Light at the bottom of the ocean.

The oceanic mystery started off with the discovery of blind shrimp that can actually see. When biologists in 1985 first spotted a new shrimp species swarming around geysers of hot water along the deep ocean floor, they named it Rimicaris exoculata, a Latin term describing a shrimp lacking eyes. Further study, however, revealed that these creatures are not blind at all. Rather, they have an unusual pair of eyes on their heads (SN: 2/11/89, p.90). That discovery led scientists to wonder what R. exoculate could be looking at in the dark ocean depths 5 kilometers below the surface.

Oceanographers who have made recent dives to these hotwater vents have now ruled out the idea that the shrimp are watching light generated by heat from the geysters. Researchers raised that theory five years ago, when divers in the deep-sea submersible Alvin photographed light emanating from the hotwater chimneys. They suspected that the light could be blackbody radiation, much like the red glow given off by hotmetal. That explanation made senses because the water spewing from the vents is a blistering 350 [degrees] C.

But the recent dives have sunk that idea, says Alan D. Chave of the Woods Hole (Mass.) Oceanographic Institution. During an expedition last year in the Alvin, researchers used photodiodes to measure th estrength of light coming from the geysers, both at their openins and 10 centimeters above the vents, where the water temperature cools to 40 [degrees] C. The theoretical calculations suggest that if black-body radiation were producing the liight, it should be a billion times stronger at the vents than in the cooler water above. But the photodiodes detected only eight times more light at the vents, indicating that the illumination must come from process other than thermal radiation, says Chave.

Last January, oceanographers, biologists, chemists, and even astrophysicists met at Woods Hole to discuss what could produce the vent light. Aside from black-body radiation, they listed a number of other possible sources of illumination: crystalloluminescence, produced when chemicals crystallize; sonoluminescence, powered by the sound of bubbles collapsing; triboluminescence, created when rock crystals crack; and Cerenkov radiation and scintillation, both caused by the radioactive decay of elements in the vent water.

Cindy Lee Van Dover from Woods Hole currently making measurements at deep-sea vents to determine what wavelengths of light are released. These results should help narrow the field of potential explanations, says Chave. Biologists are intrigued by the idea of deep-sea light because it raises the possibility that organisims may harness such energy with a type of photosynthesis completely independent of the sun.