Unorthodox pattern of microvilli and intercellular junctions in regular retinular cells of the porcellanid crab Petrolisthes

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Abstract
Retinular fine structure in compound eyes of the porcellanid crab Petrolisthes differs significantly from two paguroid anomurans Clibanarius and Pagurus which basically conform to the usual conservative decapod crustacean retinular pattern. Bidirectional orientation of microvilli has been discovered in rhabdomeres of retinular cells R1–R7 in Petrolisthes. Distally the regular rhabdom has mainly a typical banded microvillus structure (Figs. 7, 8). Proximally rhabdom banding continues but uniquely all seven regular retinular cells contribute sets of alternately orthogonal microvilli to each band (Figs. 5, 6, 12). This unorthodox pattern should reduce polarization sensitivity and enhance sensitivity to unpolarized light. In this special region microvillus layers are strongly elliptical in cross section with the minor axis parallel to the microvilli (Fig. 12). Hence the ends of the major axes protrude considerably from the central area of overlap (Fig. 6). Retinular cell eight has bidirectional microvilli (Figs. 5–7) as usual in brachyuran crabs. Unlike the latter as well as paguroid crabs, Petrolisthes has square facets and a rectangular retinular array (Figs. 1, 3) similar to other galatheids and macruran decapods generally. It also resembles macrurans (shrimps and lobsters) in having perirhabdomal vacuoles absent or much reduced. Tight junctions occur widely between adjacent retinular cells (Figs. 14, 17) especially basally immediately distal to longitudinal zonular adherentes (Figs. 6, 16) typical of compound eyes. Freeze fracture reveals in addition numerous rectangular arrays of particles on the protoplasmic face of retinular cell membrane near, but not part of, the rhabdom (Figs. 19, 20). Other authors have hypothesized polarized transfer functions for similar particle aggregates in certain vertebrate cells. Experiments and further comparative study are needed to test whether the special fine structural features of the eye depend on Petrolisthes' sedentary photophobic habits or on its particular anomuran affiliation.