Reference: Biul. Bull. 200: 9-19. (Fehruarv 2001) Three-Dimensional Analysis of Finlet Kinematics in the Chub Mackerel (Scomber japonicus) JENNIFER C. NAUEN* AND GEORGE V. LAUDER Department of Organismic ciiul Evolutionaiy Biologv, Hun-unl University; 26 Oxford Street, Cambridge. Massachusetts 02138 Abstract. Finlets, which are small non-retractable fins located on the body margins between the second dorsal and anal fins and the caudal fin of scombrid fishes, have been hypothesized to improve swimming performance. The ki-nematics of three posterior finlets of the chub mackerel, Scomber japonicus, were examined using three-dimensional measurement techniques to test hypotheses on finlet rigidity and function during steady swimming. Finlet bending and finlet planar orientation to the A:, vc, and AY planes were measured during steady swimming at 1.2 lengths s~' in a flow tank. Despite very similar morphology among the individual finlets. there was considerable variability in finlet flexure during a stroke. Several of the finlets were relatively rigid and flat (with intrafinlet angles close to 180 during the stroke), although intrafinlet angle of the proximal portion of the most posterior finlet varied considerably over the stroke and was as low as 140 midstroke. Finlets showed complex orientations in three-dimensional space over a stroke, and these orientations differed among the finlets. For example, during tail deceleration the proximal portion of the fifth finlet achieves a mean angle of approximately 75 with the A; plane, while the distal portion of this finlet is oriented at 1 10". Our data suggest that the trajectory of local water flow varies among finlets and that the most posterior finlet is oriented to redirect flow into the developing tail vortex, which may increase thrust produced by the tail of swimming mackerel. Received 30 May 2000; accepted 1 December 2000. * To whom correspondence should he addressed. E-mail: jnauen oeb.harvard.edu. Introduction Finlets are small non-retractable fins characteristic of scombrid fishes including mackerel, bonitos, and tuna (Col-lette and Nauen. 1983; Joseph el ill., 1988). The finlets are situated on the dorsal and ventral body margins adjacent to the tail (spanning the region between the second dorsal and anal fins and the caudal fin. Fig. I ). In the case of the five dorsal and five ventral finlets of the chub mackerel. Scomber japonicus (Fig. 1 ). the summed surface area of the finlets is about 15% of the surface area of the caudal fin (Nauen and Lauder, 2000). Muscles that may actively control finlet motion insert at the base of each finlet (Nauen and Lauder. 2000). Scombrid fishes are capable of high locomotory perfor-mance, including burst speeds from 18 body lengths per second (bl s~') for mackerel (Wardle and He. 1988) to up to 27 bl s~' for tuna (Fierstine and Walters, 1968; also see Magnuson, 1978), and cruising speeds from 3.5 bl s~' for mackerel (Wardle and He. 1988) to 6-10 bl s~' for tuna (Yuen. 1970; summarized in Beamish. 1978). Given the close proximity of finlets to the caudal fin. previous inves-tigators have suggested that finlets play a role in locomo-tion. Walters ( 1962) proposed that finlets direct flow longi-tudinally along the body, and Magnuson (1970) and Lindsey (1978) suggested that finlets direct flow across the caudal peduncle and caudal keels. A recent study by Nauen anu Lauder (2000) using two-dimensional (2-D) kinematic ti?\i!ysis methods to quantify the kinematics of finlets of S. jiiptmicits showed that during steady forward locomotion at speeds from 1.2 to 3.0 fork lengths (/) s~', finlet kinematics in the vertical (AT) and horizontal (.\z) plane were independent of speed. Angle of attack calculations using the kinematic measurements and the assumption that the direction of flow incident to the finlet was equal and opposite to the path of motion of the
BioStor
