Reduction of sea star predation by the burrowing response of the hard clam Mercenaria mercenaria (Mollusca: Bivalvia)

The hard clam Mercenaria mercenaria burrows deeper into the sediment when the predaceous sea star, Asterias forbesi is present. The supposition that this increase in burial depth represents an escape response designed to reduce predation was tested experimentally by regulating clam burial depth through manipulation of the amount of sediment available for burial. Mercenaria maintained at zero depth were eaten by Asterias at greater rates than those held at ordinary burial depths (2.5–3.0 cm). These clams in turn were eaten at greater rates than those maintained at escape depths (4.0–4.5 cm). The results unambiguously establish an anti-predator function for the burrowing response, as well as underscoring the protective function of the fossorial habit. They are not confounded by behavioral predator food preferences, inherent differences between prey species, or debilitating side effects of preventing prey from escaping. Mechanisms by which the burrowing response may reduce predation are discussed and observations on the unreported clam-digging behavior of Asterias forbesi are presented.