Horseshoe crabs don’t look mysterious and enigmatic, but they are. Normally, one would not expect any very deep questions to be evoked concerning creatures which resemble miniature tanks, moving with ponderous dignity across the beach. But these marine creatures with shells, these “crabs,” are not actually miniature when compared to other animals of the seashore. They weigh as much as 4.5 kg (10 lbs), and may grow to be 60 cm (2 ft) long. To find one such specimen would be memorable enough – but where there is one, there are generally thousands or hundreds of thousands. At the appropriate time in the spring, some beaches along the Atlantic seaboard, from Maine to the Yucatan Peninsula, are invaded by thousands or even millions of these apparitions.
They look sinister, these crabs with their lateral eyes projecting in a sort of perpetual frown from the smooth contours of the shell. They are not really threatening, however. These creatures have merely come to the beach to lay their eggs. But not all of them manage to retreat safely back into the sea. Through the years, many have been captured for physiological study. It was in 1926 that H. Keffer Hartline began to study electrical impulses from the optic nerve of horseshoe crab eyes. From these studies some important principles about the functioning of eyes were discovered. As a result, Drs. Ragnar Granit of Sweden, and Americans H. Keffer Hartline and George Wald were awarded the 1967 Nobel Prize in Medicine.
During all those years of study, the lateral eyes had always been removed from the crab before the experiments were conducted on electrical impulses, either in the nerve leading away from the eye, or in the eye itself. Then in the 1970’s a novel approach was tried. A team of scientists applied electrical probes to the eyes of intact animals. Imagine their surprise when they found that at night, the sensitivity of the crab’s eye to light was increased by a factor of up to one million times that of the daytime response!! (Barlow, 1990) Subsequent research showed that an internal 24-hour clock (circadian rhythm) in each crab’s brain controlled this amazing cycle. Even when crabs were kept in constant darkness for more than a year, their eyes still showed this circadian rhythm.
Not surprisingly, it has been discovered that this unique cycle of sensitivity to light has very complicated controls. For a start, the research team found that these changes involve a feature exactly opposite to other biological systems. Normally, as sensitivity to a stimulus increases, so does the background noise (signals generated at random rather than in response to a genuine stimulus). The normal situation is like what happens when you turn up the volume on your radio. The volume goes up, but so does the static (background noise). In the case of the crab’s eye, however, the noise level goes down as the sensitivity increases (Barlow, 1990)….
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