Abstract

Although crayfish commonly inhabit water systems affected by acid precipitation relatively little is known of their tolerance or of their physiological responses. At least in acute (96 h) exposures, crayfish are substantially more acid (H₂S0₄) tolerant than most fish species (LS₅₀ for Procambarus clarki and Orconectes rusticus were pH 2.8 and 2.5 respectively). Physiological responses to sublethal exposure (4 days at ambient pH = 3.8) included the development of severe hemolymph acidosis. The acidosis was largely metabolic in origin and was associated with marked depression of hemolymph bicarbonate levels. This degree of acid exposure produced little disturbance of hemolymph Na⁺, CL^-, K⁺, or MG⁺⁺ ion levels. No increase in hemolymph SO₄^-2 resulted. Hemolymph Ca⁺⁺ levels rose significantly suggesting that some invading H⁺ ions were being buffered by dissolution of exoskeletal or other carbonate stores. With the exception of the rise in Ca⁺⁺ the physiological responses observed are qualitatively similar to those of trout tested at similar external Ca⁺⁺ levels but the crayfish were exposed to substantially higher acid loads. The reasons for the greater tolerance of crayfish are not clear at this time.

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