The marine teleost Rhinecanthus aculeatus (Balistidae) has recently been shown to possess trichromatic color vision supported by a retinal combination of double and single cones. Double cones are composed of two members with different spectral sensitivity. It is not known whether a correlation exists between the chromatic wiring of double cones to the inner retina and trichromacy, nor how unmixed, chromatic information is extracted from the two members of the couple. In mammalians, bipolar cells determine color segregation by means of the midget system, central to trichromatic color vision; however, midget bipolar cells have never been described in teleosts. On the basis of its likely importance in transferring chromatic photoreceptor signals to the inner retina, we have morphologically characterized the retinal bipolar cell types of R. aculeatus using DiOlistic staining techniques to verify if an anatomical specialization of this group of cells is required to support trichromatic color vision. Thirteen cell types are described: eight putative OFF types and five putative ON types. Of these, four had axonal boutons ramifying in both sublayers (ON and OFF) of the inner plexiform layer, six had terminals restricted to the OFF layer, and three cell types had terminals restricted to the ON layer. Dendritic arbors of bipolar cells had narrower diameters (5-40 microm) in comparison to bipolar cells of other teleost species; this supports the idea that a low degree of photoreceptor to bipolar convergence is correlated with trichromacy in this retina and possibly with the function of double cones as color receptors.
(c) 2010 Wiley-Liss, Inc.