Interactive effects of shade and surface orientation on the recruitment ofspirorbid polychaetes

A series of observations and an experiment were carried out to test hypothe ses about the effects of shade on the densities of spirorbid polychaetes (N eodexiospira spp.) on intertidal pneumatophores (mangrove roots) of Avicenn ia marina. Densities of spirorbids were greater on pneumatophores surrounde d by seagrass (Zostera mucronata) than patches without seagrass. Within pat ches of seagrass, the density and survivorship of spirorbids on pneumatopho res was greater near the substratum (covered by seagrass) than high above t he substratum (not covered by seagrass). The model that these patterns of a bundance are explained by greater recruitment of spirorbids to shaded surfa ces was assessed. This was done by experimentally testing the hypothesis th at recruitment to patches without seagrass would not differ between the upp er (unshaded) and lower surfaces (unshaded) of clear plastic sheets, but wo uld be greater on the lower surfaces (shaded) than upper surfaces (unshaded ) of black plastic sheets. Recruitment was consistent with these prediction s and therefore provided evidence that differences in densities of spirorbi ds between substrata with and without seagrass may be due largely to differ ences in shading.