Competitive grazers and the predatory whelk Lepsiella flindersi (Gastropoda : Muricidae) structure a mussel bed (Xenostrobus pulex) on a southwest Australian shore

On a southwest Australian exposed rocky shore within King George Sound, the predatory muricid Dicathais orbita was observed feeding non-selectively on an array of sessile prey, including Austromega-balanus nigrescens, Galeola ria caespitosa, Patelloida nigrosulcata, Serpulorbis sipho, Siphonaria sp. and the mussel Xenostrobus pulex. On a more sheltered, contiguous, platform of the same shore, however, a mat of Xenostrobus pulex occurred which was being predated by another muricid, Lepsiella flindersi. X. pulex occupied a wide vertical range on this shore . Higher-zoned, supralittoral, individuals were small and occurred as clust ers in crevices and pits in the rock. A mat of mussels in the lower eulitto ral zone was heavily predated by the drilling L. flindersi which attacked s electively the largest individuals. Between, in the high eulittoral, the mu ssel bed was formed into patches which were isolated by the grazing activit ies of an array of herbivorous gastropods. Recruitment here by X. pulex was to the patch edges and the grazer's habits of clustering against them, fur ther kept them discrete. Little predation by L. flindersi occurred at this shore level. The Xenostrobus pulex community was, therefore, highly structured down the shore sequentially by: (a), physiological stresses; (b), competitive grazin g pressures and (c), predation.