution from the researchers is worth repeating: seeing a glow on an animal does not prove the animal made the light, because it could be carrying glowing organisms on its body. That is why diet and other explanations are still being studied.

Why Does This Discovery Matter?

• It solves an old puzzle. It confirms sightings that had been doubted for more than a century.
• It shows how little we know of the deep. Even large, striking animals can hide extraordinary features in the abyss.
• It raises new questions. If glowing is linked to diet, it may reveal how deep-sea food webs work.
• It reminds us deep-sea research takes time. The specimens were collected in 2017, and the results were published in 2026.

Fun Facts About Sea Spiders

• They have been around for at least 425 million years.
• They are not arachnids in the strict sense, so they are not close relatives of land spiders.
• Their long legs hold part of their digestive system.
• Some giant species live at depths of nearly 5,000 metres.

Frequently Asked Questions

Do sea spiders really glow?

Yes. Researchers have confirmed bioluminescence in two giant sea spider species, with a blue glow coming mostly from their legs.

Where were the glowing sea spiders found?

In a deep-sea trench off the east and southeast coast of Australia, at depths between about 1,006 and 4,977 metres.

Are sea spiders dangerous or poisonous?

No. Sea spiders are not true spiders and are not known to be a danger to people. They live in the deep sea.

Why do giant sea spiders glow?

Scientists do not yet know. Theories include a light-producing ingredient from their food and attracting mates, but neither has been confirmed.

How many sea spider species can glow?

The discovery brings the number of known bioluminescent sea spiders from two to four.

Is the glow bioluminescence or fluorescence?

Both were observed. The blue glow is bioluminescence, and the green glow under ultraviolet light is fluorescence.

Sources

• Science News: These deep-sea spiders give off an otherworldly glow
• Science News Explores: Giant sea spiders emit eerie glow deep below the ocean waves
• Phys.org: Meet the long-legged giants that light up the deep ocean
• Discover Wildlife: Deep-sea scientists astonished to find two giant creatures lighting up the deep, dark ocean
• Original study: Jérôme Mallefet et al., Abyssal glowing sea spider, first insight into pycnogonid luminescence, Deep Sea Research Part I: Oceanographic Research Papers (2026)