The Secret World of Shrimps: Polarisation Vision at Its Best
Figure 2
A) left. Diagram of a longitudinal section through an ommatidium (visual unit) in the hemispheres and mid-band rows 5 and 6 of the eye. The main rhabdom is formed by seven photoreceptors (R1–7), overlaid by a small, four-lobed ultraviolet sensitive photoreceptor, R8. right. The arrows indicate the microvillar directions within each retinal region as if looking into the eye (frontal view). The R1–7 cells are divided into group I cells (R1, R4, R5) and group II cells (R2, R3, R6, R7), which form layers of orthogonal microvilli throughout the rhabdom, see Figure 3C, and thus are sensitive to orthogonal polarisations. The overlying R8 cells in rows 5 and 6 are extraordinarily long and they produce parallel microvilli, whereas the R8 cells in the remainder of the retina produce microvilli that are both orthogonal and interdigitating (crossed arrows). {r,l} indicates sensitivity to right- and left-circularly polarised light, {a,d,h,v} indicates sensitivity to anti-diagonal, diagonal, horizontal and vertical linearly polarised light. All directions indicated in the text and subsequent figures refer to a frontal view of a right eye with the mid-band arranged horizontally. B)–D) Frontal diagrams of the R1–7 (numbered 1–7) cell body arrangement around the central light guide (rhabdom); and examples of dye-injected cells shown in the photomicrographs (scale bars 50 µm). B) The dorsal hemisphere, which analyses {d,a}, C) the ventral hemisphere, which analyses {h,v} and D), mid-band row 5, which analyses {r,l}. Group I retinular cells are stippled and group II retinular cells are plain. Bold numbers in the diagrams indicate the stained cell(s) in the accompanying photomicrographs. Arrows indicate microvillar axes, and thus the directions of linearly polarised light to which the photoreceptors respond maximally (φmax). Grey and black arrows indicate group I and II receptors, respectively. The cell arrangement in mid-band row 6 (not shown) is rotated 90° counter-clockwise compared to row 5. Circular polarisation sensitivity is not innate to the R1–7 cells, but arises from the quarter-wave retardance of the overlying four-lobed R8 cell (D top). Quarter-wave retardance is realised by increased photoreceptor length and by the formation of unidirectional microvilli, the axis of which is indicated by the black arrow. The R8 microvilli are arranged at 45° to the underlying orthogonal microvillar sets formed by the R1–7 cells (D bottom): R8 converts circularly polarised to linearly polarised light at ±45° to the R8 microvillar axis, depending on the handedness of the circular polarisation. Both dye-filled photoreceptors (row 5 R3 and row 6 R6) belong to group II receptors. The angle between the microvillar (optical) axes of the R8 cells and the microvillar directions of group II photoreceptors is −45° in both mid-band rows 5 and 6. Both stained retinular cells are therefore more sensitive to l; similarly the group I photoreceptors are more sensitive to r.