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Light in the Deep Ocean

1Below about two hundred metres, the ocean receives too little sunlight for photosynthesis, and below a thousand metres it is effectively black. Yet this enormous, dark volume — by far the largest living space on the planet — is not without light. A great many of the animals in it make their own. Bioluminescence, the production of light by a living organism, is so widespread in the deep sea that biologists now regard it as the ordinary condition there rather than a curiosity.

2The chemistry is straightforward and remarkably consistent. A molecule generically called luciferin is oxidised in a reaction controlled by an enzyme, luciferase, and the energy released emerges as a photon rather than as heat. Because almost none of the energy is lost as warmth, the process is often described as cold light, and it is far more efficient than any lamp humans have built. What is striking is that this chemistry has evolved independently many times — current estimates suggest more than forty separate occasions across the animal kingdom, and several more among bacteria and single-celled organisms. When a solution appears that often, it is usually because the problem it solves is severe.

3The problem, in the deep sea, is being seen — or not being seen. Most deep-sea light is blue, in the narrow band of wavelengths that travels furthest through seawater, and most deep-sea eyes are tuned to exactly that band. This creates a kind of arms race conducted entirely in blue.

4The commonest use of light is, paradoxically, concealment. Looking upwards from below, a fish sees the faint remains of daylight, against which any body between it and the surface appears as a dark silhouette. Many midwater animals counter this by producing light on their undersides at an intensity that matches the water above them, erasing their own shadow. This is counter-illumination, and some species adjust it continuously as the surface light changes through the day. A few can even alter the colour to match.

5Light is also used to hunt. The anglerfishes carry a luminous lure on a modified spine above the mouth, which in most species glows because of bacteria housed in a specialised organ — the fish itself makes no light. Other predators use a different trick: a small number of dragonfishes produce red light, which almost no deep-sea animal can see. They can therefore illuminate their prey without being detected, as though hunting with a searchlight nobody else can perceive.

6Defence takes several forms. Some animals eject a cloud of glowing particles, the deep-sea equivalent of a squid's ink, and disappear while the predator is dazzled. Others do the opposite and light up brightly when attacked. This appears wasteful until one considers who else is watching: a bright display attracts larger predators, which may attack the animal that is attacking. Biologists call this the burglar alarm, and the analogy is exact — the point is not to fight off the intruder but to summon something that will.

7Finally, light is used to communicate. In several groups the males and females carry different patterns of light organs, and some produce distinct flash sequences. This is best documented in shallow water — fireflies signal in codes precise enough that a species can be identified from its flash pattern alone — but comparable species-specific signalling is now recognised in deep-sea shrimps and fishes.

8Studying any of this is difficult. Bringing a deep-sea animal to the surface usually kills it, and a submersible's floodlights destroy the very darkness the phenomenon depends on. Much of the recent progress has come from cameras sensitive enough to work in near-total darkness, mounted on vehicles that sit quietly rather than sweeping the water with light. The picture they return is of an environment in which most encounters between animals are mediated not by sound or smell but by faint blue flashes — an enormous, silent conversation that no human eye has ever properly seen.

Light in the Deep Ocean

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