Vulnerability of juvenile clams to predation by shore crabs

Abstract

Predation, especially by crabs, is a common source of mortality for natural and farmed populations of intertidal clams. Consumption of juvenile “seed clams” forces aquaculturists to try to exclude predators and/or raise juveniles in hatcheries until they can reach a size refuge. We ran a variety of lab experiments assessing vulnerabilities of juvenile clams to small, common shore crabs (Hemigrapsus spp.). Crabs <1 cm width can consume hardshell Manila clams larger than those normally used for “seed”, and can readily crush even larger softshell Mya clams. We suggest that using netting to prevent consumption by shore crabs is not practical given that smaller individuals can fit through mesh openings. Raising seed in hatcheries until they are 10–15 mm will provide a size refuge from shore crabs, but not larger cancrid crabs. Farming on beaches with little habitat (e.g., cobbles) for shore crabs can likely reduce juvenile clam mortality. A better understanding of predation threats to commercially important clams is critical, especially as the invasion of the green crab Carcinus to Washington shorelines further threatens survival of juvenile clams.