Ultrastructural changes of rhabdoms of the eyes of Ocypode species in relation to different regimes of light and dark adaptation
In: Journal of Crustacean Biology. Crustacean Society: Washington. ISSN 0278-0372; e-ISSN 1937-240X, more
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| Abstract |
Ultrastructural changes of the compound eyes were compared in Ocypode species living under different daylight and annual rhythms near the equator (0. ryderi, O. ceratophthalma), in the northern (0. cursor) and in the southern hemisphere (0. quadrata). Independent of geographical latitude, the eyes of these Ocypode species undergo daily changes in morphology. Throughout the day the rhabdomeric diameter is small. A massive increase of rhabdom diameter occurs at dusk; a breakdown phase starts during the second half of the night. Before dawn the rhabdom size is near to that during daytime and seems to be prepared for the day-state. Besides the rhabdom, the size of the palisade-ER and the position of pigment granules and mitochondria within the retinula cells change. In all investigated Ocypode species the rhabdom turnover is influenced by different light and dark regimes. During extended darkness, rhabdom diameters resemble those of day-adapted animals. Extended exposures to light cause no or little change in rhabdom diameters of O. ryderi, but in O. ceratophthalma and O. quadrata rhabdom diameter of both distal and regular retinula cells grows up moderately. During the following dark adaptation the rhabdom diameter increases dramatically in the regions of both distal and regular retinula cells of O. ryderi. In O. ceratophthalma and O. quadrata the diameter of regular retinula cells doubles. If the crabs are kept under darkness during the day, the rhabdomeric diameter grows moderately but never reaches the diameter of the night-adapted state. If the crabs are brought into bright light during the night, rhabdom diameters decrease to that of day-adapted animals. Independent of different geographical habitats, in these Ocypode species rhabdom synthesis seems to be stimulated above all by the onset of darkness, whereas rhabdom diminution during the night seems to be controlled endogenously. |
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