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1. Bluegill Sunfish Kinematics: How Gaits Affect Swimming Efficiency

   Parker, J. ; Morris, C. ; Duncan, M. ; Wood, B.M. ( April 2022 , The University of Montana Western 15th Annual Research Symposium)

   Locomotion is vital to the survival and fitness of animals and dominates daily energy budgets. The main energy consuming process of locomotion is the muscle activity needed to maintain stability or generate propulsive forces. In fish, the speed of swimming is thought to depend on the gait type, which may reflect an energetically efficient locomotory behavior. Bluegill Sunfish (Lepomis macrochirus) exhibit either steady or intermittent (burst-coast) gaits when swimming in the field, but whether these gaits differ in their energetic efficiency is unknown. We analyzed the electromyography (EMG) of oxidative muscle in Bluegill swimming at low velocities to determine if steady swimming is more or less energetically efficient than intermittent swimming. EMG data were acquired using bipolar fine wire electrodes implanted into oxidative musculature at 2/3 tail length. Steady swimming EMGs were recorded in a flume (fish treadmill) at incrementally increasing speeds relative to body length, until nonoxidative muscle was recruited. As speed increased, EMG intensity increased, which corresponds to increased muscle recruitment. Fish reached maximum EMG intensity (100% oxidative muscle capacity) between 1.75 - 2.25 BL/s. Intermittent swimming EMGs were recorded while the fish swam volitionally in a pool. The burst phase consisted of 2-3 tailbeats, followed by a coast phase duration of 1 second or less. Based on preliminary results, fish in the pool swam at an average of 62.1% (n = 10) of their maximum oxidative capacity. When intermittently swimming, muscle activity was 37.9% more efficient than steady swimming at similar speeds. This demonstrates that when swimming volitionally Bluegill choose the most energetically effective gait. However, further analysis is needed to determine how individual variation affects swimming performance. Continued comparison of these methods of locomotion will broaden the understanding of energy decisions that fish make. These results suggest that intermittent swimming is the more energetically efficient form of aquatic locomotion. This work is supported by NSF grant award number 2135851. 

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2. Intermittent Swimming Kinematics of Bluegill Sunfish (Lepomis macrochirus): Energetics versus Maneuverability

   Morris, C ; Coughlin, D ; Pfister, A ; Reynolds, Z ; Ellerby, D ; Wood, BM ( July 2023 , Society of Experimental Biology)

   Locomotion is an important behavior in the life history of animals and is characterized by discrete gaits, which may be adopted for optimal energetic efficiency, fatigue resistance, or maneuverability. We evaluated the kinematics and electromyography of Bluegill Sunfish (Lepomis macrochirus) swimming at different gaits to evaluate which factors might influence gait choice. When placed in the flume, Bluegill adopted a steady swimming gait until speeds reached 2.0 BL/s. When swimming volitionally, either in a laboratory pool or the field, Bluegill adopted an intermittent swimming gait (burst phase followed by a glide phase) and swam at average speeds of 1.0-1.3 BL/s. No statistical relationship was found between the kinematics of the burst and glide phases in either the lab or the field, so the phases were considered uncoupled. Furthermore, since the kinematics (tailbeat frequency, glide-duty factor) of lab and field volitional swimming were statistically identical, the EMGs of volition swimming in the lab likely reflect field effort. When relativized to volitional swimming speeds, the EMG intensities for both gaits were statistically identical. These results suggest that intermittent swimming may not reflect a strategy for energetic efficiency. Instead, the decoupling between the burst and glide phase may improve maneuverability, since 75% of 3D tracked intermittent swimming bouts (n=129) in the field involved a directional change. Although previous research suggests that intermittent swimming may also provide fatigue resistance, we hypothesize that intermittent swimming evolved in Bluegill as an adaptive gait for navigating their densely vegetated habitat. 

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3. Intermittent propulsion in largemouth bass, Micropterus salmoides , increases power production at low swimming speeds

   https://doi.org/10.1098/rsbl.2021.0658  

   Coughlin, D. J. ; Chrostek, J. D. ; Ellerby, D. J. ( May 2022 , Biology Letters)

   Locomotion dominates animal energy budgets, and selection should favour behaviours that minimize transportation costs. Recent fieldwork has altered our understanding of the preferred modes of locomotion in fishes. For instance, bluegill employ a sustainable intermittent swimming form with 2–3 tail beats alternating with short glides. Volitional swimming studies in the laboratory with bluegill suggest that the propulsive phase reflects a fixed-gear constraint on body–caudal-fin activity. Largemouth bass ( Micropterus salmoides ) also reportedly display intermittent swimming in the field. We examined swimming by bass in a static tank to quantify the parameters of volitional locomotion, including tailbeat frequency and glide duration, across a range of swimming speeds. We found that tailbeat frequency was not related to speed at low swimming speeds. Instead, speed was a function of glide duration between propulsive events, with glide duration decreasing as speed increased. The propulsive Strouhal number remained within the range that maximizes propulsive efficiency. We used muscle mechanics experiments to simulate power production by muscle operating under intermittent versus steady conditions. Workloop data suggest that intermittent activity allows fish to swim efficiently and avoid the drag-induced greater energetic cost of continuous swimming. The results offer support for a new perspective on fish locomotion: intermittent swimming is crucial to aerobic swimming energetics. 

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4. Going in circles: Nesting kinematics of Bluegill Sunfish (Lepomis macrochirus)

   Reynolds, Z. ; Pfister, A.H.L. ; Gee, P.J. ; Wood, B.M. ( January 2022 , The Society for Integrative and Comparative Biology)

   P1-97: In early summer, nesting Bluegill Sunfish (Lepomis macrochirus) expend large amounts of energy building nests, spawning, protecting their offspring, and chasing away predators. The energetic demands of nesting are likely significant and may heavily influence their life history and reproductive success. However, the exact metabolic cost of nesting is difficult to quantify without precise information about the three dimensional position of the center of mass of nesting fish. Field observations of Bluegill Sunfish nesting in Lake Waban (Wellesley, MA) were obtained throughout June until early July by using underwater cameras, fitted with a temperature and light sensor, calibrated to allow three dimensional tracking. The positional data of nesting Bluegill Sunfish were analyzed to derive velocity and acceleration in order to calculate their metabolic rate. We chose to analyze repetitive nesting behaviors, such as rim circling and defensive chasing, due to their frequency of occurrence and consequentially high metabolic demand. Using metabolic rates calculated from Bluegill swimming in a flume, we found that rim circling is nearly 22.2 times more metabolically expensive than the average metabolic cost of swimming in a straight path for the same velocity and duration. Since rim circling is so metabolically expensive and since we estimate that rim circling occurs nearly 25,500 times during an 8 day nesting period, our results strongly suggest that the nesting cycle is one of the most critical periods in the life history of Bluegill Sunfish. The high energetic demands of nesting and the temporary bout of starvation while the fish occupies its nest results in a small margin of error for reproductive success. These conclusions deepen our understanding of male Bluegills’ true paternal investment and can serve to illuminate our understanding of their life history from a quantifiable perspective. 

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5. A biomechanical paradox in fish: swimming and suction feeding produce orthogonal strain gradients in the axial musculature

   https://doi.org/10.1038/s41598-021-88828-x  

   Jimenez, Yordano E. ; Marsh, Richard L. ; Brainerd, Elizabeth L. ( May 2021 , Scientific Reports)

   The axial musculature of fishes has historically been characterized as the powerhouse for explosive swimming behaviors. However, recent studies show that some fish also use their ‘swimming’ muscles to generate over 90% of the power for suction feeding. Can the axial musculature achieve high power output for these two mechanically distinct behaviors? Muscle power output is enhanced when all of the fibers within a muscle shorten at optimal velocity. Yet, axial locomotion produces a mediolateral gradient of muscle strain that should force some fibers to shorten too slowly and others too fast. This mechanical problem prompted research into the gearing of fish axial muscle and led to the discovery of helical fiber orientations that homogenize fiber velocities during swimming, but does such a strain gradient also exist and pose a problem for suction feeding? We measured muscle strain in bluegill sunfish,Lepomis macrochirus,and found that suction feeding produces a gradient of longitudinal strain that, unlike the mediolateral gradient for locomotion, occurs along the dorsoventral axis. A dorsoventral strain gradient within a muscle with fiber architecture shown to counteract a mediolateral gradient suggests that bluegill sunfish should not be able to generate high power outputs from the axial muscle during suction feeding—yet prior work shows that they do, up to 438 W kg⁻¹. Solving this biomechanical paradox may be critical to understanding how many fishes have co-opted ‘swimming’ muscles into a suction feeding powerhouse.

    

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