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Science Frontiers ONLINE No. 87: May-Jun 1993 |
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The vent glow and "blind" shrimp
In 1988, when the research submersible Alvin was exploring those remarkable hydrothermal vents or "black smokers." Its CCD (Charge-Coupled Device) camera detected a ghostly glow emanating from the vents. Since this mineral-laden water gushing from these cracks in the deep-sea floor exits at 350�-400�C, the simplest explanation of the vent glow is that is is simply thermal radiation from the hot fluid. Indeed, color filters on the camera recorded a spectrum close to that of a 350�C plume.
But the deep-sea shrimp camped around the vents have raised second thoughts. The shrimp, only a few inches long, live in the perpetual darkness of the miles-deep vents. They do not need and do not have ordinary eyes. Rather, they sport a mysterious organ on their backs that is connected to their brains by a nerve-fiber bundle much like an optic nerve. This organ is packed with the same light-sensitive pigments found in the eyes of surface creatures. Despite its unusual location on the shrimp, it is an "eye" of sorts. But of what use is it in the Stygian abysses? To find, perhaps, vent glows that betray the presence of chemosynthetic food sources. If this is so, the shrimps' optical organ, which is most sensitive in the blue-green portion of the spectrum, is badly mismatched to the infrared of the vent glow. It is a truism that nature is a perfectionist and would not tolerate such bad design. The eyes of animals are always well-tuned to their ways of life. Some possible conclusions: (1) The vent glows are not entirely due to thermal radiation; (2) The shrimps' organ is tuned to "something else"; and (3) The vent-shrimp link is recent, and evolution has not yet had time to fine-tune the shrimps' "eye."
(Travis, John; "Probing the Unsolved Mysteries of the Deep," Science, 259: 1124, 1993.)