Credit: E. WIDDER

It is a calm August day in the azure waters of the Bahamas, and Erika Raymond, a doctoral student in Oceanography at Johns Hopkins University in Baltimore, Maryland, has been screening hours of video footage inside a shipboard laboratory. The footage comes from a camera system that was stationed on the sea floor some 600 metres down.

Hardly a trace of sunlight makes it that deep, and trying to film in the darkness is a tricky feat. Artificial lights might scare creatures away, or attract the wrong kind. So the camera is fitted with a red light, thought to be invisible to the eyes of most deep-sea creatures. But the system does more than film the depths. Eye-in-the-Sea, as it is called, is equipped with an LED lure, a cluster of tiny lights designed to flash and flicker in very specific patterns, all in the hope that something might respond.

Raymond is getting excited about what is on the screen. She calls over Edith Widder, her PhD adviser, who becomes similarly animated. "That is so great," says Widder, co-founder of the Ocean Research and Conservation Association in Fort Pierce, Florida. Others gather round, expecting to see another good clip of one of the giant sixgill sharks that have been nosing around the rig. But the duo's excitement stems from something more subtle and more profound.

First there's a repetitive flash of light from the lure in the centre of the screen. Then, in the distance, there's a similar burst, but this one is from an animal. Raymond and Widder jump forward to other clips in which the lure was making that same flashing pattern, one of five they have been using. Again they see the response, sometimes multiple responses. It is official. Through their lure, the scientists have begun to communicate with deep-sea organisms using what might be the loudest form of expression in that dark void, the language of bioluminescence — biochemically produced light (see Living light - how it works).

Light is the most important variable in our environment, and it is probably as important in the marine environment. Edith Widder ,

What exactly had been said is another issue entirely. The response could have been the equivalent of a flirtatious wink or, more likely, a warning call. But simply making that connection for the first time is a good start in a field of study that, of necessity, operates in the dark.

True behavioural observations of bioluminescent activities in the deep are rare, because of the difficulties inherent in keeping deep-sea species alive in the lab, and the severely limited access to the depths of the oceans. Moreover, the tools of on-site exploration — remotely operated vehicles and submersibles — generally fall short of being unobtrusive. Ron Douglas, who studies deep-sea vision at City University in London, compares exploration by submersible to taking a "Land Rover and going out into the savannah in the middle of the night with the stereo on full blast, the lights on full, with a rotating siren and expecting to see normal lion behaviour".

Plumbing the depths

More restrained research such as that conducted this summer in the Bahamas, the third in a series of expeditions funded by the National Oceanic and Atmospheric Administration's Office of Ocean Exploration, is gradually revealing new information about what light can be found in the depths, how the organisms there use it, and even how they see their world. Although much uncertainty remains, one thing is abundantly clear: bioluminescence is nearly everywhere, and for inhabitants of the deep sea, it seems to play key roles in everything from eating, through mating, to staying alive. Tantalizing recent discoveries also suggest that at least some deep-sea organisms are seeing more than anyone expected.

Descending into the crystal waters of the Bahamas, observers are invariably struck by the remarkable blues not only of the water, but of everything in it. The long wavelength red light is quickly absorbed and extinguished by the uppermost layers of water. This is why most deep-sea creatures don't see red — there is hardly any of it. Travel a bit deeper, say 200 metres or so, and there is still enough light for a passable twilight. But as surface light fades, the first small flashes of bioluminescence from dinoflagellates appear. Go deeper still and the flecks become more prevalent. Larger flashes, perhaps from a shrimp or a jellyfish, punctuate the scene. Eventually, the lightshow grows into a veritable fireworks display against an ever blacker background. By 500 metres or so humans can't detect much if any sunlight. The animals equipped for this realm might still detect some light below that level, but at 1,000 metres, absolutely every trace of sunlight is extinguished.

Seeing without sunlight

Yet even beyond sunlight's reach, eyes are still common. Bottom-dwelling creatures that spend their entire lives shielded from the light of the Sun actually tend to have enlarged eyes. Those eyes have to be seeing something and that something is the bioluminescence. Indeed, ongoing surveys of bioluminescence led by Monty Priede, from the University of Aberdeen, UK, show that although its frequency decreases with depth, bioluminescence persists thousands of metres beneath the surface.

"People lose sight of the fact that light is the most important variable in our environment, and it is probably as important in the marine environment," says Widder, "but we have to understand they're not seeing like we do."

Edith Widder with the Eye-in-the-Sea submersible. Credit: D. SMITH

Researchers suggest that although other senses are clearly important, some deep-sea organisms depend on bioluminescence for every life function. Not surprisingly, given the scarcity of hiding places, one of the most common uses is simply survival.

One established theory is that certain organisms use bright bioluminescent flashes like burglar alarms to startle predators, or to attract their predators' predators1. Widder says that one of the earliest deployments of Eye-in-the-Sea may support the second option. In 2004, just more than a minute after the lure was activated in a jellyfish-like pattern for the first time at depth, a 2-metre squid arrived on scene. The next year, hundreds of miles away, a squid of the same as-yet-unidentified species responded similarly. Widder says that it was terrific proof that unobtrusive observation could capture novel behaviour. "I don't think anything can top that squid," she says.

Circumstantial evidence suggests that deep-sea animals have other uses for their bioluminescent organs, known as photophores, as well as bioluminescent tissues and various forms of 'spew'. Sometimes it's a deterrent. Some organisms seem able to light up a predator's stomach when they are eaten, broadcasting its location to other predators2. Bioluminescence similarly could be used like the colours on a poisonous snake or frog to warn would-be predators of toxicity or unpalatability3. On the 2005 Deep Scope expedition, Widder discovered the first known bioluminescent anemone, which produces a sort of glowing slime. Its use is not known, but she speculates the slime could be just such a warning. There is even evidence of batesian mimicry, where perfectly edible organisms have evolved to impersonate noxious species.

Many animals can hide from predators using light. In the ocean's twilight zone, where some light still penetrates, a dark silhouette is easily spotted from below, and is hence a dangerous liability, says Sönke Johnsen, a biologist at Duke University in Durham, North Carolina, and the 2007 Deep Scope chief scientist. So, many animals use photophores to counterilluminate, or light up their undersides and blend4.

On the flip side of bioluminescence, there is strong evidence that many animals use light to attract and find others. Lots of fish and squid, for instance, use bioluminescence as a form of searchlight. Others attract mates using sex-specific bioluminescent patterns5. And female deep-sea anglerfish are famous for their lures, filled with bioluminescent bacteria, that they dangle in front of imposing fangs.

Bioluminescent bacteria are common throughout the ocean, and researchers have proposed that the bacteria use their luminosity to find good homes. Bacteria that are lit up, whether hitching a ride on a bit of detritus or on a fecal pellet, are more likely to be eaten by fish, in whose guts their needs are well met.

Look deep into the eyes

Understanding what deep-sea animals can see is, of course, integral to understanding how bioluminescence might be used and to what degree of success. Tammy Frank, a Deep Scope leader and a visual ecologist at the Harbor Branch Oceanographic Institution in Florida, which owns the ship and submersible used, has done extensive work studying deep-sea eyes. She speculates that the size of the eyes of bottom-dwelling deep-sea creatures, relatively larger than those of the occupants of the water column above them, could be tied to energy expenditure. The advanced vision requires energy. Creatures in open water cannot afford to waste energy that might be better spent outswimming predators. Bottom-dwelling animals can use the sediment or rock structures to help them hide, which could allow them to funnel more energy towards vision.

My physics head says 'No' but my biology head says, 'Well, Why not?'. Justin Marshall ,

To date, most deep-sea animals studied seem able to see light only in the blue-green range. These shorter wavelengths penetrate the water farther and are where the majority of bioluminescence falls. But there are notable exceptions. In 2005, on the second Deep Scope expedition in the Gulf of Mexico, Frank discovered a bottom-dwelling crab species sensitive not only to the standard blue-green range, but also to ultraviolet light. The utility of this ability is not clear, but could, by some unknown means, allow the crab to detect its favoured hiding spots — soft corals — or to distinguish between different types of bioluminescent light, which does at times dip very slightly into the ultraviolet range. If such an ability is proven, it would not be the first time scientists have discovered visual skills that seemed initially bizarre or improbable.

Because of red light's weak ability to travel in water, conventional wisdom had long held that all bioluminescence in the deep was blue or green. But in 1981 Peter Herring, a now semi-retired bioluminescence pioneer at the University of Southampton, UK, reported the strange discovery of red bioluminescent 'searchlights' under the eyes of certain dragonfish6, a common family of elongated deep-sea fish with menacing fangs appropriate to their name (see picture, top). Herring, and others, have shown that these searchlights are the result of fluorescent proteins that shift bluish bioluminescence to the red range. The eyes of these fish are adapted to see red light as well, suggesting the animals can secretly attract mates or hunt using flashlights that few other fish can see7, 8.

Never seen: shrimps can spew bioluminescence as a predator deterrent. Credit: E. WIDDER

Strangely, one dragonfish species initially did not seem to have the pigments necessary to see the red light. Douglas and his colleagues eventually discovered that these fish use a chlorophyll pigment from bacteria for red vision. There are still significant uncertainties about how the pigment is obtained and how it accomplishes the energetic shift that leads to red vision. But other groups have confirmed that when the chlorophyll is added to the eyes of animals such as mice, it boosts red vision9.

Seeing red

Steven Haddock, at the Monterey Bay Aquarium Research Institute in Moss Landing, California, and others have proposed that there may be still more uses for red light in the deep. In 2005, his group made the controversial proposal that at least one fish-eating species of siphonophore, a jellyfish relative, combines bioluminescence with red fluorescent proteins to create red fishing lures on its tentacles10.

Challenges to the theory revolve around the low number of deep species so far known to see red, although the vast majority have not been tested. Physics provides another challenge. Fluorescence is inefficient, because energy is lost in the transfer from bioluminescent reactions to fluorescent proteins, meaning a relatively weak signal is produced or received.

"My physics head says, 'No,'" says Justin Marshall, another Deep Scope participant, from the University of Queensland in Brisbane, Australia, "But my biology head says, 'Well, Why not?' Biology is weird, so it could be."

Haddock, and Mikhail Matz, another Deep Scope leader, based at the University of Texas in Austin, are involved in a related research project focused on the red fluorescent protein found in the dragonfish searchlight photophores, and, according to preliminary results, also in the chin barbels of two species. There it is not associated with a bioluminescent organ, making its presence all the more intriguing. "I think red fluorescence attracts some sort of prey," says Matz, but what and how are not clear. The dragonfish protein seems to be novel, and, as with several collected from shallower-dwelling animals, Matz is exploring the potential use of the protein as a new tool for biomedical research, where other fluorescent proteins are used frequently.

Questions such as the extent to which red fluorescence might play a role in the deep are likely to remain unanswered for some time, but Widder hopes that there will soon be a new tool to advance research on a number of the deep's mysteries. Thanks to military funding, most research on bioluminescence has focused on shallow waters where it can reveal the presence of submarines and other vessels. But Widder has now secured funding from the National Science Foundation for a new version of Eye-in-the-Sea, scheduled for deployment as part of an undersea observatory array off Monterey, California, in early 2008. This system will provide researchers with the first ever perpetual, unobtrusive view of the depths. That could mean the first effective, long-term study of true deep-sea bioluminescent behaviour. Among numerous other potentials for discovery, this extended view may allow Widder and her colleagues to decode just what was said between their lure and the unidentified creatures they were communicating with in the Bahamas. "I feel like we'll be able to address some of these issues for the first time," she says.