Cystisoma: its large, pale retina minimizes its shadow when viewed from below. Widder, HBOI
Cystisoma: its large, pale retina minimizes its shadow when viewed from below. Widder, HBOI

Transparent Motives

'Visual ecologist' Sönke Johnsen pursues elusive, fragile undersea phantoms for their biological secrets.
November 30, 2005

Floating in the warm depths of the Gulf of Mexico, Sönke Johnsen is surrounded by "ghosts," swirls of ethereal entities whose glimmerings tell him he is not alone in the see-forever cerulean waters. He is enveloped in a clear-as-glass menagerie of creatures that make the open ocean their home. They survive because they have evolved to be nearly invisible.

Unlike his spectral companions, Johnsen is an all-too-obvious, tempting morsel of rubber-wrapped flesh. Suspended by safety tethers, he's like bait on a fishing line in the featureless sea. His sense of peril is heightened because he and similarly appetizing companions once fended off a marauding shark--only small plastic poking sticks between them and precisely two-zillion needle-sharp teeth.

Johnsen's vulnerability makes him appreciate all the more the survival value of the sea creatures' crystalline camouflage in a realm where there is nowhere to hide but in plain sight. The larval fish, worms, shrimp, jellyfish, and flea-like amphipods wafting past him exemplify evolution at its most ingenious. And unlike the exterior disguises worn by many land animals--a fawn's colored fur or a snake's patterned scales--the disguise these creatures embody is by definition much more than skin deep, extending even to internal organs. Some creatures have developed cunning ways of concealing the telltale signs of any undigested dinner: mirrored stomachs that hide the food by reflecting the infinite blue around them. Others have needle-shaped stomachs that swivel and can be made to point downward, minimizing shadows that would be a dead giveaway.

Like most organisms, they need light-absorbing pigments in their retinas in order to see. But to minimize the pigments' compromising effect on their camouflage, some have evolved eyes on stalks extended far from their bodies; others, compact retinas that are mere dots in the water, or even diffuse, pale retinas that show up only as faint smudges. To help carry the appearance of invisibility, even shadows must be minimized. Many of the creatures are flat and thin--some as thin as a few sheets of paper--so that light passes more easily through their transparent tissues, and any shadow they cast is only an indistinct line. Where complete physiological transparency is not available, organisms resort to tricks of the eye, evading predators by sporting bioluminescent "light bulbs" along their under surface, for example, that help minimize shadows.

The collector: Johnsen gathers animals in Gulf of Mexico

The collector: Johnsen gathers animals in Gulf of Mexico. Marshall, University of Queensland

But evolutionary ingenuity is not just the province of prey. Predators, too, have developed their own adaptations to thwart invisibility in this silent, subtle duel for survival. Some have eyes that can see polarized light, rendering prey more visible. Some carry biological flashlights--photophores--that may illuminate prey. And others have eyes on top of their heads, so they can constantly scan the waters above them, seeking subtle shadows that reveal the presence of a potential meal.

Observing this intricate evolutionary duel is another exotic species--the breed of rare, curious scientists called "visual ecologists," of which Sö;nke Johnsen, a Duke assistant professor of biology, is an exemplar. His aim, he says, is to understand the "arms race between the hiders and the seekers." His scientific perspective comprises an "outside" and an "inside": "The 'outside' is trying to understand the ecological function of optical camouflage and what animals have done to break this camouflage," says Johnsen.

The "inside" questions, he says, are those aimed at understanding the physiology of these exotic creatures. "What are they doing to their bodies to make these strange optical properties? How are they making their body clear? How are they managing to reflect light at only certain wavelengths? How are they managing to focus light so well, despite having ball-shaped lenses?" But Johnsen doesn't just study eyeballs and photophores in isolation; he tries to make sense out of how these creatures use their visual capabilities in the life-or-death thrust and parry of predator and prey.

The exotic elegance of these creatures is reason enough to study them. But there are other compelling motivations as well. Of all life on the Earth, sea life is perhaps the most ecologically significant. About half the oxygen in each breath we take originated from the photosynthetic activity of phytoplankton floating in the ocean. And these phytoplankton are part of the intricate--and fragile--ecology that includes the creatures studied by Johnsen and his cohorts. Then there's the importance of ocean ecology to our food supply. Phytoplankton are the base of an intricate marine food chain at whose apex are the fish we eat. We are blindly whacking away at this food chain--scouring vast regions with sprawling fishing nets, injecting massive pollution into the ocean, altering ocean temperatures via global warming, and increasing ultraviolet radiation reaching Earth by destroying protective ozone. To understand the ultimate effects of this vast ecological attack, we must understand in detail the biology of its victims, Johnsen says. And in so many cases, we know so little.

That's where what he refers to as the "embarrassment factor"--our ignorance of so much of the life that inhabits our planet--propels his studies of midocean creatures. "We know more about the surface of the moon than we know about the bottom of the ocean," says Johnsen. "And over 99.5 percent of the Earth's inhabitable space is the midwater of the ocean."

"It's been said that if a space alien came down with a net and scooped out an animal from a random spot on Earth, it would probably be one of these weird gelatinous animals. So, while to us they seem very unusual, and to us they have this sort of freaky unearthly appearance, we're the weird-looking ones when you get down to it."

As an example, Johnsen points to the Diel vertical migration. While the migration patterns of birds, butterflies, and other land creatures have long been the subject of research, they are inconsequential compared with the Diel migration, a gargantuan movement that occurs daily, around the globe, as ocean creatures move upward or downward with the changing daylight. Still, our ignorance about the reasons for this largest of all migrations is as deep as, well, the ocean, says Johnsen.

Johnsen with image of deep-sea octopus whose suckers have evolved into photophors, or light-emitting organs

Johnsen with image of deep-sea octopus whose suckers have evolved into photophors, or light-emitting organs. Les Todd

"In some cases we do know it's the light," he says, "but we don't know exactly what it is about the light, except in a few cases. Is it that the animals always stay in the same level of brightness, so that as the sun goes down they move up? Or is it that they start moving up when the light changes very quickly, as it does at dawn and dusk? That seems to be true for some animals. Or is it something to do with a change in color of the light?

"In some cases, it may actually not be light at all. They could be following other organisms. It could be that the phytoplankton are moving up and down, and everybody else is just following up and down for lunch. And in other cases, it may actually be just to avoid sunburn, because ocean water is very clear compared to coastal water, and UV light can get down through the surface layers."

Even beginning to understand this massive movement means understanding the basic biology of the animals--a major challenge in the ocean, Johnsen points out. "Most of the migrations on land aren't as big a mystery, because people can see both ends of the migration. They know that birds do their breeding at one site, and they do feeding at another site. They can see it happen. For us, we don't even know the reproductive ecology of most of these animals. So, we don't know what's going on."

Observing the fragile, elusive creatures, much less making scientific measurements on them, has proven an enormous challenge, says Johnsen. "On land, you can have a graduate student sit near a beaver lodge or set up a camera and observe the animals without disturbing them. You can have somebody studying forest ecology go right up to the plants or animals and observe them without disturbing them too much. And you can learn a great deal about how the whole system fits together."

By contrast, humans in the oceans are bulls in an ecological china shop. "The old technique was trawl netting, but that was sort of like flying over London with a big grappling hook, yanking up some poor guy, and trying to understand the culture of the English people. Then we developed submersibles, but that's about the equivalent of showing up in a school bus with all the lights on and the horn honking, two feet from a bunny rabbit, and expecting it to behave normally."

Now, however, visual ecologists use what they call "stealth observation technologies" to study their delicate, light-sensitive quarry. One recent innovation is the "Eye in the Sea" camera developed by Johnsen's colleague Edith Widder at the Harbor Branch Oceanographic Institution in Florida. The camera uses a red light, which is invisible to ocean creatures. "We always say that we catch the slow, the dumb, and the small," says Johnsen. "We don't see the amazing crazy megafauna, because they are smart enough, fast enough, and big enough to get away."

Light and shadow: bioluminescent emissions of galatheid crab viewed on coral under white, blue, and red light

Light and shadow: bioluminescent emissions of galatheid crab viewed on coral under white, blue, and red light. Widder, HBOI

But last fall, when Widder, Johnsen, and their colleagues deployed the Eye along with a bag of bait, to search for deep-ocean creatures in the Gulf of Mexico, they managed to capture images of the fast, the smart, and the large after all. "During the first successful deployment, we saw this two-meter-long squid that nobody has ever seen before come in and just nail this bait bag. It was this solid, muscular, monster-of-the-deep kind of animal."

Later, he continues, the scientists saw "this huge six-gill shark, somewhere in the seventeen-to-twenty-five foot range. And it just stayed there and tried to eat the tripod. So, there is some very impressive stuff down there."

Besides such dramatic fishing expeditions, the researchers also conduct more delicate experiments using new optical instrumentation to glean insights into animals and their behavior. In one set of experiments, Johnsen and his colleagues explored what colors gave creatures the best chance at invisibility. The problem was far from straightforward. He explains that in the ocean depths, there are two kinds of evasion taking place. "There's just hiding under the normal ambient light. But then, there's also hiding from all the animals that are swimming around with 'flashlights.' A lot of fish and arthropods have photophores directly under or over their eyes, so they're swimming around with headlights."

In their study, the researchers first mathematically predicted the colors of animals hiding from ambient light versus those that might be evading predators' bioluminescent searchlights. After collecting a multitude of animals and measuring their colors at all wavelengths, the researchers discovered that evolution had dictated the best strategy. Basically, the researchers found that the animals were "darkest"--reflecting the least light--at precisely the wavelength of the predators' flashlights.

In an evolutionary response, other denizens, such as the aptly named dragonfish, use bioluminescent "light bulbs" along its bottom surface as "counter illumination" to offset its shadow, as seen by predators lurking below. The problem, says Johnsen, is that "most of these light bulbs are widely spaced, which means they're not going to blend in perfectly when a predator looks up at them."

While Johnsen says he first thought that water turbidity might blend the light from the bulbs, in the depths where the creatures live, the water was quite clear because of the lack of suspended particles. After some careful calculations of the optics of water and of the images of dragonfish under various conditions, the researchers realized that it was the predators' fuzzy vision that saved the prey. "Some of these predatory animals have sharp vision, but only in a very narrow field of view right over their heads. But they're nearsighted in other directions." Unless the dragonfish happens to blunder directly overhead, Johnsen says, its rows of light bulbs blend nicely.

Ghosts of the deep: swimming worm

Ghosts of the deep: swimming worm. Widder, HBOI

Studies by Johnsen and other visual ecologists have shed only the faintest light on the puzzle of bioluminescence. "Ninety percent of the species down there are bioluminescent," he says. "We really don't know why. We have some ideas, and a few hypotheses seem well borne out, like the idea of counter illumination. But then there are these other, barely tested possibilities--that prey use light to startle a predator or even to bring in a bigger predator, called the 'burglar alarm' theory. It's energetically expensive to make light. It's obviously there for a good reason and probably serves a multitude of purposes."

Also puzzling, he says, is how predators use polarization--the plane of vibration of light--to detect prey. We humans, for example, use polarized sunglasses to eliminate glare, as off the surface of water. Since such glare consists mainly of light that is polarized horizontally, sunglasses with vertical polarized filters selectively block glare. "We're looking at polarization vision as a way to see better in the ocean," says Johnsen. "Many animals appear to have it, but we have no idea why. We're thinking it might be a way to break camouflage."

In such research, he says, "we resort to 'forensic biology,' where we figure out the best hypothesis by piecing together what we can measure from animals we collect and from measurements in the environment." For example, Johnsen uses technology to put himself into the place of his quarry. "We go down with filters and special polarization cameras to mimic the way it actually looks to these animals, which gives you a better idea of who's well-hidden and who's not; which patterns on a fish are important, and which don't matter at all."

Even as Johnsen takes knowledge from the sea, he also gives that knowledge back, in the form of insights that could protect ocean animals. One of his projects is protecting endangered sea turtles, which are often inadvertently hooked by fishermen after swordfish and marlin. "What they want is something that will be visible to the fish, but invisible to the turtles," he says. "And so, one of our projects is designing lures based on the visual differences between these animals--trying to make something visible to one and invisible to another." Alternatively, he says, they could "design a deterrent that's visible to the turtles, but invisible to the fish. And so, we're making these giant sharks of clear Plexiglas sheets that are opaque in UV light. Turtles can see UV. Sharks can't."

viper fish

Viper fish. Widder, HBOI

Working with Duke medical researchers, Johnsen is also using techniques he's learned to study cataracts in humans. "It's not completely known how the different kinds of cataracts work, what's causing the opacity," he says. "There are many changes in an older person's lens, but people argue over which changes really matter. Is it the granulation of the cytoplasm, or is it these funny little spherical balls that show up?

To find that out, you need to figure out what their optical effects are." New studies by Johnsen and medical collaborators are revealing that abnormal substances once believed to be the central causes of vision-impairing cataracts are, in fact, not the most important.

Even as Johnsen delights in the insights he's gaining through his study of creatures that inhabit the open ocean, he is aware that his life's work may well be a coda to their existence. The very creatures he studies are at the mercy of human folly, which persists in altering the nature of light that penetrates the ocean depths and blighting the purity of its waters.

"Probably the worst thing about being a biologist is watching everything die," he says. "You're basically watching humans wipe it all out, bit by bit. You know there's almost nothing you can do about it. A lot of what I do is for practical reasons, but, a lot of it is just to get people to care. We put out as many beautiful pictures, as many interesting stories, to get people to care enough to maybe slow it down some. But, it almost seems inevitable. We're growing more and more people, and everything else is dying."

Ironically, his sadness is made all the more acute by his experiences in places like the deep-ocean reef that is one of the study sites--the exhilaration over what is found there contrasting with the near certainty that it may all soon disappear, he says.

A thousand feet down, "it's this underwater Eden. There are these huge fish going back and forth--all these different kinds of strange snails and corals and weird little critters. It's neat to see everybody's faces coming up from a dive. They all come up with this big beatific smile. You know, like, Wow!"