Density-dependent predation, habitat variation, and the persistence of marine bivalve prey

The persistence of prey encountering intense predation varies by species, p rey density, and habitat type; however, the collective impact of these fact ors has rarely been tested experimentally in natural marine systems. Using the thin-shelled clams Mya arenaria and Macoma balthica as prey, and the ma in epibenthic predator of whole adult clams, the blue crab Callinectes sapi dus, we conducted a series of experiments in Chesapeake Bay tributaries tha t (1) links field abundance and distribution of bivalve prey species with h abitat-specific mortality patterns; (2) represents the first comprehensive field test of species-specific, habitat-specific, and density-dependent mor tality for subtidal, soft-bottom, deep-burrowing prey; and (3) thereby enab les development of a conceptual model to be used as a heuristic tool linkin g predator-prey dynamics, habitat type, and evolutionary defense tactics fo r marine benthos. In 15 years of field monitoring, Mya was more common in sand than mud habit ats, and Macoma was widely distributed and at higher densities than Mya in mud and sand. In field experiments, mortality of both Mya and Macoma was de nsity dependent in those habitats where the clams are common. The blue crab population in the field exhibited a type III "guild functional response" o n Mya in sand, and on Macoma in both mud and sand. Mortality was lower in s and than mud for Mya, and similar in mud and sand for Macoma, correlating w ith the high abundances of Mya in sand and Macoma in sand and mud. The pers istence of large juvenile and adult bivalves when confronted with intense p redation derived substantially from a low-density refuge from predation tha t varied in a species-specific manner with habitat type, demonstrating the species-specific importance of density and habitat to clam survival. We developed a conceptual model detailing the relative importance of behavi or, morphology, habitat features, and the basic components of predator-prey interactions to the survival of bivalve molluscs. At one extreme are bival ve molluscs, such as oysters, that emphasize morphological refuges that inc rease the predator's handling time. At the other extreme are bivalves, such as Mya and Macoma, that reduce predator encounter rates. The model is inte nded to be used as a heuristic tool to develop testable hypotheses.