Hydrodynamic function of dorsal fins in spiny dogfish and bamboo sharks during steady swimming

Abstract

A key feature of fish functional design is the presence of multiple fins that allow thrust vectoring and redirection of fluid momentum to contribute to both steady swimming and maneuvering. A number of previous studies have analyzed the function of dorsal fins in teleost fishes in this context, but the hydrodynamic function of dorsal fins in freely swimming sharks has not been analyzed, despite the potential for differential functional roles between the anterior and posterior dorsal fins. Previous anatomical research has suggested a primarily stabilizing role for shark dorsal fins. We evaluated the generality of this hypothesis by using time-resolved particle image velocimetry to record water flow patterns in the wake of both the anterior and posterior dorsal fins in two species of freely swimming sharks: bamboo sharks (Chiloscyllium plagiosum) and spiny dogfish (Squalus acanthias). Cross-correlation analysis of consecutive images was used to calculate stroke-averaged mean longitudinal and lateral velocity components, and vorticity. In spiny dogfish, we observed a velocity deficit in the wake of the first dorsal fin and flow acceleration behind the second dorsal fin, indicating that the first dorsal fin experiences net drag while the second dorsal fin can aid in propulsion. In contrast, the wake of both dorsal fins in bamboo sharks displayed increased net flow velocity in the majority of trials, reflecting a thrust contribution to steady swimming. In bamboo sharks, fluid flow in the wake of the second dorsal fin had higher absolute average velocity than that for first dorsal fin, and this may result from a positive vortex interaction between the first and second dorsal fins. These data suggest that the first dorsal fin in spiny dogfish has primarily a stabilizing function, while the second dorsal fin has a propulsive function. In bamboo sharks, both dorsal fins can contribute thrust and should be considered as propulsive adjuncts to the body during steady swimming. The function of shark dorsal fins can thus differ considerably among fins and species, and is not limited to a stabilizing role.

Passing the Wake: Using Multiple Fins to Shape Forces for Swimming

“…Also, of note is that these changes in mean forces occurred despite the dorsal and anal fins producing near-zero net thrust. This is not that unusual from a biological perspective, as some fish median fins also produce near-zero net thrust [5,48].…”

Survival of a grey reef shark Carcharhinus amblyrhynchos without a dorsal fin

“…Dorsal fin size tends to scale allometrically throughout ontogeny in carcharhinid sharks, implying that the fin's function is consistent over time (Irschick et al, ; Irschick & Hammerschlag, ). Studies of particle velocimetry in a bamboo shark Chiloscyllium plagiosum and spiny dogfish Squalus acanthias suggest that where the forward dorsal fin is located near the centre of mass, as it is in grey reef sharks, then its function is primarily one of stabilisation and the prevention of roll rather than thrust (Maia et al, ). A similar conclusion was drawn from wind‐tunnel experiments and analyses of forces acting on multiple fins (Harris, ).…”

Volitional Swimming Kinematics of Blacktip Sharks, Carcharhinus limbatus, in the Wild

“…For example, caudal, dorsal, and pectoral fins vary by species and can impact swimming kinematics. The bodies of sharks have been shown to operate as springs or brakes depending on the loading [59][60][61][62][63][64]. The mean swimming speed of bull sharks (C. leucas) is 0.58 bl/s and their tailbeat frequency is 0.78 Hz, whereas reef blacktip sharks (C. melanopterus) swim faster at 0.80 bl/s using a 1.13 Hz tailbeat [54].…”