Resistance of Four Freshwater Crustaceans to Lethal High Temperature and Low Oxygen

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Abstract
The crustaceans studied were the isopod Asellus intermedius Forbes, and the amphipods Hyalella azteca (Saussure), Gammarus fasciatus Say, and Gammarus pseudolimnaeus Bousfield.Resistance to high temperature decreased with size in A. intermedius and G. fasciatus, but no difference was found for H. azteca. Female Gammarus, were more resistant than males, there was no difference for A. intermedius, and a variable relation in H. azteca. Acclimation temperature could be raised at rates of 2.5 to 5 °C per day, at temperatures above 14 °C. Raising acclimation from 10 to 20 °C increased the lethal temperature (50% mortality in 24 hours) by 1.9 °C in G. fasciatus, 1.3 °C in A. intermedius, 0.5 °C in G. pseudolimnaeus, and apparently not at all in H. azteca. There was no seasonal variation in resistance after A. intermedius was acclimated in the laboratory.Two difficulties encountered were mortality during acclimation and short survival of controls, but careful checking showed that neither greatly affected the reported resistance.Estimates of the ultimate 24-hour lethal temperatures were 34.6 °C for A. intermedius and G. fasciatus, 33.2 °C for H. azteca, and 29.6 °C for G. pseudolimnaeus. The concentrations of low oxygen causing 50% mortality in 24 hours, with acclimation and testing at 20 °C, were.0.03 mg/l for A. intermedius, 0.7 mg/l for H. azteca, 2.2 mg/l for G. pseudolimnaeus, and 4.3 mg/l for G. fasciatus. Lethal temperatures would seldom seem to affect distribution under natural conditions, but resistance to low oxygen fits field observations fairly closely.

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