294 INVERTEBRATE PHYSIOLOGY 



on other days at noon, and on still other days it occurred in the afternoon. 

 Occasionally the crabs became their darkest both early in the morning and 

 early in the evening. A study of the change in the time of day of greatest 

 darkening showed that this occurred about 50 minutes later each day. 

 Furthermore, when they were very dark about 7 :00 in the evening, they 

 were also very dark about 7 :00 in the morning. 



It is common knowledge, of course, that high and low tides in any given 

 locality occur about 50 minutes later each day and that two low or two 

 high tides on any given day are about 121/^ hours apart. It was soon found 

 that crabs in the photographic darkroom became their darkest at the times 

 of low tide on the beach where they had been collected. When low tide 

 occurred about 7 :00 p.m. on any given day, there was also a low tide 

 occurring about 7 :00 a.m. In other words, when the crabs on their home 

 beaches were actively foraging in the bright light at the water's edge with 

 their bodies protected from both the sunlight and their predators by be- 

 coming their blackest, their captured relatives that had been in a laboratory 

 darkroom for even as long as a month were also becoming their darkest. 



One hypothesis accounting for the preceding results is that the crabs 

 possess metabolic rhythms of two frequencies, one with 24-hour cycles 

 and the other with 12.4-hour cycles. Rhythms of these two frequencies 

 would be expected to coincide in phase about every 15 days. It was soon 

 discovered that the crabs were, in parallel fashion, their darkest in the 

 photographic darkroom at 9 o'clock every 15 days. This indicated quite 

 emphatically that the frequency of the tidal or lunar rhythm of the crabs 

 was quite precisely that of the tidal or lunar cycles of the external environ- 

 ment. The crabs in the darkroom therefore possessed a clock for measuring 

 periods of semilunar length. A few simple experiments sufficed to show 

 that the crabs' ability to measure off periods of lunar frequencies was inde- 

 pendent of temperature fluctuations (Brown, Webb, Bennett, and San- 

 deen, 1954). To be of service to the crabs in their normal living, this was 

 obviously necessary. If our clocks, for example, were to double their rates 

 when the temperature rose 10° C, and halve their rates when it fell 10° C, 

 they would be useless, indeed. 



Are the tidal clocks of the crabs set by the tides on the beaches, or by the 

 moon itself ? To answer this we collected crabs from two different beaches 

 a few miles apart ; low tide on the Martha's Vineyard beach regularly 

 occurred 4 hours later in the day than on the Woods Hole beach. The two 

 groups of crabs, side by side in the same darkroom, each continued to signal 

 the time of low tide on their own home beaches. Just as it had been possible 

 to reset the 24-hour daily cycles by shifting the illumination periods, it 

 turned out to be possible similarly to reset the 12.4-hour tidal cycles. The 

 crabs behaved quite as if they normally used their daily cycle as a clock to 



