Light and time correlated migration of invasive hemocytes in the crayfish compound eye

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Abstract
The number and radial distribution of hemocytes discovered periodically invading the retinas of crayfish compound eyes (Figs. 1–3) were measured under a variety of light and dark adaptation programs (Fig. 4, Table 1). Overall the density of these cells in the eye was significantly less in light than in darkness (Figs. 5–8, Table 2). The corresponding hemocyte ebb and flow occurred in intercellular spaces between retinulas, not in blood vessels which are absent from decapod crustacean eyes everywhere distal to the basement membrane.Hemocytes under all conditions were most dense just below the basement membrane. Within the retina their occurrence ranged from maximum at the basement membrane to minimum or zero near the distal tip of the rhabdom, beyond which they rarely occurred (Figs. 5, 6, Table 3). The effectiveness of light in reducing hemocyte concentrations was maximal distally and minimal proximally and hence had a gradient inverse that for cell density (Fig. 8).Bilateral experiments with one eye covered and the other exposed showed that light could independently and unilaterally induce hemocyte emigration from the retina (Fig. 5). But in time‐of‐day experiments hemocyte movements occurring before the programmed cyclic diurnal illumination changes proved that endogenous control mechanisms must also participate (Fig. 7). Such experiments also showed the fewest retinal hemocytes (≈︁0) at 18:00 hours, when the rate of rhabdom synthesis should be near maximum.So far no direct phagocytic uptake of identifiable photoreceptor membrane has been found in crayfish retinal hemocytes. Nevertheless these cells were seen to make junctionlike contacts with retinular cells and appeared to be actively ingesting interstitial detritus in the retina at levels coincident with the radial extent of the rhabdoms (Figs. 1–3). Previously phagocytic wandering cells were known to participate in photoreceptor membrane turnover in teleost eyes and in reptile pineal organs although fixed retinal pigment epithelial cells are normally involved in this function in birds and mammals.