Approximately 71% of the Earth’s surface is covered by water with an average depth of 3 kilometres, and 85% of the seafloor lies at a depth greater than 2 kilometres (Cocker 1978). These simple facts help to convey the scale of the ocean waters occupiable beyond the commonly understood continental shelf-seas and well-lit photic zone. The deep-oceans are undoubtedly the largest habitats on Earth and only recently have efforts been made to understand the organisms that live there (Buck 1978).
Bioluminescence has evolved many times independently across an array of taxa (Rees et al. 1998) , from prokaryotes through to complex megafauna such as crustaceans, cephalopods and fish. The marine environment offers perfect conditions for the advance of such bioluminescent species. Due to the stability of large oceanic bodies of water, the relative optical clarity when compared to fresh water systems, the sheer volume of deep waters untouched by sunlight and the vast array of taxonomic groups forced to interact as predators and prey within the food chain (Haddock et al. 2010), Bioluminescence has become natural selection’s weapon of choice.
Two-hundred metres below the surface lies the largest habitat on Earth; the Deep-Sea. The twilight encountered at this depth and continuing down to around 1000 metres is known as the Mesopelagic zone (Warrant & Locket 2004). A vast, featureless expanse of ocean occupied by a uniquely adapted fauna. The difference between the <1% of naturally occurring light filtering vertically down from the surface and that produced by the organisms inhabiting the zone has fueled a spectacular optical arms race. The Cockatoo squid (Figure 1) is an inhabitant of this twilight zone, a completely transparent cephalopod but for its large opaque eyes. The animal angles its bioluminescent photophores straight down to counter-illuminate its silhouette and avoid predation.
Below 1000 metres exists the Bathypelagic zone, a realm entirely devoid of natural light. Here exploiters have evolved to benefit from bioluminescence in many different ways; from lures, and decoys, through to reproductive beacons (Buck 1978). The subtlety of the battle fought between prey and predator in the twilight above is replaced by a bioluminescent light show of bewildering complexity and diversity in the true darkness.
This review attempts to provide an insight in to the diversity of biological light, the chemistry behind it, which organisms utilise it and why.