USA (20 July 2004) -- At first blush, invisibility does not seem all that hard to manage. Harry Potter does it by putting on his father's cloak. Good waiters do it by resisting the urge to divulge their first names. Women do it just by getting older. Yet the physiological challenge of being a genuine see-through creature is among the greatest in nature, one that truly must go beyond the pale. "People assume that if you aren't pigmented or you don't absorb light, you'll be transparent by default," Dr. Sonke Johnsen, an assistant professor of biology at Duke, said. "But albinos lack pigment, and they aren't see-through. Clouds and milk don't absorb light, but they're opaque." The key, Dr. Johnsen said, is to neither absorb light, as do, say, a mauve chenille swatch or a rose petal, nor to scatter light every which way, as a cloud does. Because nature generally constructs bodies of materials that do both with abandon, it needs a very good reason to build otherwise, to make the too, too solid flesh melt into something resembling water. As it happens, that very good reason is - water. Dr. Johnsen, who studies transparency in animals, said he understood quite well its benefits every time he went scuba diving in the ocean in search of see-through species. "You feel like shark bait floating on the end of a string," he said. "There's nowhere to hide in the open ocean, and you realize how exposed and vulnerable these animals are. They're slower and weaker than their predators. They can't flee. They can't fight, and once they're seen, they're dead. Their only chance is to not be seen in the first place." The stakes are so high that even with the technical difficulties of being transparent, the ocean has got plenty of nothing. Beyond the familiarly unearthly jellyfish, there are foot-long spectral snails, worms that look like crystal candles, phantom octopuses, clear fish, crustaceans that hardly make a homeopathic dent in the water as they pass. "For any major animal group you can think of," Dr. Johnsen said, "there's a transparent representative swimming out in the ocean." So what does it take to be a nobody? Some marine animals, like the jellyfish, manage it by being built almost entirely of water. Others, like the glass catfish, do it by having tissues with cells so densely packed that they take on a quasicrystalline structure, similar to the human eyeball lens. No matter how they make the bulk of themselves disappear, transparent animals must deal with two functions that are hard to hide - eating and seeing. Food that is being digested does not line up in well-ordered, potentially crystalline stacks, and what good does it do to have a windowpane frame if dinner is on widescreen view inside? Many transparent animals tackle the problem by having as minimalist a gut as possible. "They make it into a tiny sphere that actually casts a very small shadow," Dr. Johnsen said. "That's how we find these animals. We look for strange little globes floating in the water." | | The transparent Phronima, a crustacean that Dr. Sonke Johnsen of Duke calls "really creepy looking." source: Kjell B. Sandved/Photo Researchers Inc. Other marine spirits have needle-shape stomachs tethered to muscles that, regardless of the animal's orientation, keep the pin gut pointed up and down, almost shadow free. Eyes are also hard to disguise. To see requires that one have tissue capable of absorbing light, which means the retina cannot be entirely transparent. Here, the solutions run from comic to telecom. Some creatures put their eyes on long stalks that bob about like little blobs unattached to anything. In other cases, the retina is right behind the lens, a positioning that smears the absorbent tissue into an almost imperceptible bit of fuzz. The most farsighted species, if you will, use fiber optic technology. Dr. Johnsen described phronima, a crustacean he likened to the parasitic star of "Alien." "It's a really creepy looking animal with huge eyes and a tiny dot of a retina," he said. Normally, it would be hard to focus with that combination, but phronima has transparent fiber optic cables that channel all incoming light onto the retinal speck. Handy though invisibility might be to terrestrial creatures like a fawn spotted by a puma or a child caught with a hand in the cookie jar, transparency is rare on land except for a few clear-winged butterflies and semitransparent frogs. Out of water, animals need some pigmental protection against ultraviolet radiation, and the comparatively dense bones that give land animals their heft are impossible to hide. It's a familiar dilemma: so many skeletons, not enough closets. SOURCE - CNS |
