Macrofauna associated with an introduced oyster, Pinctada radiata: Spatial scale implications of community differences

The macrozoobenthos associated with the introduced pearl oyster Pinctada radiata has been sampled at two different spatial scales of three sectors (order of hundreds of kilometres) and of eight localities (order of tens of kilometres). Moreover, the NW sector was selected to compare three localities with the presence of P. radiata (low density) and one locality where it was totally absent. The first design was hierarchical, with random localities nested within sectors; the second one was an asymmetrical factorial design, in which the presence/absence of Pinctada and hydrodynamism were considered. Similarity relationships were investigated by means of multivariate clustering, similarity percentage analysis and nm-MDS ordination; the two experimental designs have been tested by permutational MANOVA and analysis of dispersion (PERMDISP). Most of the variability of the associated zoobenthic community appeared to be mainly captured by local environmental factors; the meso-scale variability was more discriminating than differences at larger spatial scale. Large scale NW–SE biogeographic gradient may also have some effects in the assemblage composition. Although the whole arrangement of samples in the MDS plane showed a clear Bray-Curtis distance between the locality without Pinctada and all the remaining sites, pair-wise contrasts were not all significant. The factor “presence/absence” was not significant in this design, whereas the exposure was more indicative of differences in the local assemblage composition. These results may not confirm that the community structure variability is due to the impact of Pincata invasion because the potential and subtle community shift may be masked by the overwhelming influence of just the local environmental gradients. In spite of this, the introduced oyster may play the role of an engineer species at high densities, contributing to the complexity of the benthic habitat and influencing the trophic pattern of its fauna.