The spectral sensitivity of Hydra carnea L. Agassiz (1850)

The spectral sensitivity of Hydra carnea was investigated by growth rate and action spectrum studies. The rate of asexual growth was measured over a 40-day period for animals cultured under white, blue, green, and red lights of equal intensity. The growth rate was greatest under red light and slowest under blue; the rate under white light was similar to that under green light. The action spectrum, measured as the reaction times to light stimuli of various wavelengths, varied according to the light under which the animals were cultured. For H. carnea that had been cultured in the dark, the reaction time was shortest at 650 nm and longest at 450 nm when lights of equal intensity were used. This differs from the results of an earlier study with H. pirardi, which showed maximum sensitivity to blue light. Animals cultured under blue, green, and red lights showed a decreased sensitivity, indicated by an increased reaction time, to the light under which they were cultured as compared to animals cultured in the dark.