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 GeorgeSound, the predatory muricid Dicathais orbita was observed feedingnon-selectively on an array of sessile prey, including Austromegabalanusnigrescens, Galeolaria caespitosa, Patelloida nigrosulcata,Serpulorbis sipho, Siphonaria sp. and the mussel Xenostrobuspulex. On a more sheltered, contiguous, platform of the same shore,however, a mat of Xenostrobus pulex occurred which was beingpredated by another muricid, Lepsiella flindersi. X. pulex occupieda wide vertical range on this shore. Higher-zoned, supralittoral,individuals were small and occurred as clusters in crevicesand pits in the rock. A mat of mussels in the lower eulittoralzone was heavily predated by the drilling L. flindersi whichattacked selectively the largest individuals. Between, in thehigh eulittoral, the mussel bed was formed into patches whichwere isolated by the grazing activities of an array of herbivorousgastropods. Recruitment here by X. pulex was to the patch edgesand the grazer's habits of clustering against them, furtherkept them discrete. Little predation by L. flindersi occurredat this shore level. The Xenostrobus pulex community was, therefore, highly structureddown the shore sequentially by: (a), physiological stresses;(b), competitive grazing pressures and (c), predation. (Received 2 February 1998; accepted 5 January 1999)