The foveal photoreceptor mosaic in the pipefish, Corythoichthyes paxtoni (Syngnathidae, Teleostei)

The foveal and non-foveal retinal regions of the pipefish, Corythoichthyes paxtoni (Syngnathidae, Teleostei) are examined at the level of the light an d electron microscopes. The pipefish possesses a deep, pit (convexiclivate) fovea which, although lacking the displacement of the inner retinal layers as described in other vertebrate foveae, is characterised by the exclusion of rods, a marked increase in the density of photoreceptors and a regular square mosaic of four double cones surrounding a central single cone. In th e perifoveal and peripheral retinal regions, the photoreceptor mosaic is di srupted by the insertion of large numbers of rods, which reduce spatial res olving power but may uniformly increase sensitivity for off-axis rays. In a ddition to a temporal fovea subtending the frontal binocular field, there i s also a central area centralis subtending the monocular visual field. Base d on morphological comparisons with other foveate teleosts, four foveal typ es are characterised and foveal function discussed with respect to the theo retical advantage of a regular square mosaic.