PHOTOREGULATORY BEHAVIOR OF BLUEGILL, LEPOMIS-MACROCHIRA, IN A VIRTUAL LIGHT GRADIENT

Vertical movements of bluegill were monitored in gradients of light in tensity 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 intens ity at the fish's depth position. This approach separated the process of gradient formation from normal clues for photoregulation and allowe d formation of light gradients incompatible with natural taxic respons es to intensity. Hourly shifts in gradient position minimized the poss ibility of confounding photoregulation with position regulation. Obser ved patterns of movement reduced the extremes of light intensity to wh ich bluegill were exposed, compared to no movement or random movement. Seven fish were tested, producing 10 experiments. In 4 of 10 experime nts, the fish effectively photoregulated in gradients in which light i ntensity decreased with depth, as in natural habitats. In 1 of 10 expe riments, the fish photoregulated in an inverse gradient, with intensit y increasing with depth. Evidence of regulation in an inverse gradient suggests that normal taxic responses are not essential for photoregul ation in bluegill.