PREDATION ON THE BIVALVE CHOROMYTILUS MERIDIONALIS (KR.) BY THE GASTROPOD NATICA (TECTONATICA) TECTA ANTON

Abstract

The effects of a population of the boring gastropod Natica tecta on the bivalve Choromytilus meridionalis were investigated at Bailey's Cottage, False Bay, South Africa. In July 1979 the N. tecta density on the mussel bed averaged 69 m−2 and the population consisted mainly of reproductively mature individuals between 20–33 mm shell width. Laboratory experiments on N. tecta showed that prey size selection is an increasing function of predator size. The prey size range taken by large N. tecta is also greater than that taken by small individuals. The position of the borehole on the mussel shell is a function of the way in which the shell is held by the foot during the boring process. Consumption rates measured in the laboratory showed an increase from approximately 1 kJ per week in 18 mm N. tecta to 4.5 kJ per week in 28 mm individuals. Population consumption in the field was calculated as 663 kJ m−2 month−1. It was estimated that at this rate the standing crop of mussels in the pool would be eliminated within 10 months. Field measurements showed significant depletion after 6 months. New spat settlement of mussels occur every 4–6 years. The growth curve shows that after one year the population mean size exceeds 30 mm shell length, which is beyond the prey selection size range of small N. tecta. It was concluded that at the time of a new mussel settlement a niche is provided for the simultaneous settlement and growth of juvenile N. tecta in high densities. However, within one year the increase in prey size, together with depletion due to over-exploitation, limits population growth and density in N. tecta.