HYDRODYNAMIC DISTURBANCES PRODUCED BY SMALL ZOOPLANKTON - CASE-STUDY FOR THE VELIGER LARVA OF A BIVALVE MOLLUSK

Zooplankton produce hydrodynamic disturbances during swimming and feeding that enlarge their perceptive volume. From the standpoint of both prey and predators, fluid disturbances increase the probability that an organism is detected, identified and reacted to within appropriate time and space scales. Morphology and kinematics dictate the magnitude, symmetry and attenuation of disturbances in the fluid medium. Therefore, fluid disturbances may be species and age (size) specific. Normal and high-speed video microscopy was used to study flow-field generation by free-swimming and tethered bivalve larvae. These organisms swim and feed using many highly coordinated and symmetrically distributed appendages (i.e. cilia). Larvae tethered in flow at various free stream velocities (U-0), simulating swimming activity, induced particle trajectories approximately parallel to the organism's dorso-ventral axis. Velocity (v) and acceleration (a) were symmetrical in the transverse plane and asymmetrical in the vertical plane. Greatest velocity magnitudes (similar to 7, 3 and 6 mm s(-1)) occurred dorsal to the velum and attenuated with source distance (r) as 1/r, 1/r(1.9) and 1/r(2.9) at U-0 = 0, 3.1 and 6.4 mm s(-1), respectively. For a larva in flow, but with velum retracted, simulating sinking, velocity attenuated at 1/r towards the organism. Mean velocity gradients were on the order of 3, 8 and 10 s(-1) for swimming, sinking and hovering larvae, respectively. The high-frequency (22 Hz) component of particle velocity past free-swimming larvae was due to beat frequency of the velar cilia. This attenuated rapidly with r leaving only low-frequency (1-3 Hz) disturbances 0.1 mm beyond the tips of the cilia. Comparisons of the kinetic energy dissipation rate for turbulence in coastal waters with the kinetic energy of laminar now fields implied possible dominance of the flow field of hovering, but not swimming, larvae to at least three body diameters from the organism (similar to 1 mm). These differences in flow fields have important implications for larval survival. The perceptive volume of a hovering larva will be 40-foId greater than that of a swimming or sinking larva. However, a hovering larva is also more likely to be detected by a potential predator that uses mechanosensory organs to locate prey.