The world’s oceans cover 71 per cent of the planet’s surface with an average depth of 3,668m, making the deep ocean by far the largest habitat on Earth. It’s also one of the darkest, and as such we still know more about the surface of the Moon than the bottom of our oceans. Survival can be hard at the best of times, especially for those inhabiting the deep, dark and cold ocean depths, which has lead to the evolution of some unusual characteristics.
Perhaps most noticeable is bioluminescence, the production of light by a living organism, which can be produced by a chemical reaction or a symbiotic relationship with bacteria. Bioluminescence is vital for a myriad of essential biological functions, including locating mates, communication, luring prey and escaping from and camouflaging against predators.
A recent study by a collaboration of researchers from the University of Kansas, St. Cloud State University and the American Museum of Natural History sheds light on the evolutionary significance of bioluminescence in fish. Although bioluminescence does occur in terrestrial organisms, such as fireflies, most of the organisms that emit light occur in the world’s oceans, where it’s considerably more anatomically and biochemically diverse. In fact, 80 per cent of fish species that live in open water (and often at depth) bioluminesce.
The researchers developed an evolutionary tree detailing for the first time the evolutionary significance of bioluminescence in 1,500 fish species, using data collected from 11 gene fragments associated with light production. They concluded that the ability to bioluminesce independently evolved 27 times since its first appearance in the late Jurassic period 150 million years ago. Before it was only thought to have occurred in just a few instances.
“When things evolve independently multiples times, we can infer that the feature is useful,” says W. Leo Smith, assistant curator with the KU Biodiversity Institute. “The huge variety in ways fish can deploy bioluminescence — such as leveraging bioluminescent bacteria, channelling light through fibre-optic-like systems or using specialised light-producing organs — underlines the importance of bioluminescence to vertebrate fish.”
We are only just beginning to scratch the surface of the variety of life in the deep-sea, studies such as this offer a slightly higher resolution image of the biological diversity far beneath the waves. As ocean exploration technology develops throughout the 21st century, who knows what we’ll discover next from this hostile and vast habitat.