Cold Fire in the Sea
It is ten o'clock. Conditions are near-perfect for your night dive. There is no wind to disturb the calm, flat surface and the night sky is clear and cloudless, punctuated by a gibbous moon and sprinkled with sharp points of stellar light. Your outboard engine growls to life. As you race toward your dive site, the bracing chill evaporates and you are filled with the electricity of anticipation. Glancing astern, you notice a curious thing — for more than a half-kilometre behind your boat, the V-shaped wake glows an eerie, other-worldly green. A mantle of greenish 'sparks' — bright enough to fracture your night vision — erupts periodically along either gunnel. Tentatively, you lower your hand into this glowing cascade. But when you pull up a sample, your handful of water displays none of the spectral magic it did before. What's going on here?
The phenomenon of a 'glowing wake' has for centuries evoked wonder and curiosity in seafarers. In the 1850's, naturalists demonstrated that this glow was caused by multitudes of single-celled animals, particularly a dinoflagellate called Noctiluca (which can be loosely translated as "night light"). Today it is common knowledge that many creatures can produce light — but relatively few know how or why.
Bioluminescence, or 'living light', is possibly the strangest phenomenon of the marine world. One of the most striking features of bioluminescence is the enormous diversity of organisms that have developed the ability to emit light. Marine examples include: planktonic radiolarians, dinoflagellates, and comb jellies; sessile sponges, corals, hydroids, and clams; and motile creatures ranging from snails and squid to a dazzling variety of fishes. Bioluminescent organisms range in size from bacteria less than one micrometre in length to the five metre-long 'Megamouth' shark.
Despite their tremendous diversity, all bioluminescent organisms produce light via the same process. Bioluminescence is the result of a chemical reaction between a protein (luciferin) and an enzyme (luciferase) in the presence of oxygen. As anyone who has ever touched a burning electric lightbulb knows, most lightsources radiate a lot of heat, reducing the amount of free energy that can be converted into light. Another source of energy loss is the production of sound ("@#$% THAT'S HOT!!!" — which momentarily masks that annoying hum). Bioluminescence, by contrast, is nearly 100% efficient. Virtually all of the energy generated by the luciferin-luciferase reaction is converted into light with almost none lost in heat or sound production. Bioluminescence is literally a 'cold fire'.
Bioluminescent creatures can be divided into two basic categories, based on the type of light organ they possess. The first category includes those with self-luminous organs called 'photophores' — complex eye-like structures embedded in the skin. Light is produced by specialized photogenic cells and reflected through a 'lens' and clear outer covering, producing a gentle, even glow whose intensity can be precisely modulated. The second category 'borrows' the luminosity of bacterial symbionts. Subcutaneous bags of bioluminescent bacteria are maintained and nurtured by the host organism in exchange for a more brilliant level of light. The host is unable to control the intensity of light produced by the bacteria, and so organisms with bacterial light organs have evolved some fascinating mechanisms to 'turn out their lights' when they would be disadvantageous.
These complicated light-generating and modulating systems are unlikely to have evolved without strong selective pressures favoring their development. What use is all this 'light show' paraphernalia? Four major functions are known: co-ordination of schooling, predator evasion, attracting mates, and luring prey.
The greatest abundance and diversity of animals with photophores is found in the upper regions of the deep sea. Especially elaborate patterns of photophores occur on many deep-sea fishes and squids. This pattern is different for each species, and may allow individuals to recognize their own kind in the vastness of the deep sea. This may help to maintain visual contact and co-ordinate schools in the permanent twilight at depth. Dwarf lanternsharks (10-centimetre relatives of the familiar spiny dogfish) may use their pattern of photophores to co-ordinate group attacks on deep-sea squid many times larger than themselves. Some nocturnal shallow-water fishes may also rely on photophores to help co-ordinate their schools. Certain cardinal fishes, for example, have distinctive patterns of photophores which may aid them in co-ordinating their schools as they forage over the night reef.
Many deep-sea creatures undergo a nightly 'vertical migration', following their prey toward the surface at night and returning to deeper zones before dawn. Photophores may help co-ordinate these migrations and help protect deep-sea animals from predators. Seen from below, an unlighted fish or squid would be clearly silhouetted against the moonlit surface. Some use their ventral photophores to 'erase' this silhouette to blend with the moonlit upper waters. This 'counter illumination' occurs in diverse deep-sea creatures, including firefly squids, euphausiid shrimps ('krill'), dogfish sharks, hatchet- and lanternfishes. The photophores on a midshipman's belly produce a gentle, even light to match the downwelling moonlight upon the sandy bottom, eliminating its shadow as well as its silhouette.
Photophores may also be used to produce a 'blinding' flash of light, which might startle or confuse a potential predator long enough to allow escape. The firefly squid distracts would-be predators by releasing a luminescent cloud, which masks the it's escape. When threatened, many lanternfishes emit a bright flash from the caudal (rear) photophores and simultaneously swim rapidly away. The predator is likely to focus its attention on the spot where the flash occurred, giving the lanternfish an opportunity to save its tail.
Among the most extraordinary nocturnal fishes, the eerily bioluminescent flashlight fish are seen only in the dead of night — made all the more fascinating by their light organs' uncanny resemblance to pairs of demonic eyes. This nearly magical species carries a pair of highly evolved light producing organs beneath its eyes. Specialized networks of blood vessels in these organs encourage the growth of bacteria that continuously emit a cool, blue-green light. The front of each light organ is covered by a retractable fold of skin that can be snapped open and shut in a split second. By blinking them on and off, flashlight fish use their living lights to confound natural predators and net-wielding scientists alike. When a school is chased, each flashlight fish takes evasive action, first dashing in one direction with lights on and then blinking off while darting away on a completely different course. Each fish performs this 'flash and dash' behavior up to 75 times a minute. Flashlight fish are the brightest of bioluminescent creatures. Their schools produce so much light that along reefs of the politically tense Red Sea, that they have been shot at and bombed, apparently mistaken for night-operating frogmen equipped with diving lights.
Bioluminescence may also facilitate finding potential mates of the appropriate species — especially among a host of other vertical migrators. Many creatures increase the frequency and intensity of bioluminescent displays during their mating period. As Columbus approached the coast of North America for the first time, he reported seeing what appeared to be "candles moving in the sea". Some biologists suspect that Columbus was witnessing the mass mating ritual of the bioluminescent Bermuda fire worm. This small, bottom-dwelling polychaete swarms near the ocean surface on summer evenings for a few nights following a full moon. In late spring, firefly squid migrate toward the surface to breed. These small, deep-sea squid have a large number of photophores located on the mantle surface, around the eyes, and on the ventral arms. These photophores give off a brilliant white light that the squids flash during their nocturnal mating activities. In certain fishes — such as lantern fishes — the pattern of photophores differs between males and females. At night these fishes migrate to the upper layers where they mate, taking advantage of surface currents to distribute their fertilized eggs.
A final major function of bioluminescence is luring prey. Since many deep-sea creatures incorporate bioluminescence into their sexual displays, it is understandable why so many predators use light to attract prey. There are many deliciously 'devious' methods employed by these 'come hither' predators. Perhaps the most familiar of these are the angler- and viperfishes, with their huge eyes, snaggle-toothed jaws, and elongate luminous fin spines. But since most divers are unlikely to encounter or be threatened by one of these pint-sized predators, I will focus on a lesser-known bioluminescent predator which occasionally does complicate the lives of people who work in the sea.
The Cookiecutter Shark (Isistius brasiliensis) also undertakes a nightly vertical migration — not to feed on its fellow deep-sea creatures, but to gouge pieces from large surface-dwellers. Evidence suggests that the bright green bioluminescence of this 30-cm-long dogfish lures large fishes and cetaceans closer to investigate the glow as a potential meal. This allows the shark to launch a surprise frontal attack at close range. The cookiecutter gloms onto its prey with highly specialized suctorial lips and drives in its saw-like lower teeth. The shark then twists about — aided by the flow of water over its moving prey — and slices out a conical plug of flesh. Cookiecutter bites often look like they were made with a razor-edged ice-cream scoop. These 'crater wounds' have been documented on a wide range of large pelagic creatures, including tunas, marlins, elephant seals, humpback and pilot whales, dolphins, and even the bioluminescent 'Megamouth' shark.
You will be relieved to hear that, to date, no human beings have been parasitized by the cookiecutter shark . . . at least not directly. Recently, some U.S. nuclear submarines returned from patrol with neat cuts in the neoprene shields of their sonar domes, causing the oil to leak out and rendering the sonar useless. Naval engineers must have had nightmare visions of unknown enemy weapons until the cookiecutter was revealed as the villain. It is ironic that mega-buck, mega-tonne high-technology can be incapacitated by a tiny shark with Mick Jagger lips. I don't know about you, but I'm going to start wearing a jock strap when I go night diving!