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

Floating in the warm depths of the Gulf of Mexico, Sö;nke 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 Photo: 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.