FATTY-ACID COMPOSITION AND ULTRASTRUCTURE OF PHOTORECEPTIVE MEMBRANESIN THE CRAYFISH POCAMBARUS-CLARKII UNDER CONDITIONS OF THERMAL AND PHOTIC STRESS

The ultrastructural state of the crayfish visual membrane is correlate d with its fatty acid composition during times of photic and thermal s tress and the period over which the dynamic events occur is investigat ed. Crayfish kept at 4 degrees C under constant darkness contain in th eir rhabdoms significantly increased amounts of unsaturated fatty acid s such as 16:1, 18:1, 20:5, and 22:6 compared with individuals kept at 25 degrees C. The ratio of unsaturated/saturated fatty acids (UFA/SFA -ratio) amounts to 2.17 in the cold-water- and 1.46 in the warm water- acclimated animals. The visual membranes of crayfish suddenly transfer red from 4 degrees C to 25 degrees C exhibited ultrastructural modific ations such as membrane collapse and disappearance of microvillar dens e core-filaments most clearly 3 h post-transfer. Parallel to the struc tural changes a significant increase in fatty acid 18:0 was observed, while the amounts of 16:1 and 20:1 decreased. When 4 degrees C, dark-a dapted crayfish were exposed to light alone and not a temperature incr ease, only fatty acid 22:6 showed a significant reduction to 10% of it s pre-experimental level within 2 h of exposure. Thereafter, it slowly increased again. In cold water-acclimated crayfish that had been expo sed to light of 5000 1x for 3 weeks no significant change of the UFA/S FA ratio was observed, although fatty acid species 18:0, 20:4, and 20: 5 had increased at the expense of fatty acids 14:0, 16:0, 16:1, 18:1, 20:1, and 22:6. The total amount of fatty acids, however, had become s ignificantly smaller (from 0.058 ng g(-1) body weight in the dark-adap ted to 0.048 ng g(-1) in the light-adapted crayfish). Morphologically the rhabdom volume had decreased by approx. 20%, but ultrastructurally rhabdom microvilli remained almost unchanged. The amount of peroxidiz ed lipids in the retina following irradiation with bright white light in the cold-adapted crayfish fell during the first 2 h of exposure fro m 0.4 nmol g(-1) to 0.32 nmol g(-1), but after 12 h of exposure had re ached a level of 0.48 nmol g(-1). Greatest structural abnormalities to the visual membranes occurred when dark-adapted, cold-acclimated cray fish were suddenly subjected to bright light and an increase in water temperature. Under such conditions the microvillar arrangement was dis rupted and membrane collapse and disappearance of core-filaments were apparent. Our results provide evidence that the fatty acid composition of the membranes determines to a considerable extent the structural i ntegrity of the photoreceptor, but that it is too simplistic a model t o think that peroxidation of membrane lipids alone is responsible for the disintegration of the photoreceptive membranes in the crayfish eye following exposure to bright light.