Invertebrate drift: Behavioral experiments with intertidal meiobenthos†

Abstract

Water column drift of benthic invertebrates may be the result of either active migration from the bottom or of erosion by water currents. Recent studies have shown that marine meiobenthos occur regularly in the water column; however, whether this is due to active (behavioral) processes or passive (erosional) processes is not known. A series of experiments were performed that showed meiobenthos do not regularly emerge, suggesting that drift of meiobenthos is due to erosion. The behavior of the animals can, however, influence their susceptibility to suspension. Fauna most active on the sediment surface (primarily harpacticoid copepods) were most susceptible to passive suspension. A multi‐factorial experiment tested the effects of time of day (light vs. dark) and tide (high, low, flow) on number of animals active on the sediment surface. Animals frequenting the sediment surface did so only in the absence of currents and regardless of time of day. Animals began to burrow as soon as flow increased. If they were suspended, copepods (except Microarthridion littorale) and turbellarians quickly oriented and swam back toward the sediment surface using geotactic and phototactic cues. Greater sinking rates of active vs. anesthetized animals indicated positive geotaxis. Negative phototaxis was demonstrated experimentally when copepods oriented toward dark if given a light/dark choice. Suspended nematodes and ostracods relied solely on passive settlement. In contrast to some freshwater stream‐drift invertebrates, marine meiobenthos in this study did not have nocturnal activity patterns; however, as in stream fauna, species‐specific behavior probably plays a crucial role in determining abundances and diversity of meiobenthic drift fauna.