Comparisons between the diurnal changes in the concentrations of Rh-LpOps1-4 and Rh-LpOps5 coexpressed in retinular cell rhabdoms provide clear evidence that the turnover of coexpressed opsins can be regulated differently.
The diurnal change in the concentration of Rh-LpOps1-4 in both LE retinular cells and giant VE photoreceptors probably contributes to observed diurnal changes in LE and VE sensitivity to light (Barlow et al., 1977; Kass and Renninger, 1988), but the functional relevance of LpOps1-4 and LpOps5 coexpression and diurnal changes in their relative concentrations in rhabdoms is unclear.
However, as in the rhabdoms of LE retinular cells and giant VE photoreceptors, the ratio of the opsins coexpressed in small VE photoreceptor rhabdoms changes with time of day.
Electrophysiological studies also provided the first hint that the opsin expressed in visible light-sensitive ME photoreceptors might be different from that expressed in LE retinular cells and giant VE photoreceptors.
LpUVOps1 is also the only opsin detected in LE eccentric cells where it concentrates in dendrites, which form electrical junctions with retinular cells.
But the coexpression of opsins from different clades--LpOps5 with LpOps1-4 in LE retinular cells and giant VE photoreceptors and LpOps5 with LpUVOps1 in smaller VE photoreceptors--could be a plesiomorphic characteristic that Limulus has retained from its euchelicerate ancestor.
Studies of LE retinular cells showed that this is, in part, because the coexpressed opsins differ in how their turnover is influenced by signals from an internal circadian clock.
Once again, the retinular cell axons are grouped together to form a distinctive optic nerve.
The ommatidia of most decapod eyes each consist of a corneal facet secreted by two corneagenous cells, a single crystalline cone formed by four cone cells, eight retinular cells forming a single rhabdom, and a number of tapetal cells.
Species from deeper water, such as Acanthephyra purpurea, have very little retinular cell screening pigment, and the rhabdoms interdigitate to form a continuous layer with no optical isolation of rhabdoms (Gaten et al., 1992).
The volume density of rhabdoms within the retinular cell layer varies from 62% in Alvinocaris markersis to 75% in 'Chorocaris'.
As a result, any incident rays at angles up to 12.5 [degrees] from the ommatidial axis will be retained within the rhabdom, assuming that the refractive indices of the rhabdoms and the surrounding retinular cells are similar to those of other decapods (e.g., Gaten, 1994).