Growth and mortality of the predatory intertidal whelk Morula Marginalba Blainville (Muricidae): The effects of different species of prey

Morula marginalba Blainville is a common predatory gastropod on rocky shores of southeastern Australia. Mean size of Morula varied in accordance with the species of prey present, both within a shore where a variety of prey were available, and among shores each dominated by a single species of prey. Growth of tagged whelks varied among populations both with respect to the asymptotic size reached by the adult whelks, and the rate of approaching this size. Sampling and short-term experiments on growth in an area with a mixed assemblage of prey confirmed these trends. Mortality also varied significantly among populations, but independently of the pattern of growth. The size structure of a population can be understood as an interaction of the mean and variance of the asymptotic size, and the rate at which animmals in any population approach this size, in comparison with the rate of mortality. Since these can vary independently in relation to different species of prey, a great range of size structures is possible in different habitats. Recruitment of Morula can vary greatly from year to year, and the success of recruitment into any one population apparently bears no relationship to success in others. These findings are discussed with regard to generalizations about population models, life-history characteristics, and predator-prey interactions.