Passage IV

NATURAL SCIENCE: This passage is adapted from Richard Monastersky's article "Deep-See Shrimp," which was published in Science News magazine (©1989 by Science Service, Inc.).

The term "jumbo shrimp" has always tickled oxymoron-lovers. But scientists are chuckling at a new morsel of shrimp humor—the name Rimicaris exoculata.

In 1985, when researchers discovered these shrimp swarming around deep-sea geysers of super-heated water, they named the species exoculata, meaning "without eyes." It seemed a fair and accurate title for a shrimp that lacked the eyestalks and corneas other shrimp use for vision.

But marine biologist Cindy Van Dover from the Woods Hole Oceanographic Institution and her colleagues report that the eyeless R. exoculata does indeed have eyes. For some reason, the forces of evolution have granted this shrimp a pair of unusual visual organs located on the animal's back.

The story of R. exoculata's eyes began in 1986 when Van Dover obtained some specimens of the shrimp in order to study their diet.

While studying the contents of the shrimps' stomachs, Van Dover began to focus on a strange patch located on the backs of the animals. This patch was hardly noticeable on the dead specimens that had been fixed in preservatives or frozen for lab study. But in videotapes of the live animals in their natural habitat, taken from the deep submersible Alvin, the patches were reflective. The submarine's lights glinted off them as car headlights might set a cat's eyes aglow.

When Van Dover took a closer look at one of the specimens, she saw that the reflective patches seen on video were actually two lobe-shaped structures sitting underneath a thin, transparent layer of carapace, or shell-type material. A dissection showed that these lobes hooked into the shrimp's brain via a bundle of neurons that looked suspiciously like an optic nerve. She wondered: Could this be an eye?

She asked biochemist Ete Z. Szuts at Marine Biological Laboratory in Woods Hole to look for characteristic visual molecules in the lobes. Meanwhile, bioengineer Steven C. Chamberlain, a specialist in the structure of invertebrate eyes from Syracuse University, examined the back organ to determine whether it was actually organized like eyes.

Chamberlain was able to detail the anatomy of the patch, but he could not pin down the function of this novel structure. "I wasn't willing to say it was a sensory organ; it could have been a gland," he says. Ultimately, it would be up to Szuts' lab to provide the key piece of proof that the organ must be a pair of eyes.

Szuts was looking for a visual pigment known as rhodopsin, which is the light-sensing molecule in all known types of eyes. Rhodopsin molecules are the switchboard in the eye, absorbing photons of visible light and initiating a neurologic message to the brain.

At the start, Szuts did not expect to find any rhodopsin when he ground up several of the organs for analysis. An animal's back just seemed to be the wrong place to put a pair of eyes. After all, he thought, other species of deep-sea shrimp have eyes in the normal place.

Besides, even if the back patch turned out to be two visual organs, other problems would seem to hinder the detection of any pigment. Normally, biochemists need concentrated extracts from some 50 to 100 shrimp in order to detect any visual pigment molecules, says Szuts. But the number of available R. exoculata was limited, and he was dealing with extracts from 5 to 10 shrimp.

Szuts' skepticism turned to surprise when he found the "eyeless" shrimp did have rhodopsin. "It turns out R. exoculata has a visual pigment, and it has it in very large quantities一at the very least five times more than the usual amount of pigment in other shrimp," he says. Because the bright lights of the submersible most likely damaged much of the pigment in the collected shrimp, Szuts believes the pigment he found must be only a small fraction of the amount the animal truly possesses.

It doesn't take a microscope to see some of R. exoculata's adaptations for sensing extremely weak light. Perhaps the most striking feature about the eyes is their size. Chamberlain believes they developed on the animal's back because that is the only spot where such large organs would fit.

R. exoculata's eyes have no lenses, so they cannot see actual images of an object. Just about all they can do is sense the strength and direction of a light source. Yet while they seem limited by the standards of animals living in bright light, the strange eyes are particularly well adapted for their dark environment.