Sensitivity to light in the sea-anemoneMetridium senile(L): adaptation and action spectra

The sea-anemone Metridium senile responds to illumination by local contraction of the longitudinal musculature, particularly of the parietal system. No specific receptors have so far been identified for this response. The phenomena of light and of dark adaptation are shown by the photoreceptive mechanism. The action spectra of white individuals show a maximum at about $\text{490 to 520}$ $\text{m}\mu $. Relative to this the sensitivity in the region $\text{550 to 600}$ $\text{m}\mu $ varies in different individuals. The possible presence of two distinct photosensitive systems is mentioned. Sensitivity decreases very rapidly beyond 600 $\text{m}\mu $. Action spectra of the white, red and brown coloured varieties of the species are compared in relation to the absorption spectra of tissues obtained from the three types. It is concluded that the pigments which give the characteristic body colour in the red and brown varieties act as filters. Comparison with the white variety indicates that these pigments probably do not act as specific photosensitive substances. It is pointed out that the differential sensitivity of the coloured varieties appears to have no selective influence on the occurrence and distribution of the different coloured varieties.