Edinburgh Napier University

  Many soft-bottom benthic invertebrates display sediment-associated patterns of dispersion. Habitat selection experiments have shown that the larvae of some species can choose to settle in favourable habitats, and this process could establish patterns observed in the field. However, many soft-bottom infauna, including the bivalves Cerastoderma edule and Macoma balthica, show postlarval relocation. Such movements could obviate patterns established at initial settlement, and active habitat selection in these postlarval stages may be responsible for adult distributions. We investigated sediment selection in postlarval stages of these bivalves on an intertidal mudflat using a field survey and experiment. Our 1225 m2 grid survey showed a strong spatial correlation between the smallest individuals of both species, which showed no correlation with sediment characteristics. Larger size groups showed increasingly strong associations with sediment type: C. edule with sandy sediments and M. balthica with muddy. In our sediment choice experiment, the numbers of bivalves recruited into muddy, sandy and muddy depression (to detect small-scale hydrodynamic effects) treatments after 2 days in the field were recorded. There were no significant treatment effects in either C. edule or M. balthica. All bivalves were too large to be initial settlers; they were thus relocating individuals. Neither the survey nor the experiment provided any evidence that relocating individuals of these two species could choose the sediment in which they alight. The sediment-related patterns observed in adults of these species must therefore be due to other mechanisms, possibly postsettlement predation by Carcinus maenas and/or an ability to leave less-preferred sediments in a series of migrations until the preferred habitat is found.