A Tail of Four Fishes: An Analysis of Kinematics and Material Properties of Elongate Fishes

Abstract

The elongate body plan is present in many groups of fishes, and this morphology dictates functional consequences seen in swimming behavior. Previous work has shown that increasing the number of vertebrae, or decreasing the intervertebral joint length, in a fixed length artificial system increases stiffness. Tails with increased stiffness can generate more power from tail beats, resulting in an increased mean swimming speed. This demonstrates the impacts of morphology on both material properties and kinematics, establishing mechanisms for form contributing to function. Here, we wanted to investigate relationships between form and ecological function, such as differences in dietary strategies and habitat preferences among fish species. This study aims to characterize and compare the kinematics, material properties, and vertebral morphology of four species of elongate fishes: Anoplarchus insignis, Anoplarchus purpurescens, Xiphister atropurpureus, and Xiphister mucosus. We hypothesized that these properties would differ among the four species due to their differential ecological niches. To calculate kinematic variables, we filmed these fishes swimming volitionally. We also measured body stiffness by bending the abdominal and tail regions of sacrificed individuals in different stages of dissection (whole body, removed skin, and removed muscle). Finally, we counted the number of vertebrae from CT scans of each species to quantify vertebral morphology. Principal component and linear discriminant analyses suggested that the elongate fish species can be distinguished from one another by their material properties, morphology, and swimming kinematics. With this information combined, we can draw connections between the physical properties of the fishes and their ecological niches.