Are swimming fish dual oscillator systems? A case study using free-swimming smooth dogfish sharks (Mustelus canis)

Fish swimming has classically been modeled as a rearwardly propagating wave of increasing amplitude and fixed frequency, based on kinematic data from large numbers of species in captivity. However, recent work on sharks swimming in natural environments has suggested that anterior and posterior body segments oscillate at different frequencies from each other. We attached accelerometer, gyroscope and magnetometer data loggers to the anterior and posterior body sections of smooth dogfish, Mustelus canis (n=4), and released these individuals in the wild. In over 25 h of recording, the anterior and posterior frequency estimates were within 0.1 Hz of each other for 97±1.9% of the time. Additionally, the phase differences between the anterior and posterior body regions were narrowly distributed, indicating that the anterior and posterior were phase locked at the same frequency, demonstrating that smooth dogfish are not dual oscillating locomotor systems. We highlight the fact that when comparing the frequency of oscillation along the body, emphasis needs to be placed on analyzing the phase difference between body parts with sufficient accuracy to test for significant differences, not just the dominant frequency.