Light in the Deep

Below about two hundred metres, sunlight fades to almost nothing. Yet this vast dark region is not lightless. A remarkable proportion of the animals living there produce their own light, a phenomenon known as bioluminescence. Surveys carried out off the coast of California suggest that more than three quarters of the animals observed between two hundred and one thousand metres are capable of producing light.

The chemistry involved is broadly similar across very different species. A molecule called luciferin reacts with oxygen, assisted by an enzyme, and the reaction releases energy as light rather than heat. Because almost none of the energy is lost as warmth, biologists sometimes describe the process as cold light. What varies between species is the precise form of the luciferin, and this variation is one reason researchers believe bioluminescence has evolved independently many times.

The uses to which animals put their light are strikingly diverse. Some species use it to attract prey: the anglerfish dangles a glowing lure in front of its mouth. Others use light defensively. Certain squid eject a cloud of luminous fluid that confuses a predator in the way that ink clouds the water in shallower species. A few animals use light for camouflage, a strategy that seems paradoxical until one considers the problem from below. An animal swimming above a predator appears as a dark silhouette against the faint light from the surface; by producing light on its underside that matches the brightness above, it can erase that outline. This is called counter-illumination.

Communication is a fourth function. Some deep-sea shrimp release light in patterns that appear to be species-specific, and several fish have light organs arranged in distinct configurations along their flanks. Whether these constitute a signalling system in any meaningful sense remains difficult to establish, since observing undisturbed behaviour at depth is extremely hard.

That difficulty explains why the field advanced slowly for much of the twentieth century. Animals brought to the surface in nets are usually damaged and rarely luminesce normally. The introduction of remotely operated vehicles fitted with low-light cameras changed this. Researchers could observe animals in place, and several behaviours that had been inferred from anatomy were confirmed directly for the first time.

Bioluminescence has also proved unexpectedly useful in laboratories far from the sea. Genes for light-producing proteins, originally isolated from jellyfish, are now routinely inserted into other organisms so that researchers can see when a particular gene is active. The technique has become standard in cell biology, an application that no one anticipated when the proteins were first described.