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The Knysna Estuary is a large, clear-water embayment and one of the most important estuaries for conservation in southern Africa. The estuary is detritus-dominated and the benthic food webs have low diversities of carnivores, which may make the estuary vulnerable to invasions through the empty niche hypothesis. In the last two decades, the Knysna Estuary has experienced an increase in the population of the cryptogenic and apparently omnivorous estuarine moonshine worm (Diopatra aciculata). Our goal was to describe the trophic role of D. aciculata. We measured carbon and nitrogen stable isotopes in all the common macrophytes and macroinvertebrates across four seasons to 1) establish the isotopic landscapes of three disparate sites within the Knysna Estuary where D. aciculata occurs, 2) establish whether the trophic level of D. aciculata matches those of other common macroinvertebrates, and 3) establish whether the isotopic niche of D. aciculata overlaps with those of syntopic macroinvertebrates. Communities at the different sites differed in food web width, most likely due to differences in primary producer diversity. Our findings suggested that D. aciculata is a facultative carnivore, and some sub-populations are sustained primarily by animal tissues. Isotopic niche overlap analysis showed that direct competition with other common macroinvertebrate consumers is doubtful and D. aciculata occupies a unique niche within the estuary. [Display omitted] • Trophic level and isotopic niche of baitworm across space and time. • Spatio-temporally variable niche with minimal overlap with common syntopic consumers. • Estuarine moonshine worm is facultative carnivore. [ABSTRACT FROM AUTHOR]

Dispersal and observed genetic structure of sessile marine invertebrates such as the alien polydorid polychaete, Polydora hoplura , which is associated with cultured molluscs, may be influenced by biogeographic barriers and human-mediated dispersal. To test this hypothesis, we compared the population genetic structure of P. hoplura to that of Boccardia polybranchia since both species have similar larval developmental modes (and consequently natural dispersal potential) and distribution ranges in southern Africa. However, since B. polybranchia is not a pest, its population structure should reflect the effects of only biogeographic barriers on natural dispersal. Boccardia polybranchia was collected from five wild sites along the South African coast, and P. hoplura from seven wild sites and four oyster farms; two each were on-shore and off-shore. Analyses of mtDNA Cyt b sequence data indicate two distinct lineages for B. polybranchia , supporting Cape Point as a biogeographic barrier to the dispersal of larvae. In contrast, no distinct geographic lineages were identified for P. hoplura occurring in the wild, and haplotype sharing suggest possible cross infestation between farm and nearby wild sites. At farms the genetic diversity of P. hoplura is lower but also more structured, suggesting separate anthropogenic introductions at each site possibly from genetically distinct source populations. Furthermore, more limited genetic sharing was detected between worms from on-shore and nearby wild sites, whereas the opposite was observed for off-shore farms and associated wild sites. This suggests that populations in on-shore farms are more contained with most of the individuals carrying parasites corresponding to the source of introduction while those on off-shore farms display much more genetic exchange with wild populations. Thus oysters in off-shore facilities are probably more vulnerable to infestation by polydorid pests than are those in on-shore facilities. Statement of relevance Movement of oysters spreads associated pests [ABSTRACT FROM AUTHOR]