Interactions between Zostera capensis and Callianassa kraussi:: influences on community composition of eelgrass beds and sandflats

Eelgrass beds have been postulated to support different faunal communities from unvegetated sandflats, in part because of the stabilisation of sediments in eelgrass beds and their destabilisation in sandflats due to bioturbation. More specifically, eelgrass beds have been hypothesised to support disproportionately greater numbers of small, flexible and non-burrowing species, and bioturbated sandflats to support more large, hardbodied burrowers. These concepts were tested in Langebaan Lagoon on the west coast of South Africa, where beds of the eelgrass Zostera capensis and sandflats dominated by Callianassa kraussi bioturbation are mutually exclusive. In eelgrass beds, sediment penetrability was significantly reduced compared to that in sandflats. Distinctly different faunal communities occurred in the two habitats, and the suite of species characteristic of each was consistent between years and between sites. Overall, density was higher in eelgrass beds than in sandflats. In contrast to previous studies, however, richness and diversity in sandflats were greater than, or equal to, values for eelgrass beds. There is convincing support for the hypothesis that burrowers are more prevalent in sandflats and less prevalent in eelgrass beds. Conversely, body flexibility does not differ between habitats and sizes are greater in the eelgrass beds than in the sandflats.