Effects of oyster death and shell disarticulation on associated communities of epibiota

Oyster mortality and subsequent degradation of shell matrices may influence associated epibiota by modifying processes of filtration and biodeposition and by changing habitat structure. In the Hawkesbury River, NSW, Australia, QX disease devastated aquaculture populations of the native Sydney rock oyster, Saccostrea glomerata, and threatened wild populations. To ascertain effects of this oyster mortality on associated epibiota, we compared epifaunal communities occupying 100% oyster shell cover among 5 sites along the estuary, ranging from 25 ppt salinity and live oyster density of 96 ± 13 m^{− 2}, to 32 ppt salinity and live oyster density of 3187 ± 233 m^{− 2}. Epifaunal richness was greatest closest to the estuarine mouth, where live oyster abundance was greatest. Epifaunal abundance, by contrast, generally increased with distance upstream, as oyster matrices were increasingly dominated by the shells of dead oysters. To ascertain the role of live and dead oysters as habitat providers, we carried out a manipulative experiment to test the hypothesis that the epifaunal assemblages that settle over a 4 month period will significantly differ among constructed matrices comprised of live, whole dead and/or degraded (single valve) oysters. Treatments containing a component of single valves, which increase the availability of interstitial spaces and provide greater surface area for attachment, supported the greatest numbers of epibiota. Matrices comprised solely of live oysters supported fewest species and numbers of organisms. Results demonstrate that death and degradation of oysters alter the structure of associated communities, even where 100% cover of shell matrix is maintained. These results have important ramifications for management strategies and retaining estuarine biodiversity in the event that disease such as QX causes local oyster extinctions.