The roles of intertidal marine gastropod shells as secondary substrate for macroalgae

Abstract

Primary space within the marine benthos is by far the most limited resource for which all sessile organisms must compete. It only ever becomes available when the organisms occupying it dies or are forced to leave the area. It has long been established that herbivorous molluscs play an important role in freeing up space on rocky intertidal shores. However, their grazing activities, although freeing up space. often result in diminished species diversity because most intertidal algae and invertebrates are either consumed or displaced. The exploitation of microhabitats (secondary space) therefore becomes important under such scenarios and is known to increase the biodiversity of a particular area. Ironically, mollusc shells are knom1 to provide such microhabitats as they increase the available surface area for the settlement of a variety of epizoic organisms. While the international literature abounds with examples of the importance of mollusc shells as secondary substrata, virtually nothing is known of these associations within a South African context. Surveys were conducted on five more or less equidistant intertidal rocky shore communities (Kalk Bay. Rooiels, Sandbaai, Pearly Beach, Stinkbaai) within the South African western biogeographic transition zone. At all sites, invertebrate densities and biomass, and algal and lichen percent cover abundance were sampled along transect lines running more or less parallel to the shoreline from mean highwater spring (MHWS) tide mark to mean low water spring (MLWS) tide mark.