Effects of epibenthic predators in flow: transport and mortality of juveniles of the soft shell clam Mya arenaria

摘要

During the early post-settlement period, dispersal and predation can drastically alter patterns of distribution and abundance of benthic invertebrates. These 2 processes may interact if predators cause disturbance and alter rates of transport of juvenile clams. In this study, experiments were conducted in an annular flume to examine the effect of 2 epibenthic predators, the seven-spine bay shrimp Crangon septemspinosa and juveniles of the green crab Carcinus maenas, on rates of erosion and mortality of juvenile soft shell clams Mya arenaria. Clams were exposed to 2 flow speeds (u = 9 cm s~(-1), shear velocity u_* = 0.30 cm s~(-1) and u = 27 cm s~(-1), u_* = 0.97 cm s~(-1)) in the presence and absence of predators. Erosion of sediment and clams, and formation of ripples occurred at the high-flow velocity only in the presence of shrimp. This also occurred when shrimp disturbed the sediment prior to the experimental run, suggesting that shrimp activity changed bottom topography and the erosion threshold of the sediment. Juvenile green crabs caused less disturbance of the sediment and their activities did not result in transport of sediment or clams. Gut content analysis indicated that juvenile green crabs preyed on juvenile M. arenaria. Shrimp preyed on clams in 1 of 2 experiments, probably due to a difference in size of clams between experiments (shell length = 1.0 vs. 3.0 mm). This study demonstrates that, in addition to causing mortality, predators can indirectly affect their prey by causing transport of sediment and associated small benthic invertebrates.