GROWTH-RESPONSES OF SUSPENSION-FEEDING BIVALVE MOLLUSKS TO CHANGES IN WATER-FLOW - DIFFERENCES BETWEEN SIPHONATE AND NONSIPHONATE TAXA

Experiments were conducted in a multiple-flume apparatus to determine the effects of water flow (current speed) on individual growth of the infaunal siphonate Mercenaria mercenaria (hard clam) and the epibenthic nonsiphonate Crassostrea virginica (eastern oyster). During summer 1990 there were no significant differences in shell growth rate or final soft tissue weights of either species over a 24-day period for mean flow treatment levels of almost-equal-to 2, 4 and 8 cm s-1 mid-depth (almost-equal-to free-stream) speeds. The overall trends were a positive relation between growth and flow speed for clams, and a negative relation for oysters. A 28-day experiment in summer 1991 tested for the effects of four flow speed levels: 0, 1, 2 and 4 cm s-1. There were substantial and marginally significant differences in shell growth rates for oysters (ANOVA, P = 0.042) and clams (ANOVA, P = 0.063). Growth response patterns were different for the two taxa, with clams showing a consistent positive relation between flow speed and growth (maximal growth at 2 to 4 cm s-1), and oysters showing maximal growth (average three-fold increase over other flow speeds) at 1 cm s-1 with decreased growth at 0 cm s-1 and > 1 cm s-1. We hypothesize that the differences in growth response patterns may be explained by differences in inhalant pumping speeds, with maximal individual growth expected for suspension-feeding bivalves at ambient flow speeds that approximate the inhalant pumping speed. Because siphonate taxa typically have greater inhalant pumping speeds, we predict that ambient flow speeds optimal for individual growth will generally be several cm s-1 greater than those for nonsiphonate taxa.