LABRIFORM PROPULSION IN FISHES - KINEMATICS OF FLAPPING AQUATIC FLIGHT IN THE BIRD WRASSE GOMPHOSUS-VARIUS (LABRIDAE)

Labriform, or pectoral fin, propulsion is the primary swimming mode fo r many fishes, even at high relative speeds. Although kinematic data a re critical for evaluating hydrodynamic models of propulsion, these da ta are largely lacking for labriform swimmers, especially for species that employ an exclusively labriform mode across a broad range of spee ds. We present data on pectoral fin locomotion in Gomphosus varius (La bridae), a tropical coral reef fish that uses a lift-based mechanism t o fly under water at sustained speeds of 1-6 total body lengths s(-1) (TLs(-1)). Lateral- and dorsal-view video images of three fish swimmin g in a flow tank at 1-4 TLs(-1) were recorded at 60 Hz. From the two v iews, we reconstructed the three-dimensional motion of the center of m ass, the fin tip and two fin chords for multiple fin beats of each fis h at each of four speeds. In G. varius, the fin oscillates largely up and down: the stroke plane is tilted by approximately 20 degrees from the vertical. Both frequency and the area swept by the pectoral fins i ncrease with swimming speed. Interestingly, there are individual diffe rences in how this area increases. Relative to the fish, the fin tip i n lateral view moves along the path of a thin, inclined figure-of-eigh t. Relative to a stationary observer, the fin tip traces a sawtooth pa ttern, but the teeth are recumbent (indicating net backwards movement) only at the slowest speeds. Distal fin chords pitch nose downward dur ing the downstroke and nose upward during the upstroke. Hydrodynamic a ngles of attack are largely positive during the downstroke and negativ e during the upstroke. The geometry of the fin and incident how sugges ts that the fin is generating lift with large upward and small forward components during the downstroke. The negative incident angles during the upstroke suggest that the fin is generating largely thrust during the upstroke. In general, the large thrust is combined with a downwar d force during the upstroke, but the net backwards motion of the fin a t slow speeds generates a small upward component during slow swimming. Both the alternating sign of the hydrodynamic angle of attack and the observed reduced frequencies suggest that unsteady effects are import ant in G. varius aquatic flight, especially at low speeds. This study provides a framework for the comparison of aquatic flight by fishes wi th aerial flight by birds, bats and insects.