HYDRODYNAMIC CONTROL OF FILTRATION IN PLACOPECTEN-MAGELLANICUS

Filtration responses of the individual giant scallop, Placopecten magellanicus, at the same seston concentration are a unimodal function of velocity when these responses are assessed over a sufficiently wide range of current speeds. Based on ambient velocities measured just in front of the inhalant opening the following responses occurred when 10(4) cells.ml-1 of Chroomonas salinus were offered as food: (a) at <3 cm.s-1 localized seston depletion around the scallop occurred and small increments in velocity partially removed depletion, resulting in enhanced feeding; (b) at optimum velocity (approximately 3-6 cm.s-1), seston depletion was absent and filtration was unaffected by increments in ambient velocity; (c)at >6 cm.s-1, further increments of velocity reduced the pump power and hence filtration rate; and (d)at high velocities (>30 cm.s-1), the scallop completely closed its valves and mantle openings. Four competing theories to explain the mechanism of velocity-inhibited feeding (c, in the above scheme) were experimentally examined. Results support two hypotheses derived from those theories: (1) that valves and mantle wall partially close/open in response to ambient velocity, thereby avoiding overloading the filtration surfaces and limiting filtration; (2) that valves and mantle wall partially close/open in response to the ambient seston concentration available, there being a characteristic optimum concentration. Our results imply central physiological control of filtration and hence feeding in the giant scallop in response to environmental changes in velocity and seston concentration.