LIGHT-INDUCED PHOTORECEPTOR SENSITIVITY LOSS AND RECOVERY AT 4-DEGREES-C AND 14-DEGREES-C IN MYSIS-RELICTA LOVEN (CRUSTACEA, PERACARIDA) FROM POJOVIKEN BAY (FINLAND)

Specimens of Mysis relicta from Pojoviken Bay (Finland) were divided i nto two groups, one kept at 4 degrees C, the other at 14 degrees C, in total darkness. Immediately following a 1 h exposure to noon sunlight both 4 degrees C and 14 degrees C animals displayed strongly reduced visual sensitivities. In both groups pre-exposure levels were regained in about two days, but apparently along slightly different routes. Sl opes of V/log I curves hardly changed throughout the time of observati on in the 14 degrees C material, suggesting an adaptation to brighter light levels without undue stress responses. In the 4 degrees C animal s, however, there appears to have been not only a longer initial delay before recovery commenced, but slopes of V/log I curves indicated tha t these animals had reacted with depression to the bright light and ne eded time to regain their pre-exposure value. The results suggest that recovery is a two-stage process in which biochemical reactions and st ructural phenomena interact. The results, when compared with similar o bservations on Lake Paajarvi specimens, underline the view that differ ent localities may have populations of Mysis relicta which differ from each other in photophysiological characteristics.