How Bioluminescent Animals Light Up the Deep Ocean

Between 200 and 1,000 metres below the ocean’s surface, sunlight fades rapidly until it is almost absent. Yet this environment is far from entirely dark: around 80% of the animals living within this section of the water column can generate light through bioluminescence, a chemical reaction that helps them survive in one of Earth’s most challenging habitats.

How can so many animals produce their own light?

Bioluminescence is created by a chemical reaction inside an organism. In broad terms, a light-emitting molecule known as luciferin takes part in a reaction that may involve the enzyme luciferase or specialised proteins. The released energy is given off as light, while producing comparatively little heat.

Fish, squid, jellyfish, siphonophores and comb jellies are among the groups in which this ability is widespread. As not every species uses precisely the same compounds, the capacity to emit light appears to have evolved independently several times.

• Deep-sea fish may have light-producing organs;
• Squid use flashes in a range of situations;
• Jellyfish can create luminous patterns;
• Siphonophores use structures that can emit light;
• Different species have evolved their own chemical mechanisms.

Why is there so little light between 200 and 1,000 metres?

This area is called the mesopelagic zone, or twilight zone. Beyond the first 200 metres, the available amount of sunlight drops sharply, leaving insufficient illumination to support photosynthesis. As the depth approaches 1,000 metres, only a tiny fraction of the brightness present at the surface remains.

What is bioluminescence used for in the darkness?

Making light can decide which animal finds food and which one is detected by a predator. Some animals use glowing points to lure prey, while others release sudden flashes to confuse or frighten an attacker. Certain species rely on light to identify mates, and others use it to communicate.

One especially unusual strategy is counter-illumination. Some bioluminescent animals have light organs on the underside of their bodies, adjusting their brightness to match the limited light still arriving from above. This makes their outline less visible to predators below them.

• Drawing prey close to the mouth;
• Confusing or frightening predators;
• Camouflaging the silhouette against light from the surface;
• Recognising members of the same species;
• Finding mates during reproduction.

Why is almost all of this light blue?

Water does not transmit every colour equally. Wavelengths associated with blue travel through water more effectively than most other visible colours. As a result, most bioluminescent marine organisms produce blue or blue-green light, although violet, green, yellow and even red emissions also occur.

• Blue travels further through seawater;
• The eyes of many deep-sea animals are particularly sensitive to this range;
• Red light from the surface disappears quickly;
• Some species have evolved exceptions and can produce other colours.

The deep ocean has its own lighting system

The widespread presence of bioluminescence between 200 and 1,000 metres challenges the idea that the depths are simply an empty, dark expanse. Light produced by organisms creates brief signals, shining spots and flashes that may determine a hunt, prevent an attack or enable two individuals to find one another in a place where vision has very little dependence on the Sun.

Below 1,000 metres, conditions become even more extreme because sunlight no longer reaches the environment. In this space, virtually all visible light must be made by living organisms themselves. The chemical ability to glow demonstrates how animals have turned one of the deep ocean’s greatest limitations into a tool for hunting, defence, communication and reproduction.