Two- and three-dimensional geometries of batoids in relation to locomotor mode

Abstract

Batoid fishes possess unique body and pectoral fin morphologies that directly affect swimming performance. Two main swimming modes are distinguished within the order Batoidea: oscillatory (mobuliform) and undulatory (rajiform) locomotion. Morphological differences between rays using these two modes were examined by comparing the two- and three-dimensional geometries of the pectoral fins. Two-dimensional planforms and sectional geometry of the pectoral fins of various batoid species were measured photometrically and using computed tomography (CT) scans, respectively. The two-dimensional data showed that mobuliform rays have high aspect ratio pectoral fins (>2.60) with a wing-like planform and an anteriorly placed center of mass. Rays that swim in the rajiform mode had a circular or diamond-shaped planform with a low aspect ratio (<2.60) and center of mass at the position of maximum fin span. From CT scans, undulatory rays exhibited an increased thickness ratio (TR=maximum sectional thickness/fin chord), indicating a body form for reduced drag, whereas oscillatory rays had fin sections with an increased shoulder position (SP=distance from rostrum to maximum thickness/fin chord). Undulatory rays showed greater cambering of their body and pectoral fins compared to the more symmetrical foil shape of oscillatory rays. The distinct morphology of oscillatory and undulatory rays is associated with the locomotory modes and the concomitant restraints of lifestyle and habitat.