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Standard and Backward Swimming

The bluegill sunfish relies heavily on the flexibility of its fins to maintain maneuverability in response to fluid forces. The bluegill’s segmentation in its fins allows flexibility that mitigates the effects of fluid forces on the fish’s movement. The bluegill has a variety of unusual adaptations that allow it to navigate different environments. In conditions where the bluegill is deprived of its various sensory abilities, it utilizes its pectoral fins in navigation. If the bluegill’s visual input or lateral line input were to be compromised, its pectoral fins are then able to be utilized as mechanosensors through the bending of the fin(s) when the fish comes into contact with its environment. In standard swimming the bluegill sunfish relies on its caudal (tail) fin, dorsal fin, and anal fin. The bluegill’s caudal fin muscles are important in the fish' slow swimming and also important in the beginning stages of the fish increasing its swimming speed. The dorsal and anal fins are two types of median fins that work in parallel with each other to balance torque during steady swimming.

When swimming backwards, the bluegill utilizes a plethora of fin muscles located in various parts of its body. Backward swimming in the bluegill is more complex than steady swimming, as it is not just the reversal of forward swimming. The fish utilizes its pectoral fins to provide a rhythmic beat while the dorsal and anal fins produce momentum to drive the fish backwards. The pectoral fins’ rhythmic beat is asymmetric and aids the fish’s balance in its slow, backward movement.

C-Start Escape Response

The bluegill, amongst a wide array of other fishes, exhibits the C-start escape response, which is generated by large neurons called Mauthner cells. Mauthner cells operate as a command center for the escape response and respond quickly once the neural pathway has been activated by an initial stimulus. The cells trigger a contraction of muscle that bends the fish body into a ‘C’ to then aid in the propulsion away from a predator. The C-start trajectory is highly variable, allowing the fish to alter its escape response each time. Because of this high variability, predators have a lower chance of learning a successful predation technique to capture the fish. The C-start escape response produces other evolutionary advantages, including the ability to use the quick, unpredictable nature of propulsion to capture prey.

Hydrodynamically, the bluegill exhibits specific flow patterns that accompany its C-start escape response. The caudal (tail) fin is a main source of momentum in typical kinematic models of the C-start escape response but the bluegill draws a majority of its momentum from the body bending associated with the response, as well as its dorsal and anal fins. The dorsal and anal fins’ roles as propulsors during escape response suggest that the size of the fins could lead to an evolutionary advantage when escaping predators.