Ontogenetic changes in the retinal photoreceptor mosaic in a fish, the black bream, Acanthopagrus butcheri

The morphological development of the photoreceptor mosaic was followed by l ight and electron microscopy in a specific region of dorsal retina of the b lack bream, Acanthopagrus butcheri (Sparidae, Teleostei), from hatching to eight weeks of age. The retina was differentiated when the larvae reached a total length of 3 mm (3-5 days posthatch). Single cones, arranged in tight ly packed rows, were the only morphologically distinct type of photorecepto r present until the larvae were 6 mm (day 15) in standard length (SL). At t his time, the rad nuclei had become differentiated and the ellipsoids of se lected cones began to form subsurface cisternae along neighbouring cone mem branes. In this way, double, triple, quadruple, and occasionally photorecep tor chains of up to 10 cones were formed. At 8 mm SL, there was little appa rent order in the photoreceptor mosaic. However, concomitant with subsequen t growth, quadruple and other multiple cone receptors disappeared, with the exception of the triple cones, which gradually reduced in both number and retinal coverage to be restricted to central retina by 15 mm SL (days 40-55 ). Following this stage, the arrangement of double and single cones periphe ral to the region of triple cones in dorsal retina was transformed into the adult pattern of a regular mosaic of four double cones surrounding a singl e cone. These results demonstrate that an established photoreceptor mosaic of rows of single cones can be reorganised to form a regular square mosaic composed of single and double cones. J. Comp. Neural. 412:203-217, 1999. (C ) 1999 Wiley-Liss, Inc.