Size-related hydrodynamic characteristics of the giant scallop, Placopecten magellanicus (Bivalvia: Pectinidae)

Abstract

The giant scallop, Placopecten magellanicus (Gmelin, 1791), exhibits a three-stage life history: an early sedentary phase with byssal attachment (1–30 mm shell height), a motile phase (30–100 mm), and a later, sedentary phase, recessed in the bottom (100–160 mm). If hydrodynamics have an adaptive advantage, ontogeny of form and functional kinetics should reflect changes in life stage, and medium-sized Placopecten would be the best swimmers, with optimum hydrodynamic characteristics. Comparison of morphological characteristics of P. magellanicus in relation to size (5–160 mm) suggested that peak hydrodynamic efficiency for this species occurs between 40 and 80 mm shell height. Fineness, aspect ratio, wing loading, and potential power relationships were optimal in this range. Underwater video sequences of swimming Placopecten revealed that the angle of valve opening during sustained level flight averaged 9° per contraction and calculated jet volume expelled per unit weight peaked at 55 mm shell height. Swimming Placopecten between 30 and 100 mm exhibited a steep angle of attack only during takeoff and then swam level at velocities of 30–60 cm/s (4–9 body lengths/s), with associated Reynolds numbers of 0.6–7.2 × 104. Swimming speed, both absolute and adjusted for body length, was maximal between 50 and 70 mm shell height. Calculated Reynolds numbers increased for individuals between 30 and 70 mm but remained constant for those over 70 mm.