Science Column

The Crab's Clock

Recent research suggests that the rhythmic behaviour of many marine creatures is not directly' due, as might be supposed, to the influence of such obvious external rhythms as the tide and the day and night pattern produced by the earth's rotation. It seems to be due, rather, to the action of internal "clocks." These are usually in phase with one or more of the environmental rhythms but the "clocks" continue to work when the environmental stimuli are not present, so there is no simple cause—and—effect relationship.

Common green seashore crabs move about the beach when the tide covers it but remain still and out of sight when the tide goes out and the shore is exposed. When removed to the laboratory, where they were kept at a constant temperature In a room continuously lit by a dim red light, they continued to show the same regular bursts of activity followed by periods of quiescence.

There appear to be two separate cycles. Bursts of activity occur every 124 hours, coinciding with the high tide on the shore from which the crabs were collected. Other bursts coincide with the hours of darkness. When high tide occurs at night, activity reaches a maximum. Interestingly, the same tidal rhythm was found in crabs of the same species which were found in docks not affected by tides, suggesting that this behaviour might be Inherited. Similar Inherited rhythms are known in some non-marine creatures.

How do the "clocks" of animals work and in which parts of their bodies are they located? Since they are often independent at temperature changes, it is unlikely they are controlled by simple biochemical processes. In crabs, there is some evidence that the sinus gland in the eyestalk may have an important effect on the rhythm. Attempts to learn more about this phenomenan are being made by subjecting crabs to cyclical changes of light and tide which are different from those they would normally experience. If timing mechanisms are widespread In marine animals, they may help to explain such things as the semilunar spawning periodicity of oysters and other shellfish.