LOSS AND GAIN OF HEAT-TOLERANCE BY THE CRAYFISH W. A. SPOOR Department of Zoology, University of Cincinnati, and Stone Institute of Hydrobiology, The Ohio State University It has been well established that a fish becomes increasingly tolerant of heat as it becomes acclimatized to higher temperatures within the range of thermal tolerance of its species, and that its acclimatization to low temperature entails a loss of heat-tolerance (Doudoroff, 1942; Brett, 1944, 1946). Inasmuch as aquatic arthropods are exposed to the same fluctuations in environmental temperature as fish, one might reasonably expect to find that they, too, are capable of gaining and losing heat-tolerance. The matter has not been explored as thoroughly for aquatic arthropods as for fish, however, and there are but few instances in which it is clear that individual animals have gained or lost heat-tolerance, i.e., that they have undergone "physiological" acclimatization, as the term is used by Prosser et al. (1950). It is true that aquatic arthropods from warm waters have been found to be more heat tolerant than related species, or even members of the same species, inhabiting cooler waters (Mayer, 1914; Huntsman and Sparks, 1924; Fox and Wingfield, 1937; Mason, 1939; Whitney, 1939; Park. 1945; Walshe, 1948; Marlier, 1949; Bovbjerg, 1952), but in most cases the differences in toler-ance can be attributed as well to selection as to physiological acclimatization (Fox, 1939). With one possible exception, the literature bearing directly upon the problem of physiological acclimatization among aquatic arthropods indicates that they do become increasingly tolerant of heat as their environmental temperatures are in-creased. Huntsman (1924) showed that lobster larvae raised at temperatures between 20 and 25 C. were more heat resistant than those raised at 15, Edwards and Irving (1943) reported the thermal death point of the sand crab, Emerita talpoida, to be about 10 higher in summer than in winter, and Marlier (1949) found indications that the lethal temperature of larvae of the caddis fly, Hydro-psyche angustipennis, increased from 31 in the spring to 32 in early summer. Furthermore, Bovbjerg (1952) observed that two species of crayfish, Orconectcs propinquns and Cambarus fodiens, became increasingly tolerant of temperatures between 34 and 35 as the advancing season warmed their habitats, or after they had been maintained in warm water in the laboratory for five or six weeks. The possible exception was reported by Whitney (1939), who found nymphs of the mayfly, Bactis rhodani, to be no more heat tolerant after 40 hours at 15 than controls maintained between 10 and 11. In view of Brett's (1946) experience with the goldfish, however, in which it was shown that the development of an increased heat-tolerance required a latent period of from one to seven days, the latent period being longer the lower the acclimatization temperature, it may be that the absence of acclimatization in Baetis was more apparent than real. It seems quite possible that more than 40 hours of acclimatization were required to increase the heat-tolerance enough to be detected by the method employed. 77
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