Mortality and Stability in a Low Diversity Subtropical Intertidal Community

The Crib Island, Queensland, intertidal community is dominated by three species: the filter—feeding mactrid bivalve Notospisula parva Petit, and the naticid gastropods Polinices sordidus Swainson and Polinices conicus Lamarck. Empty drilled shells suggest naticid predation as a source of mortality for all three species. Field sampling and laboratory experiments indicate that, of a total N. parva mortality of 46% during the year December 1965 to December 1966, only 4% per year was caused by naticid predation. A mortality of 28% was caused by shell—destroying predators such as skates. Interaction between high densities and rapid seasonal growth is suggested as an important source of density dependent mortality. The naticids are probably regulated by the density of their prey (N. parva), and compensate for fluctuating N. parva abundance by breeding year—round and preying on smaller individuals of their own genus.