Light and retinal damage in Nephrops norvegicus (L.) (Crustacea)

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Abstract
Nephrops norvegicus is a burrow-dwelling marine crustacean normally only active in dim light. The eye has typical crustacean rhabdoms each consisting of alternating layers of microvilli. On light adaptation, proximal shielding pigment moves up from the bases of retinula cells to surround the rhabdoms. In dark-adapted eyes the proximal pigment moves proximally to form a band just above the basement membrane. In this position the tapetum is unshielded and it reflects light back into the eye. The only other detectable difference between light- and dark-adapted eyes is a night-time increase in rhabdom volume. Creel-caught animals raised to the surface of Loch Torridon (NW Scotland [UK]) were exposed to ambient surface light for periods ranging from 9 min to 5 h. A short exposure (9 min, average intensity 380 .mu.mol min-2 s-1) is sufficient to cause damage to the retinula cell layer. It is histologically detectable one month later. Animals fixed immediately after 15 min exposure show evidence of retinula cell breakdown with swelling of cell bodies and nuclei, escape of proximal shielding pigment from the retinula cells and vesiculation of the rhabdoms. After 2 h of illumination the microvilli of the rhabdom are completely disrupted with only membrane whorls remaining; proximal shielding pigment is found deep within the rhabdom. After 6 h of illumination the retinula cell body layer is absent and there is a massive invasion of the eye by haemocytes. By using animals acclimated to a 12 h light-12 h dark cycle (green light, 0.24 .mu.mol m-2 s-1) we were able to test the effects of natural day-light (average intensity 180 .mu.mol m-2 s-1) on dark- and light-adapted eyes of known physiological state. The animals were kept alive for two weeks after exposure and the percentage area of the retina destroyed was measured from serial wax sections. Dark-adapted eyes have substantial damage (76.74%) after only 15 s but light adaptation prevents damage with a similar exposure. After 5 min exposure, destruction is almost total (light-adapted, 97.16%; dark-adapted, 98.97%). Intensities of 1000 and 250 .mu.mol m-2 s-1 with an artificial tungsten light source gave similar results. Light-adapted eyes are less sensitive than dark-adapted ones and longer exposure cause greater damage. At these relatively high intensities the damage caused by the two light levels is not very different. At lower intensities (10-250 .mu.mol m-2 s-1) the amount of damage is proportional to the intensity. By using the tungsten source and 10 s exposures we found that dark-adapted eyes are damaged at 25 .mu.mol m-2 s-1 and that light-adapted eyes are affected at 100 .mu.mol m-2 s-1.