Effects of illumination changes on rhabdom synthesis in a crab

Summary

The rhabdoms ofLeptograpsus variegatus photoreceptors are several times larger at night than in the day. In animals kept in the laboratory and accustomed to a cycle of fluorescent light turned on and off at approximately natural dawn and dusk times, photoreceptors generally form new rhabdomeres within 30–60 min of the beginning of the dark period. At the beginning of the light period a large proportion of these rhabdomeres are removed by pinocytosis, leaving the smaller day rhabdomeres which persist until the following ‘dusk’, when they are rapidly broken down and replaced by newly synthesized phototransductive membrane. It is shown that;

• (i) Synthesis occurs if crabs are dark adapted several hours before the usual dark period, but the final rhabdom size is reduced.

• (ii) Crabs maintained in light through the usual dark period synthesize a new rhabdom.

• (iii) Synthesis as a reaction to early darkness occurs unilaterally if one eye is blinded.

An unusual turnover schedule recently proposed forLimulus lateral eye is discussed in relation to some earlier studies onLimulus light and dark adaptation and recent work on turnover in other arthropods. It is concluded thatLimulus very probably complies with the turnover schedule emerging as normal for animals that have enlarged rhabdoms at night.