Camp. t J10ck r m. Phr \ lol.. Vol. 66A. pp. 311 to 384 0 Pergamon Press Lfd 1980 Prmted in Great Britain THERMAL ACCLIMATION IN THE CRAYFISH, zyxwvutsrqponmlkjihg ORCONECTES RUSTICUS AND 0. VIRILIS DENNIS L. CLAUSSEN Department of Zoology, Miami University, Oxford. OH 45056 U.S.A. zyxwvutsrqponmlkjihgfedcba (Received 18 Sepvn~her 1979) Abstract-l. Both Orconrctrs rusticus and 0. M rilis show a pronounced thermal acclimation response. Their heat resistance is in the upper portion of the range of literature values for crayfish and other crustaceans. 2. The thermal acclimation time courses for the two species are somewhat similar. The smaller magnitude of the response for the Northern species (0. uirilis) stems from an increased heat resistance for cold acclimated animals rather than from a lowered heat resistance for 25°C acclimated crayfish. 3. Two methods (CTM and LTsO) of measuring heat resistance in 0. rusficus yield similar acclimation rates and time course patterns, but markedly different rates for reverse acclimation. 4. Ouantitative methods for acclimation time course analysis are described and applied both to the crayfish data and to derived literature values. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHG INTRODUCTION Crayfish are among the most conspicuous and energe- tically important invertebrate members of fresh-water communities (Momot et a/., 1978). More than 275 species occur in North America, and many of these have extensive geographic distributions (Hobbs, 1972). Crayfish are found in Canada, in Mexico, and in all 48 contiguous states of the U.S.A. Nevertheless, the literature on thermal acclimation and heat resist- ance of these crustaceans is surprisingly sparse. Thermal acclimation involves a compensatory change in response to temperature which occurs dur- ing the lifetime of an individual organism (Edney, 1964). Such adaptive changes in heat resistance have been demonstrated for a wide variety of organisms, including crayfish. Bovbjerg (1952) compared heat re- sistance in Orconectes propinquus and Cambarus fodiens based on median survival times at 35°C. Both species showed evidence of thermal acclimation when maintained at high temperatures. Resistance adapta- tion in crayfish was more conclusively demonstrated for 0. rusticus by Spoor (1955) in a study based on median 12 hr thermal tolerance limits. More recently, K. Bowler and his colleagues have compared survival times at various temperatures in their investigations of thermal acclimation in hstropotarnobius (Astacus) pallipes (Bowler, 1963; Bowler et al., 1973; Gladwell et al., 1976). Caine (1978) compared the upper lethal temperatures of four epigean and three hypogean species, but used only a single acclimation tempera- ture and thus did not investigate thermal acclimation per se. Becker et al. (1977) studied cold resistance in Pacifastacus Irrziu.scu/us. These few studies apparently represent our total knowledge of resistance acclima- tion in intact crayfish. Some investigators monitor percentage survival as a measure of heat resistance (Method I of Precht, 1973). It is convenient to subdivide Precht’s Method 1 into Method la (the LT,, method), where the time for 50% mortality at a given exposure temperature is recorded, Method lb (T,,, method), where the tem- perature at which SOY/,of the animals survive for a given exposure time is determined, and Method Ic (%S method), where the investigator determines the percentage survival at a specified temperature and ex- posure time. Other investigators heat the organisms at a constant (and usually quite rapid) rate until cer- tain specified symptoms appear (Method 2 of Precht, 1973). This latter method is the basis for the critical thermal maximum or CTM concept (Cowles & Bogert, 1944; Hutchison. 1961). There have been few attempts to compare Methods I and 2 within a single study. Although the CTM approach has been extensively used in studies of ectothermic vertebrates, it has rarely been applied to crayfish or most other inverte- brate groups. The methodology can nonetheless be readily extended to aquatic invertebrates and it is well suited for investigations of thermal acclimation. I initiated the present study to compare heat resist- ance of the crayfish, Orconectes rusticus and 0. tlirilis. Although I additionally compare the CTM method with the LT,, approach, this study is primarily con- cerned with quantification of the rate and magnitude of crayfish thermal acclimation and with comparisons of data from other organisms. MATERIALS AND METHODS The Orconectes rusricus were collected from Indian Creek in Butler County, Ohio. The 0. virilis were collected from Lake Butte des Mortes, Wisconsin and supplied by a commercial dealer. Both species were maintained in suit- able laboratory aquaria prior to testing. Animals selected for testing were acclimated within a variety of incubators maintained at 5, 15, 25, or 35°C & 0.5”C. The cravfish were acclimated for IO-1 I davs at 5”C, for 6-X days a; I5 and 25°C (IO days for the LTio tests), and for 5-7 days at 35-C. They were not fed during the last 3 days of acclimation, and they were maintained in constant darkness except for brief exposures to light during periodic inspections or when animals were removed, All CTM and LT,, tests were initiated between I I : 15 and I3 : I5 hr. 377