Retinal Adaptation of Japanese Common Squid (Todarodes pacificus Steenstrup) to Light Changes

The response of retinae of the Japanese common squid (Todarodes pacificus Steenstrup) was recorded in relation to various light intensities.In the light-adapted eye of common squid, the black pigment ascends to the external limiting membrance of the retina.Conversely, in the dark-adapted eye the black pigment descends toward the center of the black pigment layer.To express the degree of adaptation, the authors give the ratio of the height (thickness) of the black pigment to the total height (thickness) of the retina as a percentage (%).The results of this investigation can be summarized as follows:1) The movement of the black pigment is not very fast.2) If the illumination is brighter than 25 lux, and exposure lasts less than 2 hours, the black pigment does not migrate.This suggests that with high intensily and short exposure time, the shutting of the iris directly controls the amount of light coming in contact with the visual element in the retina.3) The intensity of the illumination in which good control of retina sensitivity by movement of the black pigment corresponds to the extreamely lower intensity, so they lives deeper layer in daytime and migrate to near-surface in night time.The process of the black pigment ascending where photopic vision occurs, and black pigment descending where scotopic vision occurs, takes place at an intensity of illumination of about 1.0 lux.4) A thickness of less than 40% of retinal black pigment in the common squid immediately after capture by jig is the normal value.