Listening Part 4: Living light in the deep sea

Lecturer: This morning I'm going to talk about bioluminescence, the production of light by living organisms. On land, the best-known example is the firefly. But the real home of bioluminescence is the deep ocean.

Lecturer: Below about two hundred metres, almost no sunlight reaches. And yet a survey of animals in the waters off California found that around three-quarters of the species observed could produce their own light.

Lecturer: So how is the light made? In most cases, it's a chemical reaction. A molecule known as luciferin reacts with oxygen, and the reaction is speeded up by an enzyme called luciferase. The energy is released as light rather than heat, which is why it's sometimes described as cold light.

Lecturer: Most deep-sea light is blue or green. That's not a coincidence. Blue-green light travels furthest through seawater, and the eyes of most deep-sea animals are sensitive only to those colours. Most of them can't see red at all.

Lecturer: Now, what is the light for? Let me give you four functions.

Lecturer: The first is camouflage. Many fish in the middle depths have light organs on their bellies. From below, a predator looking up would normally see their dark outline against the faint light from above. By producing a matching glow, they make that outline disappear. This is called counter-illumination, and the hatchetfish is a good example.

Lecturer: The second function is attracting prey. The anglerfish has a long spine on its head with a glowing tip, which acts as a lure. Interestingly, the light isn't made by the fish itself, but by bacteria living inside the lure.

Lecturer: Third, defence. Some shrimp and squid release a cloud of glowing fluid when attacked, rather like a smoke screen, which confuses the predator while they escape.

Lecturer: And fourth, a rather clever trick. Certain species of dragonfish produce red light. Since most other animals can't see red, the dragonfish can light up its prey without being noticed. It's essentially a private torch.

Lecturer: Finally, this isn't just of academic interest. Genes involved in producing light have become valuable tools in medical research, where they're used to make particular cells or processes visible under the microscope.