Size-specific predation by dominant consumers maintains a 'trophic cul-de-sac'

Manipulations of fully grown (>60.0 mm.) detritivorous Sydney mud whelks Pyrazus ebeninus Brugiere 1972 suggest that the species serves as a 'trophic cul-de-sac', limiting flow of carbon from producers to top consumers. However, although large P. ebeninus individuals experience negligible predation, smaller individuals that do not cause the same depletion of primary and secondary producers may suffer predatory losses. To assess the role that predation on the small whelks might play in weakening the trophic cul-de-sac provided by larger conspecifics, we investigated size- and habitat-specific patterns of predation on P. ebeninus through a series of field and laboratory experiments. Field tethering of 3 size classes of snail indicated that irrespective of seasonal differences in predation intensity, small (30.1 to 40.0 mm. shell height, SH) individuals experienced significantly greater predatory mortality (25% over 7 wk) than medium (50.1 to 60.0 mm. SH: 7.5%) or large (70.1 to 80.0 mm SH: 2%) conspecifics. Predatory mortality was largely attributable to naticid predators, although several tethered snails were crushed, perhaps by elasmobranchs, toadfish or crabs. For some size classes there were differences in predatory mortality between low shore mudflats and higher shore mangrove forests, but the direction of these differences varied between autumn and spring, and the differences disappeared altogether when densities of predators were held constant, indicating that they were not driven by habitat per se. In laboratory experiments, the relative contributions of small, medium and large P. ebeninus individuals to the total prey consumed by the naticid gastropod Conuber sordidus Swainson 1821 were similar between choice and no-choice experiments, and even in the absence of small prey items, large P. ebeninus snails were not consumed. Thus, even when there is depletion of small size classes of P. ebeninus, it is unlikely that common benthic predators would consume sufficient numbers of large snails to prevent deleterious effects of this species on primary and secondary production, To the contrary, predatory mortality of small P. ebeninus individuals may ensure that its populations continue to be dominated by the large, damage-causing size classes.