A flume study of drift in marine infaunal amphipods (Haustoriidae)

Amphipods of the infaunal family Haustoriidae are characteristic of high-energy marine sands and occur both in the sediment and the overlying water column. Sediments in this habitat are subject to constant reworking by tidal currents, suggesting that resident amphipod populations are affected by this disturbance in a phenomenon similar to freshwater invertebrate drift. A controlled-velocity laboratory flume was used to examine the effect of haustoriid density, current velocity, illumination, and food availability on drift rates to determine a causal basis for drift. Drift is densityindependent and greatest at night and during high current flow; it is also greater at night from sterile sediment than from untreated sand. Flume transport was usually less than 10% of amphipods present in the sediment. Haustoriids captured downstream were mostly adults occurring in a 1:1 sex ratio, suggesting no obvious function of drift in reproduction. Current-induced displacement of haustoriids may produce the patchiness in distribution observed in nature. Disturbance of the bed could also function to keep amphipod densities below the carrying capacity of the local environment. In certain cases, food limitation may cause amphipods to actively leave the substrate. Under all conditions, greater drift rates in darkness are probably adaptive in avoidance of predators. Despite the nature of sediment movement in a high-energy environment, haustoriid drift may have an active as well as passive component.