Abstract
Despite that boxfishes have a rigid carapace that restricts body undulation, they are highly manoeuvrable and manage to swim with remarkably dynamic stability. Recent research has indicated that the rigid body shape of boxfishes shows an inherently unstable response in its rotations caused by course-disturbing flows. Hence, any net stabilizing effect should come from the fishes' fins. The aim of the current study was to determine the effect of the surface area and orientation of the caudal fin on the yaw torque exerted on the yellow boxfish, Ostracion cubicus, a square cross-sectional shaped species of boxfish. Yaw torques quantified in a flow tank using a physical model with an attachable closed or open caudal fin at different body and tail angles and at different water flow speeds showed that the caudal fin is crucial for controlling yaw. These flow tank results were confirmed by computational fluid dynamics simulations. The caudal fin acts as both a course-stabilizer and rudder for the naturally unstable rigid body with regard to yaw. Boxfishes seem to use the interaction of the unstable body and active changes in the shape and orientation of the caudal fin to modulate manoeuvrability and stability.
Highlights
Boxfishes (Ostraciidae; Tetraodontiformes), a family of marine fishes, have an exceptional morphology [7,8,9,10]
The aim of the current study was to determine the effect of the surface area and orientation of the caudal fin on the yaw torque exerted on the yellow boxfish, Ostracion cubicus, a square cross-sectional shaped species of boxfish
Ostracion cubicus inhabits the waters of tropical coral reefs and manoeuvres in crevices and around reef, rock and sand patches feeding on a variety of benthic organisms [24]
Summary
Boxfishes (Ostraciidae; Tetraodontiformes), a family of marine fishes, have an exceptional morphology [7,8,9,10]. The abilities of boxfishes to hover, turn in place and even swim upside down are of great importance for the boxfishes’ fitness and survival Despite this unique bauplan, boxfishes are able to execute low recoil motions with near-zero turning radius [11,13,21,23]. The hypothesis concerning overall course stabilization by the carapace was rejected by Van Wassenbergh et al [11] They showed that the body-induced vortices exist, but that the overall impact on the pitch or yaw torque balance is relatively small: the body (i.e. carapace with a truncated part of the caudal peduncle) of both the yellow boxfish Ostracion cubicus Linnaeus, 1758 and smooth trunkfish Lactophrys triqueter (Linnaeus, 1758) generates destabilizing hydrodynamic torques (i.e. are naturally hydrodynamically unstable). The caudal fin is closed when turning and rotating [19], and the tail rotates in opposite direction compared with the body [20]
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