A world of creatures that hide in the open

A world of creatures that hide in the open

Making up more than 90 per cent of the earth’s space, the oceans are full of almost invisible animals.

Sönke Johnsen scuba dives in the middle of the ocean, far from land, miles above the seafloor. There are no shipwrecks to explore, no reefs to admire, just a disorienting oasis of blue. What he looks for is hard to see. Indeed, he and his companions often stare intently at one another, searching for distortions passing among them, slightly more visible against the dark colour of a wet suit. And then they carefully catch them and place them in glass jars.

“You’d be surrounded with all these animals,” said Dr Johnsen, a professor of biology at Duke. “But you could barely see them, because they were transparent.” The oceans, which make up more than 90 percent of the earth’s livable space, are full of almost invisible animals. That is because life there is different from everywhere else.To illustrate why, Dr Johnsen began a recent talk with a macabre scenario. Suppose just then a gunman burst into the room, shooting at the audience. Naturally, people would scramble for cover behind chairs and walls. On land, many animals camouflage themselves amid the foliage and terrain; in coastal waters, sea creatures blend into the sand or find refuge among coral or rocks. But in the deep ocean, Dr. Johnsen’s realm of expertise, creatures floating in the water have nowhere to seek refuge from larger creatures that would eat them.

Sharks and whales can swim with impunity, but many other creatures need to hide in plain sight. Transparency is the most obvious strategy — if light passes straight through, no one can see you — and the one Dr. Johnsen first began researching almost 20 years ago. In graduate school, he had been studying clear biological tissues like the lenses in eyes. 

“I wanted to try to understand why they were clear,” he said. By chance, his doctoral adviser mentioned that the open ocean was full of transparent animals. “Which was totally news to me,” he said. He shifted his research focus from transparent tissues to transparent creatures, beginning with a postdoctoral fellowship with the marine biologist Edith Widder at the Harbor Branch Oceanographic Institute in Florida.

 Transparency is not just a lack of pigmentation. Albinos, Dr. Johnsen points out, are not invisible; rather, the entire body must absorb or scatter as little light as possible.Scattering, in particular, is a challenge. When light passes into a material of a different index of refraction, which is often proportional to the density, part of the light reflects and part of it bends. That largely explains why one could search long and wide for a transparent cow or a transparent pigeon and not find one: The density of air is so much less than that of flesh that even a crystal-clear, see-through animal or bird would probably be easily spotted from its reflections.

Water is much denser, and body tissues are roughly the density of water, greatly reducing the amount of scattering. But some organs are denser than others, and the transparent animals pack their insides differently to minimize the variations and the reflections. Dr. Johnsen’s measurements of the see-through creatures that he brought up found that 20 to 90 percent of the light passed through, undisturbed.

An eel larva is almost flat, and its see-through body is almost featureless except for the bones. “These guys can actually absorb some of their nutrients through their skin, so I don’t think there’s much of a gut,” Dr. Johnsen said. But transparency can complicate life in other ways. Transparent creatures near the surface could be sunburned, not only on the skin but inside, too. To protect themselves from ultraviolet light, “these guys basically have suntan lotion in their transparent tissues,” Dr. Johnsen said. But that then allows predators with eyes sensitive to ultraviolet light to see them after all. “There’s this evolutionary arms race,” he said. “I call it ‘Fry or die.’ ”Stealth technology

The esoteric knowledge of deep-sea transparency could have practical applications. “A lot of what I learned about transparent animals, I then applied to human cataracts, which could ultimately help people out,” Dr. Johnsen said. Evolution has come up with two other forms of stealth technology: mirrors and little biological light bulbs. Many predators find their food by looking for silhouettes above. “You see many animals with upward-looking eyes, and even a squid with one big eye looking up and a ‘normal’ eye looking to the side,” said Steven Haddock, a scientist at the Monterey Bay Aquarium Research Institute in Moss Landing, Calif.

The silvery sides of fish like herring and sardines are systems of mirrors: They reflect the downwelling light, much the way a part of the sky is sometimes reflected by a glass skyscraper and blends into the rest of the sky. Thus, a predator from below would see the blue water, not a fish, swimming above. “A tuna is exquisitely adapted for camouflage,” Dr. Johnsen said.

Eric Denton, a British marine biologist, studied mirrored fish in the 1960s and figured out that the mirrors were vertical, maximizing the illusion.

The third strategy, called counterillumination, also seeks to mimic the downwelling light. But instead of mirrors, the animal generates its own glow, much as fireflies do with light-producing organs known as photophores.

Dr. Haddock witnessed the effectiveness of counterillumination in a dimly lit laboratory with a midshipman fish swimming in a transparent tank. He lay beneath the tank and looked up at the fish, about two feet away. “I couldn’t see the fish against the background when it turned on its special belly lights,” he said.He suggested that the black band was a lure, looking like a small fish that a tuna would like to eat. When a tuna closed in, the shark swiveled around and ripped a chunk of flesh from the fish. Some creatures use more than one form of camouflage. For one thing, it is impossible to be entirely transparent. Certain tissues, like retinas, need to absorb light to function. The otherwise transparent squid Chiroteuthis has a light organ to hide its large, opaque eyes, Dr. Haddock said.

A transparent squid also has an opaque gut, because what it eats often is opaque or, worse, glows, which would attract the attention of a larger predator. To minimize its visibility to predators swimming below, the squid’s gut is long and thin, like a needle, and swivels so it points vertically. The gut itself can have mirrors as supplemental camouflage. Some creatures have evolved ways to defeat the camouflage. Species of squid and shrimp have eyes that can differentiate between the two polarizations of light, something that many insects can do too.

Photons — particles of light — can be thought of as arrows with tail fins representing the oscillating magnetic and electric fields, and the polarization represents the orientation of the fields. To human eyes, the colour of reflected light is unchanged. When reflected, the angle of the polarization changes.

As the sun moves across the sky, the polarization of light filtering down to the depths changes, and to an eye that can tell the difference between the polarizations, a mirrored fish suddenly sticks out like a sore thumb. “It turns out while the camouflage is really good, you can really break it with polarization vision,” Dr. Johnsen said. That might, for instance, allow a squid to spot an approaching hungry tuna and flee. 

“We’re catching the small, the slow and the stupid, because anything else just goes away,” Dr. Johnsen said. “We’re just surrounded by an entirely mysterious world. And the fact we can’t see it means we ignore it most of the time.”