Day and night posture of the bluegill sunfish (Lepomis macrochirus)

Abstract

AbstractMany animals assume characteristic postures when resting or sleeping. These postures are often stable and can be maintained passively, thus reducing the energy cost for maintaining an unstable posture. For example, many tetrapods lay prone on the ground and some negatively buoyant fishes are also able to rest on the substrate. Other fishes rest suspended in the water column. Counterintuitively, hovering this way can be of similar energetic cost to swimming. Even if the fish is perfectly neutrally buoyant, any displacement between its center of mass and center of buoyancy will produce destabilizing pitching torques that the fish must constantly work to counteract if they wish to maintain that posture. We hypothesized that a neutrally buoyant fish could rest at an equilibrium – a posture at which no destabilizing torques are produced by the body --to minimize the metabolic costs associated with hovering. Specifically, we studied the bluegill sunfish (Lepomis macrochirus), which is unstable in a horizontal posture. However, by pitching their bodies up or down they may be able to attain a less costly equilibrium posture, one which vertically aligns their center of mass and center of buoyancy. To test this hypothesis, we measured pitch angle of bluegill over the course of 24 hours. We also measured the pitch angles of the body that correspond to stable and unstable equilibria. We found that the stable equilibrium was a belly-up posture, and the unstable equilibrium is a dorsal side up posture pitched 53±26° head-down. The fish rested at a head-down pitch of -10.7±0.4° degrees, which is significantly steeper than the average pitch during the day of -3.4±0.8° degrees head down. These results show that bluegill do not rest at unstable or stable equilibrium. However, they do rest closer to unstable equilibrium at night than during the day. This may allow them to decrease destabilizing torques generated from the relative locations of the COM and COB while maintaining maneuverability.