When picturing the dead of night or the deepest depths of the ocean, we may be inclined to think of pure, impenetrable darkness. Yet nature has quite a different image in mind – one where darkness is pierced by flashes of light, warming glows and pulses of colour. These vibrant light displays are the result of a phenomenon known as bioluminescence – the production and emission of light from a living organism, which occurs when a molecule called luciferin combines with oxygen in the presence of the enzyme, luciferase. Apart from being undeniably beautiful to watch, there is increasing evidence to suggest bioluminescence has a number of important functions for an organism.
Bioluminescence can occur in a broad range of organisms, from bacteria to fish (with the exception of higher vertebrates, e.g. reptiles and mammals), and is found across a variety of land and, more commonly, marine environments. In fact, around 60–80% of deep sea fish are thought to be bioluminescent. The pattern, frequency and wavelength (i.e. the colour) of light emitted can also differ by species and habitat. For instance, while violet or blue light is more common in deep water, bioluminescent organisms found on land tend to produce green or yellow light.
Bioluminescence lends itself to a number of functions – the first being for reproductive success. The most prominent examples of bioluminescence’s advantage in this area are in fireflies and glow-worms. In species of firefly where only the males are able to fly, the females attract their mate by emitting a constant glow which can be spotted by the males as they fly overhead. In other species, the male fireflies are also bioluminescent and produce a flashing light in response to the female’s glow. This results in a kind of “courtship conversation”. It has been suggested that the female’s preference may be determined by the frequency of male flashes, with higher flashing rates being more desirable. There are also a variety of fish in the deep ocean which appear to use bioluminescence to facilitate reproduction. Black dragonfish, for instance, are unusual in the fact that they emit a red infrared light, rather than the blue light common to deep-sea organisms. In doing so, however, dragonfish are able to use this light to seek out a mate in the darkness without alerting prey to their presence.
In contrast to hiding from prey, as with the dragonfish, bioluminescence can also be used to attract prey. Deep in the Te Ana-au caves of New Zealand, fungus gnat larvae construct luminescent “fish lines” to lure other insects. These insects then become trapped on the sticky lines and become a tasty meal for the lurking larvae. Back in the ocean once more, there is evidence that a type of jellyfish, known as the “flowerhat” jellyfish, also uses bioluminescence to attract small fish (e.g. young rockfishes) on which it preys. These jellyfish have fluorescent patches on the tips of their tentacles. In experiments studying how tip visibility influences predation, it was found that significantly more rockfish were attracted to the jellyfish with visible fluorescence than when the fluorescence was indiscernible, highlighting the importance of this attribute to this organism in acquiring food.
Alternatively, bioluminescence may also be useful to protect an organism from predation. This can work in a variety of ways, from providing camouflage to acting as a warning signal to predators against the dangers of attacking. A good example of the latter of these functions can be seen in glow-worm larvae. Unlike adult glow-worms whose fluorescence aids courtship, glow-worm larvae emit light to warn predatory toads of their unpalatability. This has been demonstrated by researchers in Belgium who found that wild nocturnal toads were more reluctant to attack if dummies resembling the larvae were bioluminescent. In addition, bioluminescence can work to protect against predation by acting as a diversion technique. Some species of squid, for example, are able to release a luminescent secretion when under threat, confusing the predator so they can escape.
For now, the role of bioluminescence seems to be clearer in animals than in those organisms outside the animal kingdom, such as fungi or bacteria. There is, however, a recent study which has suggested a potential role for bioluminescence as a method of spreading spores for a certain variety of mushroom found in Brazilian coconut forests. The investigators in charge of this study were able to attract nocturnal insects using an artificial light which replicated the mushroom’s green bioluminescence. This did not happen when the light was switched off, suggesting light may be used to help this type of mushroom entice insects which can then disperse its spores.
Bioluminescence can provide its creator with a light in the darkness. It can help an organism to seek out, attract and successfully court a mate; lure unsuspecting prey to their doom, and warn off or divert the attention of predators when under attack. Yet while there are a many instances where the function of bioluminescence is fairly clear, as discussed here, scientists remain very much “in the dark” in other cases. This is particularly true for those organisms, such as fungi and bacteria, which do not belong to the animal kingdom. Nevertheless, with continued research and new discoveries forever being made, it is only a matter of time before these elusive functions are brought to light.
Post by: Megan Freeman