Have you ever dipped a paddle or an oar into the sea on a dark night, or watched the wake of an outboard motor, and seen the swirl of light churned up by the turbulence? If we notice, we usually find momentary delight in the luminescent patterns in the disturbed water as they gradually subside. The tiny, one-celled organisms that make that light are presumably less than delighted — the flashes of light from each one are likely to be signs of fright, perhaps a means of deterring a possible predator.
A great many marine organisms are bioluminescent — proportionately more than in fresh water or on land. In large, multicellular marine organisms, there are two basic ways to emit light. One is intrinsic to each organism, consisting of chemical reactions controlled by that organism (think of fireflies on land and those one-celled organisms disturbed by a paddle). The other way is via mutualism with luminescent bacteria, and this way is especially common in the sea. Most bioluminescence is in the range of blue and green wavelengths.
The capacity to produce light has evolved many different times and serves many functions in different species — defense, warning, communication, attraction, camouflage, mimicry, illumination (as listed by one reference). It seems that most bioluminescent species have not been studied enough to know which possible functions pertain in each case. And since there are many thousands of these species (e.g., fishes, tunicates, snails, worms, squid, crustacea, bacteria and others), there is a long way to go in understanding the ecological and behavioral uses of this capacity.
However, I found some stories about bioluminescent marine organisms for which a few details are known. Here are three stories, one long one and two short ones.
There are several species of small squid (molluscs that are not-very-close relatives of octopuses) called bobtail squid or sometimes dumplings: they are only a few centimeters long, have no evident tail, and a round (dumpling-like) shape when their tentacles are not extended. They live in shallow waters of the Pacific and Indian oceans, spending the days buried in sand and becoming active at night, when they hunt for shrimp and other small prey. They bear numerous light organs on the mantle (think of it as the body covering that encloses the viscera) that house luminescent bacteria.
This relationship between squid and bacteria is mutualistic — both parties benefit. The bacteria get fed on sugar and amino acids from the squid. The squid gets a special form of camouflage: when the bacteria emit light, which they do only at the high density in the crowded light organs, the squid’s shape is no longer easily distinguished by visual predators. This is called counter-illumination; the night-active squid, when viewed from underneath against the background of a moon- or star-lit sky, is not seen as a black silhouette but as a pale form that tends to blend with the night sky.
The light organ housing the bacteria does several things. It filters the bacterial wavelength to more closely match moonlight and starlight, diffuses the light, and reflects it downward. The squid can manipulate the emitted light to match the night sky.
The best studied of the bobtail squids is the Hawaiian species and most of this information comes from research on that species. Bobtails are short-lived, usually living only about a year. As soon as a baby squid hatches, it is colonized by bacteria floating around in the sea water (and not luminescent there, because they are at low density).
The juvenile light organ is quite different from the mature form, lacking the filter, diffuser, and reflector. The opening of this organ is surrounded by little flaps covered with hairlike projections that wave, creating a current that draws water and bacteria over the opening, where the bacteria get temporarily stuck in mucus.
The squid’s light organs create conditions that allow only certain bacteria to enter — those that are strong enough to swim down the entrance and also be resistant to the unusual microenvironment within the organ (such as the presence of hydrogen peroxide, toxic to most microorganisms). By such means the squid selects the “best” illuminators and rejects the poor ones. The selection can be very specific indeed — not only the right species of bacteria but even the right strain or variety.
The juvenile light organ only lasts a few days, however. The colonizing bacteria lose their ability to swim and become smaller, and they induce many changes in the organ. The mature light organ is larger and has more chambers; the little flaps with waving hairs are lost, but it has the filter, diffuser, and reflector that make the whole thing work. Interestingly, the population of bacteria in the light organ changes daily! Every day, the squid ejects over 90 percent of the bacteria, perhaps because the “old” ones don’t shine as well. The remaining bacteria, augmented by some refreshers taken anew from the sea, quickly rebuild their population and again begin to glow. A complicated mutualism, indeed!
Then there’s the widely distributed, deep-sea fishes known as stoplight loose-jaws and their close relatives, which are small dragonfishes with huge mouths, with lower jaws like rat-traps (picture the spring-loaded steel wire that snaps down on the neck of the rat). These fishes are unusual because some of their light organs produce long-wavelength, red light. Since most deep-sea creatures can’t see red, this may be a way for the predator to see prey that cannot see it coming.
The special thing about this is that the red light is produced by a mechanism involving chlorophyll (the green pigment of plants), which the fish obtain by eating copepods (small crustaceans that eat phytoplankton). But one does not need a huge rat-trap mouth to slurp up tiny copepods! One possible explanation of the conundrum is that the fishes survive lean times by swallowing copepods and occasionally feast hugely on bigger prey snapped up in the rat-trap mouth.
A recent discovery (in 2009) is of small deep-sea worms called “green bombers.” When they feel threatened, they release a cloud of tiny, green, glowing “balloons,” which only glow when released. The luminescent cloud is thought to confuse and distract predators, while the worms escape.
There isn’t space to write about the angler fishes, some of which dangle a lighted lure over their capacious mouths, or the cookie-cutter shark that is said to use lights to entice prey to come closer (my dear!) or … There is always more to learn. But isn’t it fun to think about these nifty arrangements?
• Mary F. Willson is a retired professor of ecology.
