Fine structural changes induced by circadian light—dark cycles in photoreceptors of Dalyelliidae ( Turbellaria: Rhabdocoela)

Abstract

The circadian light—dark cycles in the dalyelliid flatworm eyes induce two changes of particular interest. One involves the metabolic membrane system, its turnover, and the localization and appearance of mitochondria and glycogen; the other is connected with retinomotor response. In the second process the dark-adapted eye presents an empty eye cup. The microvillar border of the photoreceptors and the “lens” projections come out of the cup and the cell bodies are displaced laterally with respect to that cup. An attempt was made to elucidate the development within time of the retinomotor response. The plot of the ratio of empty area/microvillar area in median sections of several eyes shows a long-lasting dark-adaptation, completed in 6–9 hr and a short-term light-adaptation completed 5–10 min after light onset. Some hypotheses on the regulation of this response are discussed.