Water Flows Around the Comb Plates of the Ctenophore <i>Pleurobrachia</i> Plotted by Computer: A Model System for Studying Propulsion by Antiplectic Metachronism

Abstract

ABSTRACT Patterns of water flow around steadily beating comb plates of Pleurobrachia pileus were tracked using suspended plastic beads. The positions of the beads and the comb plates in the plane of the central longitudinal axis of the comb row were digitised from high-speed ciné films covering several beat cycles. All of the data from each sequence were combined using a computer program which integrated them into a standard cycle, and the resulting data were plotted by a second computer program to produce charts for different stages in the beat cycle showing the flow velocity at a grid of points. On these charts, contour maps were drawn to indicate the speed and direction of the water flow. Water is drawn towards each comb row from ahead and from the sides and accelerates strongly backwards in a fairly narrow stream which joins those from the other seven comb rows at the rear of the animal. At a beat frequency of 10Hz the comb plates move with a tip speed of up to 70mm s−1 in their effective stroke; they have an estimated Reynolds number of 9 in this stroke. Changes in inter-plate volume between adjacent antiplectically coordinated plates are very important in propulsion, particularly near the end of the effective stroke when pairs of adjacent plates close together and cause the high-speed water from around the ciliary tips to be shed into the overlying stream as a series of jets at speeds of 50mms−1 or more. The antiplectic coordination of the comb plates makes a major contribution to the efficiency of propulsion.