How Bluegill Sunfish Maneuver: Analysis of Body and Fin Motion Under a Horizontal Obstacle

Abstract

This study investigates how bluegill sunfish control fin motion to deal with the added locomotor complexity of swimming under a horizontal obstacle. While previous studies have focused on upright swimming in the forward and backward direction, in the current study blue gill sunfish rotate their bodies ninety degrees and swim on their sides in order to pass under a horizontal obstacle. The goal of this research is to develop an understanding of the orchestration of fin mechanics employed by bluegill sunfish that involve changes in speed and body rotation to navigate under a horizontal obstacle. This study analyzes how roll motion is produced and the interplay between pectoral, dorsal, anal, and caudal fins in complex maneuvering during rolling. Analysis of high-speed videography revealed a series of coordinated movements of all fins to achieve rolling, propulsion under the horizontal obstacle, and unrolling. While all fins were utilized to adjust pitch, roll, and yaw, pectoral fins were the primary locomotive fins in creating rolling and unrolling via a complex fin motion described as cupping. The fish switched from median and paired fin swimming during upright swimming to body and caudal fin swimming during sideways swimming, which drove propulsion under the horizontal obstacle. There are a variety of ways to maintain stability of the center of mass in non-linear, reverse, and sideways swimming, and these are accomplished via alterations in fin surface, beat frequency, and coordination with other fins. Beyond the application of these biomechanical principles to the field of robotics and underwater exploration, a deeper understanding of fish locomotion will help inform our knowledge of the evolutionary biology that has shaped these creatures.