| Summary: | Dorsal fins have diverse morphologies across shark species. However, little is known about the ability of sharks to control fin movements and the nature of the balance of forces during swimming. White-spotted bamboo sharks, Chiloscyllium plagiosum, and spiny dogfish, Squalus acanthias, are two unrelated species that inhabit different habitats and have strikingly different dorsal fin morphologies. Previous studies that have used a shark model in a wind tunnel have hypothesized that median fins function in stabilization during steady swimming. The dorsal fin anatomy of benthic white-spotted bamboo sharks and benthopelagic spiny dogfish reflects their different swimming habits. White-spotted bamboo sharks live in complex reef habitats and thus have requirements for high maneuverability. Spiny dogfish occupy a variety of coastal and continental shelf habitats and are known to form schools, spending more time in open water environments. Principal component analysis separates these two species in morphospace. Bamboo sharks are characterized by a second dorsal with larger area and proportionally more muscle inserting onto first and second dorsal fins in comparison to spiny dogfish. The two species also differ in the morphology of the skeletal support of the dorsal fin. Basals and radials in spiny dogfish are almost indistinguishable, while radials in bamboo sharks are more numerous and are arranged in two rows of elongate, plate-like elements that articulate with one another. The muscle of the dorsal fin in spiny dogfish is composed of one large muscle mass that extends to all of the proximal radials in spiny dogfish, while in bamboo sharks distinct individual muscle bundles insert onto the ceratotrichia overlying the distal and proximal skeletal elements. During steady swimming, the first dorsal fin in spiny dogfish move independently of the body with larger amplitudes at lower speeds, indicating a stabilizing function. The dorsal fins in bamboo sharks move in synchrony with the trunk, with a larger range of motion than dogfish at both speeds. The first and second dorsal fins of bamboo sharks move laterally in phase with the axial portion of the body, although with larger lateral displacement. Electromyography data shows simultaneous muscle activity on both sides of the first dorsal fin in spiny dogfish which is independent of speed, indicating active stiffening of this fin. The second dorsal fin in spiny dogfish and both dorsal fins in bamboos show alternating muscle activity on each side of the fin, indicating active flapping. Flow acceleration and vortex shedding were identified at the wake of the second dorsal fin in spiny dogfish and both dorsal fins in bamboo sharks using particle image velocimetry. Flow in the wake of the second dorsal was considerably decelerated. These data demonstrate that contrary to the traditional hypothesis, not all dorsal fins function as stabilizers; the first dorsal fin in spiny dogfish contributes to stability while the second dorsal fin, as well as both dorsal fins in bamboo sharks, contribute to thrust production. During maneuvers, the dorsal fin muscles are active on both sides in spiny dogfish and white-spotted bamboo sharks. Spiny dogfish have longer burst duration on the contralateral side, consistent with opposing resistance to the medium. In bamboo sharks, activation might be insufficient to counteract water resistance. The function of the dorsal fins is more flexible and diverse than first thought and might explain the morphological diversity in dorsal fins found among sharks.
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