Daily changes of structure, function and rhodopsin content in the compound eye of the crabHemigrapsus sanguineus

Summary

The compound eye of the crabHemigrapsus sanguineus undergoes daily changes in morphology as determined by light and electron microscopy, both in the quantity of chromophore substances studied by HPLC and in visual sensitivity as shown by electrophysiological techniques.

• 1. At a temperature of 20 °C, the rhabdom occupation ratio (ROR) of an ommatidial retinula was 11.6% (maximum) at midnight, 8.0 times larger than the minimum value at midday (1.4%) (Figs. 2, 6).

• 2. Observations by freeze-fracture revealed that the densities of intra-membranous particles (9–11 nm in diameter) of rhabdomeric membrane were ca. 2000/μm² and ca. 3000/μm² for night and daytime compound eyes, respectively (Fig. 3).

• 3. Screening pigment granules migrated longitudinally and aggregated at night, but dispersed during the day. Reflecting pigment granules migrate transversally in the proximal half of the retinula layer i.e. cytoplasmic extensions containing reflecting pigment granules squeeze between neighbouring retinula cells causing optical isolation (Fig. 4). Thus the screening pigment granules within the retinula cells show longitudinal migration and radial movement so that the daytime rhabdoms are closely surrounded by the pigment granules (Fig. 2).

• 4. At 20 °C, the total amount of chromophore of the visual pigment (11-cis and all-trans-retinal) was 1.4 times larger at night than during the day i.e. 46.6 pmol/eye at midnight and 33.2 pmol/eye at midday (Fig. 9). Calculations of the total surface area of rhabdomeric membrane, total number of intra-membranous particles in rhabdomeric membrane and the total number of chromophore molecules in a compound eye, indicate that a considerable amount of chromophore-protein complex exists outside the rhabdom during the day.

• 5. The change in rhabdom size and quantity of chromophore were highly dependent on temperature. At 10 °C both rhabdom size and amount of chromophore stayed close to daytime levels throughout the 24 hours (Figs. 6, 9).

• 6. The intracellularly determined relative sensitivity of the dark adapted night eye to a point source of light was about twice as high as the darkadapted day eye (Fig. 10). Most of the increase in the sensitivity is attributed primarily to the effect of reflecting pigment migration around the basement membrane (Fig. 4) and, secondarily, to the changes in the amount and properties of the photoreceptive membrane.

The results form the basis of a detailed discussion as to how an apposition eye can function possibly as a night-eye.