The photobiology of Hydra's periodic activity

Hydra's response to a light pulse is a phase shift of the state of bioelectric activity correlated with the periodic shortening-elongation behaviour. The direction and absolute value of a phase shift depend on intensity, direction, application phase (along the periodic activity state), and wavelength of the light pulse. Repetitive pulses entrain the behavioural cycle. The period of the behavioural cycle depends on intensity and wavelength of steady background illumination; however, the light effect is not exerted isotropically along all the phases of the behavioural cycle. Inferences are drawn on light influence on the behaviour pacemaking mechanism. By using polyclonal antibodies against squid rhodopsin, an opsin-like protein has been identified in the ectodermal layer, presumably in sensory cells.