DISPERSAL DYNAMICS OF THE BIVALVE GEMMA-GEMMA IN A PATCHY ENVIRONMENT

The purpose of this study was to analyze the dispersal dynamics of the ovoviviparous bivalve Gemma gemma (hereafter referred to as Gemma) in an environment disturbed by the pit-digging activities of horseshoe c rabs, Limulus polyphemus. Gemma broods its young and has no planktonic larval stage, so all dispersal is the result of juvenile and adult mo vement. Animal movement was measured using natural crab pits, hand-dug simulated crab pits, and cylindrical bottom traps in the intertidal z one at Tom's Cove, Virginia, USA. This study demonstrated that horsesh oe crabs create localized patches with reduced densities of Gemma, tha t all sizes and ages of Gemma quickly disperse into these low density patches, and that the mechanism of dispersal is passive bedload and su spended load transport. Freshly excavated natural pits had significant ly lower Gemma densities than did undisturbed background sediment, but there were no significant differences in total density of other speci es, number of species, and species diversity (H'). Equitability (J') w as greater in pits than in controls because of the reduced abundance o f Gemma, the numerically dominant species. Newly dug simulated crab pi ts also had significantly lower Gemma densities than controls and retu rned to control levels by the next day. Density recovery trajectories for individually marked pits showed consistent responses in summer and fall, but not in winter when low Gemma abundance resulted in greater variability among pits. Significant positive correlations between the volume of sediment and the number of Gemma collected per bottom trap s upport the hypothesis that Gemma dispersal is a passive transport phen omenon. Assuming no active, density-dependent movement, the product of the Gemma density frequency distribution in undisturbed background se diment and the frequency distribution of sediment volume collected per trap created a predicted Gemma frequency distribution in traps that m atched the actual distribution. Absolute dispersal rates and relative dispersal rates (absolute dispersal rate divided by background density in undisturbed sediment) into pits and traps were greater in summer t han winter. Dispersal rate results suggest that increased horseshoe cr ab disturbance in summer may cause an increase in Gemma transport. Bec ause Gemma individuals are dispersed by hydrodynamic action, it was ex pected that small, young individuals would be most easily transported in the bedload. There was, however, little evidence that movement into pits and traps was size- or age-selective. Most recent benthic disper sal research has focused on the large-scale movement and settlement pa tterns of invertebrate larvae. The results from this study illustrate that dispersal of bottom-dwelling juveniles and adults plays an import ant role in regulating the local distribution and abundance of Gemma. Previous workers have shown that young Gemma live in dense aggregation s and that growth and fecundity are reduced at such high densities, le ading to population crashes. This study demonstrated a mechanism by wh ich Gemma disperses into low-density patches where intraspecific compe tition may be mitigated, possibly resulting in enhanced individual rep roductive success and population fitness.