The present study examined the ability of shortterm physiological measurements to predict growth rates of a gastropod, Hydrobia ulvae, in response to pollutant exposure. Whereas growth rates of snails collected from Munkebo Harbor, Funen, Denmark in 1989 were substantially reduced by exposure to cadmium after ca. 3 wk, we could detect no change in total carbon balance measured in short-term radioisotope experiments. At 23 %0 S, the reduction in percent growth after 3 wk was 186.4 for snails exposed to 100 ggCd1-1 and 225.7 for snails exposed to 200 gg Cd 1-1. At 13 %0 S, the reduction in percent growth was 102.6 at 100ggCdl -a and 106.4 at 200 gg Cd 1-a. We found no effect of cadmium on sediment egestion rate over 18 d of exposure. Nor could we detect effects on carbon absorption efficiency or metabolic loss of absorbed carbon during the first 24 h following its ingestion. The fractional loss of 14C from uniformly-labeled snails showed a decrease in CO2 and an increase in dissolved carbon in snails exposed to 200 lag Cd 1-1. Such changes in the partitioning of carbon loss from body stores suggest a partial shift from aerobic to anaerobic metabolic pathways which could reconcile the seeming disparity between growth and carbon balance in response to cadmium exposure. Widdows and Johnson 1988). The relevance of such studies relies on the assumption that short-term physiological measurements of energy uptake and expenditure have ecological correlates in the form of reproductive output, individual and population growth.
The application of 'scope for growth' to investigate pollutant effects has centered on studies of the bivalve, Mytilus edulis (Bayne et al. 1979, Poulsen et al. 1982, Widdows and Johnson 1988). Full evaluation of the usefulness of this approach in ecological and toxicological studies requires validation with a greater variety of species. This can entail the use of new or different physiological techniques, which themselves will need to be assessed.
The purpose of the present study was to examine the ability of short-term physiological estimates to predict growth rates of a gastropod, Hydrobia ulvae, in response to pollutant exposure. We selected cadmium for study, as previous investigations with a related species, H. ventrosa, revealed significant reductions in growth rate in response to exposures of 100 to 200 gg Cd 1-1 (Forbes 1991). Thus, a second aim of this investigation was to determine the relative effects of cadmium on various components of the energy budget to gain insight into the mechanistic basis of cadmium toxicity.