Malcolm S. Gordon

Hydrodynamic interactions in bony fishes between respiratory fluid flows leaving the opercular openings and simultaneous flows generated by movements of downstream pectoral fins are both poorly understood and likely to be complex. Labriform-swimming fishes that swim primarily by moving only their pectoral fins are good subjects for these studies. We performed a computational fluid dynamics investigation of a simplified 2D model of these interactions based on previously published experimental observations of both respiratory and pectoral fin movements under both resting and slow, steady swimming conditions in two similar labriform swimmers: the bluegill sunfish (L. macrochirus) and the largemouth bass (M. salmoides). We carried out a parametric study investigating the effects that swimming speed, strength of opercular flow and phase difference between the pectoral fin motion and the opercular opening and closing have on the thrust and sideslip forces generated by the pectoral fins during both the abduction and adduction portions of the fin movement cycle. We analyzed pressure distributions on the fin surface to determine physical differences in flows with and without opercular jets. The modeling indicates that complex flow structures emerge from the coupling between the opercular jets and vortex shedding from pectoral fins. The jets from the opercular openings appear to exert significant influence on the forces generated by the fins; they are potentially significant in the maneuverability of at least some labriform swimmers. The numerical simulations and the analysis establish a framework for the study of these interactions in various labriform swimmers in a variety of flow regimes. Similar situations in groups of fishes using other swimming modes should also be investigated.

A few broad general questions have been central to the study of the dynamics and energetics of animal swimming and flying since the field began. A partial list includes:

Boxfishes (family Ostraciidae) are tropical reef-dwelling marine bony fishes that have about three-fourths of their body length encased in a rigid bony test. As a result, almost all of their swimming movements derive from complex combinations of movements of their median and paired fins (MPF locomotion). In terms of both body design and swimming performance, they are among the most sophisticated examples known of naturally evolved vertebrate autonomous underwater vehicles. Quantitative studies of swimming performance, biomechanics, and energetics in one model species have shown that (i) they are surprisingly strong, fast swimmers with great endurance; (ii) classical descriptions of how they swim were incomplete: they swim at different speeds using three different gaits; (iii) they are unusually dynamically well controlled and stable during sustained and prolonged rectilinear swimming; and (iv) despite unusually high parasite (fuselage) drag, they show energetic costs of transport indistinguishable from those of much better streamlined fishes using body and caudal fin (BCF) swimming modes at similar water temperatures and over comparable ranges of swimming speeds. We summarize an analysis of these properties based on a dynamic model of swimming in these fishes. This model accounts for their control, stability, and efficiency in moving through the water at moderate speeds in terms of gait changes, of water-flow patterns over body surfaces, and of complex interactions of thrust vectors generated by fin movements.