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In CUriosity, experts across the CU ��«����Ƶ campus answer pressing questions about humans, our planet and the universe beyond.

This week, Jingchun Li, associate professor in the Department of Ecology and Evolutionary Biology at CU ��«����Ƶ, answers: “How do sea creatures make light?”

From shallow reefs to pitch-black depths, the ocean is alive with sparkling lights. Fish, squid, clams and plankton have found a wide range of ways to glow, shimmer and flash, lighting up the dark water like stars in the night sky.

Scientists estimate that in the deep ocean where sunlight cannot reach, can produce some kind of light.

“Light is important for signaling,” said Li, who has spent much of her career working in oceans around the world to study marine life. “It helps animals of the same species communicate and recognize each other, and it can also serve as a warning to other animals.”

 

  Previously in CUriosity

Do animals have emotions?

Bioluminescence is one of the most common methods animals use to do this. By triggering a chemical reaction between oxygen, a molecule called luciferin and an enzyme, luciferase, in their bodies, they can light up.

On land, fireflies use bioluminescence to emit their electric green light. In the deep ocean, anglerfish, a terrifying antagonist seen in Finding Nemo, use the same method to shine in the abyss.

The fish has a bony structure on its forehead that lights up like a lantern, thanks to the large number of bioluminescent bacteria living inside the fish.

But not all animals are born with the right chemical ingredients to generate light on their own. Some still find ways to shine.

The disco clam (Ctenoides ales), a mollusk living in the shallow waters of the Indo-Pacific Ocean, has evolved a unique strip of tissue on their mantle that can reflect ambient light. has found that the clam’s reflective strip contains silica, the main component in glass. When disco clams furl and unfurl their mantles quickly, they reflect sunlight, creating an effect reminiscent of a disco ball.

Li said scientists are still working to understand how disco clams developed their reflective tissue through evolution. Other closely related species, such as rough file clams (Ctenoides scaber), lack the silica structure in their tissue.

The disco clam’s dazzling display might serve as a defense mechanism. Disco clams regularly open and close their shells, but Li and her team found that when the clams sense a shadow looming over them—a sign that a predator might be approaching—they flash much faster, up to six times per second, like a strobe light.

Anglerfish, on the other hand, light up to draw smaller fish toward them, helping to attract prey in the dark. The ostracod, a tiny, bioluminescence crustacean that looks like a shrimp inside a pod, glows to attract mates. Males spit out a glowing mucus to create a special pattern during mating rituals.

The question of why deep-sea animals produce light remains an intriguing scientific mystery.

“To survive in extremely dark and cold water, every bit of energy matters. But having a vision is energetically demanding,” Li said. “From an evolutionary perspective, it’s surprising that so many animals in the deep ocean retained the ability to see and even evolved ways to illuminate their surroundings.”