Swimming in needlefish (Belonidae): anguilliform locomotion with fins

The Atlantic needlefish (Strongylura marina) is a unique anguilliform swimmer in that it possesses prominent fins, lives in coastal surface-waters, and can propel itself across the surface of the water to escape predators. In a laboratory flow tank, steadily swimming needlefish perform a speed-dependent suite of behaviors while maintaining at least a half wavelength of undulation on the body at all times. To investigate the effects of discrete fins on anguilliform swimming, I used high-speed video to record body and fin kinematics at swimming speeds ranging from 0.25 to 2.0 L s(-1) (where L is the total body length). Analysis of axial kinematics indicates that needlefish are less efficient anguilliform swimmers than eels, indicated by their lower slip values. Body amplitudes increase with swimming speed, but unlike most fishes, tail-beat amplitude increases linearly and does not plateau at maximal swimming speeds. At 2.0 L s(-1), the propulsive wave shortens and decelerates as it travels posteriorly, owing to the prominence of the median fins in the caudal region of the body. Analyses of fin kinematics show that at 1.0 L s(-1) the dorsal and anal fins are slightly less than 180 degrees out of phase with the body and approximately 225 degrees out of phase with the caudal fin. Needlefish exhibit two gait transitions using their pectoral fins. At 0.25 L s(-1), the pectoral fins oscillate but do not produce thrust, at 1.0 L s(-1) they are held abducted from the body, forming a positive dihedral that may reduce rolling moments, and above 2.0 L s(-1) they remain completely adducted.