The effect of temperature on Lirceus brachyurus (Crustacea: Isopoda)

The thermal tolerance of the aquatic isopod Lirceus brachyurus (Harger) was determined, and the effect of temperature on photoperiodicity and respiration was investigated. Survival, as measured by the time to 50 percent mortality (LT50), increased with increasing acclimation temperature. A thermal trapezium was constructed which defined the range of temperatures (0 to 23°C) tolerated by the isopod for two weeks. Upper lethal temperatures were lower than recorded for other isopod species, but did correspond to temperatures recorded in the study creek. The animals tolerated cyclic temperature fluctuations (ΔT) of ± l0°C (between 5 and 30°c) for two weeks. There was no significant difference in tolerance of an elevated temperature among animals exposed to three photoperiod regimes. However, animals acclimated to a long-day photoperiod survived a little longer due to increased metabolic activity used for evaporative cooling.

Metabolic rates, which were measured by a differential respirometer, were highly modified by the movement of animals within the experimental chambers. Mean oxygen consumption per hour decreased with increased time spent in the respirometer, regardless of temperature. Oxygen consumption was similar for animals exposed to low (5°C) and high (25 and< 30°C) temperatures, and to intermediate (10, 15, and 20°C) temperatures, regardless of acclimation. This result implies that brachyurus may regulate its metabolism without changes in energy expenditures, and that increased metabolism occurs at both elevated and reduced temperatures.

The ecological significance of these results with respect to the biology and habitat of Lirceus brachyurus is discussed.