Photoregulatory behavior of bluegill,Lepomis macrochira,in a virtual light gradient

Synopsis Vertical movements of bluegill were monitored in gradients of light intensity to assess this fish's photoregulatory ability and mechanisms. A computerized monitoring and control system created virtual gradients of light intensity by adjusting an overhead lamp's output in response to fish movements, in a vertical tube, to produce a programmed intensity at the fish's depth position. This approach separated the process of gradient formation from normal clues for photoregulation and allowed formation of light gradients incompatible with natural taxic responses to intensity. Hourly shifts in gradient position minimized the possibility of confounding photoregulation with position regulation. Observed patterns of movement reduced the extremes of light intensity to which bluegill were exposed, compared to no movement or random movement. Seven fish were tested, producing 10 experiments. In 4 of 10 experiments, the fish effectively photoregulated in gradients in which light intensity decreased with depth, as in natural habitats. In 1 of 10 experiments, the fish photoregulated in an inverse gradient, with intensity increasing with depth. Evidence of regulation in an inverse gradient suggests that normal taxic responses are not essential for photoregulation in bluegill.