The roles of predation and postlarval transport in recruitment of the soft shell clam (Mya arenaria)

Abstract

Rates of mortality and transport of newly settled benthic invertebrates can be very high and potentially have profound effects on patterns of recruitment. We used the soft shell clam Mya arenaria as a model organism to compare the magnitude of rates of postlarval transport and predation on juveniles. We hypothesized that there is a critical size below which spatial variation in abundance of juvenile Mya is hydrodynamically controlled and above which abundance is mainly influenced by mortality, particularly due to epibenthic predators. To test this hypothesis, we examined transport and predation of early juvenile Mya in Barnstable Harbor, Massachusetts, USA. A caging experiment demonstrated that exclusion of epibenthic predators had a large impact on the density and size distribution of juvenile Mya within a few weeks of settlement and disproportionately affected juveniles of >2‐mm shell length. Enclosure of the green crab Carcinus maenas changed the size distribution of Mya but did not significantly affect abundance. We attributed the effect of predator exclusion primarily to juvenile green crabs and fish. Current meter measurements suggested that tidal currents were strong enough to cause bedload transport of sediment and Mya. Measurement of gross and net rates of transport of Mya, using bedload traps and pans of defaunated sediment, confirmed that juveniles up to 5‐mm shell length were routinely redistributed by tidal currents, particularly during spring tides. The number of postlarval Mya transported varied among sites and dates and was significantly related to ambient density of Mya and sediment flux. The loss of Mya during the caging experiment was compared to the rate of net flux of Mya due to transport during an intermediate tide. This comparison suggested that the loss due to predation was considerably larger than the flux due to transport for individuals >2 mm, but that rates of the two processes were more similar for individuals <2 mm. These experiments indicate that, while predation is likely the ultimate factor controlling abundance and size of juvenile Mya at this site, transport often redistributes individuals, at least at the scale of a tidal flat (10s to 100s of m).