Hemolymph Gases, Acid-Base Status, and Electrolyte Concentration in the Freshwater ClamsAnodonta anatinaandUnio tumidusduring Exposure to and Recovery from Acidic Conditions

Abstract

The effects of stress caused by low ambient pH (4-5) were studied in two freshwater unionid molluscs (Anodonta anatina and Unio tumidus). Under soft water conditions (3.7 mg Ca/L, 30.0 ppm CaCO₃), a continuous exposure of 8 d and a recovery period of the same duration were studied and the levels of hemolymph gases (total CO₂, Pco₂, Po₂), pH, and electrolytes (Ca²⁺, Mg²⁺, Na⁺, K⁺, C⁻, and HCO⁻₃) were followed. In A. anatina, low water pH significantly decreased the pH of the hemolymph, while in U. tumidus acid stress did not change the acid-base balance to the same extent. Significant changes in hemolymph pH, HCO⁻₃, Ca²⁺, Na⁺, and Cl⁻ levels were recorded in A. anatina, whereas in U. tumidus only hemolymph [Na⁺] declined. As a result of acid stress, a significant decrease (approx. 60%-80%) in hemolymph oxygen tension occurred after only 6 h of acid exposure. During the first 24 h of recovery, an alkalosis was recorded in A. anatina, but at the end of the 8-d recovery period all hemolymph parameters were again at the control level, except for an elevated [HCO⁻₃]. In U. tumidus, acid-induced changes in Po₂ and [Na⁺] of the hemolymph were not normalized during the 8-d recovery period. Results of this study indicate that unionids have a relatively good capacity to resist short-term acid stress under soft water conditions. Of the two species studied, U. tumidus appeared to be less sensitive to acidification.