296 INVERTEBRATE PHYSIOLOGY 



used by the crab, the diver increased in weight by one gram. Such a res- 

 pirometer as this is very sensitive to barometric pressure and appropriate 

 corrections had to be made for the pressure changes. 



When the rate of oxygen consumption by fidder crabs was followed 

 continuously for several months, there seemed superficially to be only 

 random fluctuations, though large ones, from hour to hour and day to day. 

 It was soon noted, however, that, whenever the barometric pressure was 

 rising, the rate of oxygen consumption was reduced in proportion to the 

 rate of rise; and, when the barometric pressure was falling, it was in- 

 creased in proportion to the rate of fall. It has been known for some time 

 that there are unequivocal daily and lunar rhythms of barometric pressure. 

 Although the hour by hour and day by day changes tend to show a large 

 random component, the barometric pressure tends to fall shortly after 

 midnight, rise rapidly in the late morning, fall rapidly in the afternoon, and 

 rise rapidly again in the early evening. The form of the daily rhythm be- 

 comes quite evident when one averages the values for each hour of the day 

 for a period of two to four weeks ; usually only 3 to 4 days suffice to give one 

 the general form of the cycle. The fiddler crabs had a clear daily rhythm 

 of Oo-consumption, with a low rate about 6 :00 or 7 :00 in the morning and 

 another low rate about 6 :00 or 7 :00 in the evening ; both are, on the aver- 

 age, times of rapid barometric pressure rise. In the afternoon, when the 

 pressure tends to fall rapidly, there was the highest rate of Oo-consump- 

 tion for the day in the crabs (Brown, Webb, Bennett, and Sandeen, 1955). 



If now, instead of averaging values for each hour of the day for several 

 solar days, a comparable average is calculated for the hourly values of 15 

 or 30 lunar days, it is apparent that the fiddler crabs also possess a lunar 

 rhythm of Oo-consumption. In the lunar day of approximately 25 hours, 

 a low rate in Oo-consumption preceded by 3 or 4 hours the time the moon 

 was to be at its zenith in the sky, and another low rate occurred 3 or 4 hours 

 before the time the moon was to be at its nadir. Here, then, was a rhythm 

 of Oo-consumption in the crabs with cycles of tidal frequency. A rhythm 

 of barometric pressure of lunar frequency, though existent, is of such low 

 amplitude as to be incapable of resolution even with several months of 

 pressure data for a temperate zone region. 



The question then arose : Is this metabolic rhythmicity peculiar to fiddler 

 crabs ? Studies of the Oo-consumption of a salamander, a common seaweed, 

 a snail, a sea cucumber, and even an Irish potato showed them all to possess 

 similar daily and lunar rhythms of 02-consumption and for the hourly 

 fluctuations in rates of Oo-utilization a correlation with the rate and direc- 

 tion of the concurrent barometric pressure change, just as in the crabs 

 (Brown, Freeland, and Ralph, 1955; Brown, Webb, Bennett, and San- 

 deen, 1955). 



