Rhabdom breakdown in the eye of Cirolana borealis (Crustacea) caused by exposure to daylight.

The effect of daylight on the compound eye was investigated in the deep-water crustacean isopod Cirolana borealis Lilljeborg. The animals were captured and fixed at night ('dark-exposed', i.e. not exposed to light) and day ('daylight-exposed'), respectively. Changes in light and darkness have an effect on the retinula cells; the ultrastructure of dark-exposed eyes is characterized by well-preserved organelles and cytoplasm. The photoreceptor membranes covering the microvilli are regularly aligned, and the outline of the villi is smooth. Electron-dense pigment granules are evenly distributed in the cytoplasm of the retinula cell outside the rhabdom. Daylight-exposed eyes differ from the dark-exposed eyes in the following aspects: (i) the microvilli are disrupted, (ii) retinula-cell pigment is found in the rhabdom, and (iii) the cytoplasm of retinula cells is vesiculated. These results are interpreted as retinal damage caused by excess exposure to light.