THE INTERACTING EFFECTS OF SIPHON NIPPING AND HABITAT ON BIVALVE (MERCENARIA-MERCENARIA (L)) GROWTH IN A SUBTROPICAL SEAGRASS (HALODULE-WRIGHTII ASCHERS) MEADOW

Seagrasses may provide protection from predation while also influencing food supply for suspension-feeders by reducing water flow and changing rates of particle deposition. However, prior studies have shown both increased and decreased rates of bivalve growth in seagrass habitats relative to unvegetated substrata. We examined the hypothesis that variation in rates of siphon nipping and its consequences might partially explain these conflicting results. A 2-month field experiment was undertaken to examine the interacting effects of habitat (sand vs. grass) and siphon nipping on the growth of the suspension-feeding bivalve Mercenaria mercenaria (L). To simulate the effects of siphon nipping on growth we anesthetized clams and nipped (excised) a standardized portion of their siphons. We compared shell growth in length, axis of maximum growth and breadth of siphon-nipped vs. unnipped clams in enclosures both in seagrass (Halodule wrightii Aschers) and adjacent sand habitats in inshore Alabama waters. Significant growth differences in mean clam length and axis of maximum growth were observed with growth greatest in unnipped sand individuals, followed by unnipped grass individuals, nipped sand individuals and nipped grass individuals. By two-way ANOVA (main effects of siphon condition and habitat type) only siphon condition was significant. For covariate-adjusted means using initial size as the covariate, ordered results were identical, except for nipped clams in grass which did not differ significantly from those in sand. Minor differences were detected among treatments using clam breadth. Because excised siphon tissue accounted for only a few percent of the total dry tissue weight, we hypothesize that regeneration costs and reduced feeding efficiency together may explain the significant differences observed between nipped and unnipped treatments. Our results may explain previous conflicting accounts which did not control for potential differences in siphon nipping.