Stress and growth response of juvenile crayfish to rhythmic and arrhythmic temperature fluctuations

Growth, molting, and survival of juvenile crayfish, Procambarus a. acutus, were recorded under constant and variable temperature regimes. Crayfish, were exposed to four rhythmic regimes: Constants temperatures of 10, 17.5, and 25 degree C, and a daily square-wave cyclic regime of 10 to 25 degree C (mean = 17.5 degree C); and to one arrhythmic temperature regime: A daily square-wave acrylic regime varying within 10 to 25 degree C limits (mean = 17.8 degree C). The direction and magnitude of temperature changes in the acyclic variable temperature chamber were selected randomly to make temperature changes unpredictable within the specified range. Rhythmic and arrhythmic temperature regimes had different effects on growth, molting, and survival of crayfish. Growth under both variable regimes was similar to growth at a constant temperature (17.5 degree C) equal to the mean of the variable regimes. Molt frequency was directly related to temperature. Growth rates approximately paralleled changes in the adjusted molt rate for each regime. Mortality under variable regimes approximated mortality at a constant temperature (25 degree C) equal to the upper extreme of the variable regimes. The cyclic regime tended to be less optimal for growth and survival than the acyclic regime, possibly because of differences in average daily temperature fluctuations and relative time spent at temperature extremes.