Influences of predatory polychaetes and epibenthic predators on the structure of a soft-bottom community in a maine estuary

Field manipulations were used to determine the importance of two predatory polychaetes, Nereis virens Sars and Glycera dibranchiata Ehlers, and epibenthic predators in structuring an intertidal soft-bottom community in Maine. Epibenthic predators were excluded from portions of the soft bottom using cages which also enclosed elevated densities of the predatory polychaetes. The experiments ran 10 wk and 20 wk beginning in June 1979. Exclusion of epibenthic predators had no effect on infaunal densities after 10 wk but produced ≈ 1.5-fold increase in total density after 20 wk. Since gulls (Larus spp.) avoided all cages, including those not designed to exclude epibenthic predators, the effect of gull predation on infaunal abundances was not tested using exclusion cages. Crabs, Carcinus maenas (Linnaeus) and Cancer irroratus Say were observed in cages not designed to exclude predators.Densities of Nephtys incisa Malmgren, Polydora ligni Webster, Streblospio benedicti Webster, Scoloplos robustus Verrill, phyllodocids, and bivalves were highest in cages containing elevated Glycera dibranchiata density and lowest in cages containing elevated Nereis virens density. N. virens was the only taxon whose abundance was reduced in the presence of Glycera dibranchiata which may account for high infaunal densities in the G. dibranchiata treatment. Laboratory experiments demonstrated that G. dibranchiata are capable of preying on Nereis virens. Additional field experiments revealed that the presence of N. Virens reduced the abundance of some taxa within the first 10 days of colonization. N. virens may have reduced infaunal densities by predation and/or disturbance at the sediment surface. These results suggest that complex interactions within the infauna are important in structuring marine soft-bottom communities.