Dispersal Dynamics of the Bivalve Gemma Gemma in a Patchy Environment

Abstract

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 crabs, Limulus polyphemus. Gemma broods its young and has no planktonic larval stage, so all dispersal is the result of juvenile and adult movement. Animal movement was measured using natural crab pits, hand—dug simulated crab pits, and cylindrical bottom traps in the intertidal zone at Tom's Cove, Virginia, USA. This study demonstrated that horseshoe crabs create localized patches with reduced densities of Gemma, that all sizes and ages of Gemma quickly disperse into these low density patches, and that the mechanism of dispersal is passive bedload and suspended load transport. Freshly excavated natural pits had significantly lower Gemma densities than did undisturbed background sediment, but there were no significant differences in total density of other species, number of species, and species diversity (H'). Equitability (J') was greater in pits than in controls because of the reduced abundance of Gemma, the numerically dominant species. Newly dug simulated crab pits also had significantly lower Gemma densities than controls and returned 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 support the hypothesis that Gemma dispersal is a passive transport phenomenon. Assuming no active, density—dependent movement, the product of the Gemma density frequency distribution in undisturbed background sediment and the frequency distribution of sediment volume collected per trap created a predicted Gemma frequency distribution in traps that matched 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 than winter. Dispersal rate results suggest that increased horseshoe crab disturbance in summer may cause an increase in Gemma transport. Because Gemma individuals are dispersed by hydrodynamic action, it was expected 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 dispersal research has focused on the large—scale movement and settlement patterns of invertebrate larvae. The results from this study illustrate that dispersal of bottom—dwelling juveniles and adults plays an important role in regulating the local distribution and abundance of Gemma. Previous workers have shown that young Gemma live in dense aggregations and that growth and fecundity are reduced at such high densities, leading to population crashes. This study demonstrated a mech...

Effects of sediment source and flow regime on clam and sediment transport

“…Although Mya arenaria and Gemma gemma have been found in the water column in some studies (M. arenaria, Möller 1986;G. gemma, Sellmer 1967), our data and other studies (Emerson & Grant 1991, Commito et al 1995a, Hunt & Mullineaux 2002 suggest that if sediment transport at a site can be predicted, the likelihood of transport of juvenile M. arenaria, and other bivalves whose dispersal is generally linked to sediment transport, also can be predicted. It is likely that some dispersal of juvenile M. arenaria and G. gemma does occur at low flow speeds when individuals do not burrow into the sediment.…”

“…Less information was available for G. gemma than for M. arenaria, and no relationship was detected between the weight of sediment and the proportion of G. gemma transported at Sites 1 and 3, possibly due to the smaller range of sediment erosion. Previous field studies on M. arenaria (Emerson & Grant 1991) and G. gemma (Commito et al 1995a) found positive relationships between the number of clams and the amount of sediment collected in bedload traps. At the shear velocities employed in this study, clams and sand grains rolled along the bottom as bedload and were collected in the trap.…”

Spatial and temporal variability in juvenile bivalve dispersal: effects of sediment transport and flow regime

“…Results of previous studies suggest that dispersal of Gemma gemma and Mya arenaria, which were expected to be common in the Navesink estuary, is generally linked to bedload transport (Emerson & Grant 1991, Commito et al 1995a, Hunt & Mullineaux 2002. Cylindrical traps were installed flush with the sediment surface to measure the flux of clams and sediment transported as bedload or falling out of suspension in the water column.…”

“…Emerson & Grant 1991, Commito et al 1995a,b, Turner et al 1997, Hunt & Mullineaux 2002, Lundquist et al 2006). On intertidal flats, there are positive relationships between wind conditions and flux of sediment and bivalves in sediment traps (Emerson & Grant 1991, Commito et al 1995b, Armonies 1996, Turner et al 1997.…”

Responses of infaunal populations to benthoscape structure and the potentialimportance of transition zones

“…There are few studies of infaunal population dynamics over extended spatial scales in relation to benthoscape structure, and it is not clear whether the populations form metapopulations and can be studied within a metapopulation framework. Benthic ecologists are beginning to understand processes that link populations by transport of larvae and/or adults (e.g., Dobbs and Vozarik 1983;Committo et al 1995;Bradbury and Snelgrove 2001). Larval mixing and the degree to which populations interact via larval transport can differ depending on the type of coastal environment (Keough and Black 1996).…”