Effects of chronic cadmium exposures on growth of larvae of an Hawaiian bivalve, Isognomon californicum

Young bivalve larvae (3 d old) were exposed to environmentally realistic concentrations of cadmium (2, 10, 20, and 50 μg l−1) under static exposure conditions for 28 d, a period that spans most of their larval lifespan. The effects on growth were evaluated in terms of both size and weight parameters. Cd effects on size growth (length and height) were significantly different, but subtle, mostly characterized by a slow gradual decline, but remaining at ≥80 % of control sizes. However the effects of Cd on weight (total dry weight, shell weight, and tissue weight) were much more dramatic. After only 4 d, Cd-exposed larvae weighed < 80 % of the controls (total dry weight), and tissue weights were between 35 and 51 % of controls. With continued exposures, there was some improvement, but then growth stabilized at levels significantly lower than the controls or continued to decline. This response of initial severe effects, readjustment, and restabilization represents an adaptive response mediated by homeostatic mechanisms. However, the mechanisms were unable to ameliorate the toxic effects. Therefore, weight was more sensitive than size as an indicator of adaptive response and stress. The capacity for recovery was evaluated by exposing some larvae to Cd for 14 d and allowing them to depurate for 14 d. Only larvae initially exposed to 2 μg l−1 Cd showed significant recovery. The effects on growth are discussed with regard to Cd tissue concentrations and regulatory mechanisms.