Adaptive Value: Why Bioluminesce?

Bioluminescence is not entirely inexpensive. If an animal is not bioluminescing with the help of symbiotic bacteria, it must depend on its own chemoluminescence (see Mechanisms for a detailed explanation) at its own expense. In order to initiate the luminescence reaction, an animal must expend ATP. Given that nutrient energy is hardly in excess in the deep sea—and that crafty predators use every mechanism to get to it—the question arises: why bioluminesce?

Perhaps this question is better illuminated (hur hur) by considering another: why not, in the cover of darkness and a medium so dense as deep sea water, try to make oneself as undetectable as possible? It's not as if it isn't a tried and true method, after all; on terra firma, the majority of survival adaptations involve masking one's presence. The answer lies in the uses to which the denizens of the deep have put their luminosity; they encompass camouflage, predation, and mating behaviors, and vary broadly in execution.


The cookie-cutter shark uses counter-illumination to create the illusion of a smaller fish

How, you may ask, does making oneself a beacon of light in the deep sea constitute camouflage? It doesn't... but shooting a flare of phosphorescence into the water several yards away to fool a predator does! Many copepods and other small crustaceans use this technique to make their location ambiguous. Though not common in the deep sea (which is characterized by little to no surface light penetration), an astounding camouflage bioluminescence occurs frequently in the “twilight zone”, the sea zone in which there is enough light from the surface to make animals' silhouettes detectable by predators lurking below. It is called counter-illumination; animals use their luminous regions to exactly match the light quality above them, causing them to “disappear” quite seamlessly. This cookie-cutter shark even leaves a patch on its stomach (shaped to resemble a smaller fish) un-illuminated (1), leading to some unpleasant surprises for hungry would-be predators from below!

Predator Repellent

What could be more abhorrent to a predator than to become obvious prey? In much the same way as octopuses and squids 'ink' on their enemies, some dinoflagellates and jellyfish can expel a slurry of bioluminescent ink on would-be attackers, which not only confuses them but leaves them quite exposed to even larger predators. Predators can also be repelled by a luminous flash on its own, which may startle or threaten them.

Prey Attraction/PredationAHHHHHhhhhh

On the other side of the coin, luminescence is an extremely useful tool for the deep-sea predator; the archetypal example, of course, being the deceptive luminescent lure—trademark of the angler fish. Such lures are quite common in the deep, as are similar methods by which predators masquerade as prey: most fish such as anglers imitate flashing copepods and squids. Predators can also, in the same way as prey, use luminous flashes as a confusion tactic. And while most predators are just as easily hoodwinked as their prey by flashing lights in the black abyss, some are not so quickly deceived—such as the flashlight fish (pictured), who possesses a particularly devious adaptation: two large, luminous organs on either side of its head, creating beams of light which illuminate prey (2). But this isn't just any sort of light; it's red light, and it's red for a reason.

Though almost no light penetrates to the depths that constitute the deep sea, there is still some trace blue light: the shortest wavelength range in the visible spectrum, and thus the most energetic and penetrating. Since even the ghost of a visual cue could spell doom in the deep, almost all deep-sea animals are either black or red, the latter of which appears black since red light is filtered out in much shallower depths. But under the glow of red luminescence, a red animal becomes vividly visible... and an easy meal for the flashlight fish!  


Mate Attraction

All in all, the deep sea would seem a fairly bleak horizon in which to pursue a social life. However, bioluminescence is the preferred medium of communication for everything from ostracods to angler fish, be it flashes (believed to be a form of signaling between conspecifics not unlike "smoke signaling") or extravagant mating displays. Though both come at a potentially fatal expense—again, predators could be incited by the smallest visual cue—they are necessary for locating ones' cohort and reproducing (3).

Descriptive cartoon of various uses for bioluminescence, courtesy of  The Bioluminescence Website.