Water flow analysis and particle capture in ciliary suspension-feeding scallops (Pectinidae)

Abstract

Gill structures in scallops were examined by scanning electron microscopy (SEM), and gill function and particle capture were studied by means of video-microscope observations and numerical calculations of flow patterns. Observations of particle movements were made on intact gills of Aequipecten opercularis (with the adductor muscle cut through) in order to map paths and speeds, and on isolated gill preparations (stimulated with serotonin [5-HT] to restore the activity of the water-pumping lateral cilia) in order to study cilia activity and interaction of particles with cilia. Video recordings of intact plicate gills of A. opercularis showed that particles approach the frontal side of the ordinary gill filaments at angles near 90° and that particles may be deflected from the through flow to 'jump' down across these filaments towards the principal filament. Observations of isolated gill filaments indicate that the pro-laterofrontal cilia ahead of the water-pumping cilia may 'push' suspended particles, or fluid with particles, back against the current, either onto the frontal side of the filament, or out into the downward-directed water flow in the funnel between 2 crests. In order to set up a computational model to illustrate flow patterns, the volume flow rate through the interfilamental canals was estimated from experimentally determined filtration rates of undisturbed scallops. The filtration rate per cm lateral ciliary band was ∼0.25 ml h -1 , implying a mean velocity in canals of ∼0.35 mm s -1 (5°C). Two-dimensional numerical flow calculations illustrate the global flow pattern in the funnel between 2 plicate gill crests, which show the contraction and acceleration of the central downflow towards the principal filament, while the inflow to the interfilament canals in the absence of pro-laterofrontal cilia is smoothly aligned at angles near 90°. However, when cilia are present, in various modes of simulated beat, local calculations show significant disturbances reaching well into the downflow. These results suggest potential flow-driven mechanisms responsible for transfer of particles to the frontal sides of ordinary filaments or into the main downward-directed flow between 2 plicate gill crests, contributing to the observed 'jumps'.