Abstract
A marked pattern of short-term variation in heart activity is described in Scrobicularia plana (Da Costa). This is unique to Scrobicularia among the bivalves so far investigated. Heart activity, shell movement, and pumping have been recorded during suspension- and deposit-feeding activity and during inactivity. Heart activity in Scrobicularia responds markedly to gross changes in pumping level found during feeding, both heart rate and amplitude, as measured by impedance techniques, increasing rapidly when pumping commences after short periods of cessation. Heart amplitude falls rapidly and heart rate more gradually once pumping ceases. Scrobicularia shows considerable flexibility in behaviour under any given set of conditions. Tidal bradycardia occurs consistently in Scrobicularia only when the water level in the mud falls below the level of the animal in its burrow, pumping activity being maintained if surface water is present.
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