In the crab Hemigrapsus sanguineus, maintained under a 12 h:12 h light:dark cycle, the amount of vesicular smooth endoplasmic reticulum (vesicular sER) in the photoreceptor cell body increases after the light is turned off. This paper demonstrates that actin filaments in the photoreceptor cell body are involved in the transport of vesicular sER towards the rhabdom. To specify the time of actin contribution to rhabdom synthesis, we disrupted the organization of actin filaments in the cell body with cytochalasin D at various time around dusk. We then measured the rhabdom size and also examined the ultrastructure of the photoreceptor cell body 3 h after extinguishing the light. When cytochalasin D was applied from either 1 h before or immediately after extinguishing the light, the rhabdom size did not increase, whereas vesicular sER accumulated in the cell body. In contrast, cytochalasin D applied to the eyes from 20 min after turning the light off did not inhibit rhabdom synthesis. These results indicate that the first 20 min after the light is turned off is particularly important for the transport of vesicular sER towards the rhabdom by the cell body actin filaments.

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