Retinal photoreceptors of paleognathous birds: the ostrich (Struthio camelus) and rhea (Rhea americana)

Microspectrophotometry was used to determine the absorbance spectra of both rod and cone visual pigments and oil droplets from the retinae of the ostrich (Struthio camelus) and rhea (Rhea americana). Light and fluorescence microscopy of whole fresh tissue mounts were used to determine the relative numbers and distribution of oil droplets in the retinae. Both species possessed rods, double cones and four classes of single cone identified by their oil droplets. The rods had λmax at about 505 nm, whereas three cone pigments were recorded with λmax at 570, 505 and 445 nm. The P570 pigment was located in both members of the double cones and in a class of single cone containing an R-type oil droplet (λcut at 555 nm). The P505 and P445 cone pigments were found in populations of single cones containing Y-type and C-type oil droplets (λcut of 500 and 420 nm, respectively). The fourth class of single cone contained a T-type droplet and in the ostrich contained a visual pigment with λmax at about 405 nm. Double cones possessed a P-type droplet in the principal member and an A-type droplet in the accessory member. The complement of visual pigments and oil droplets, and the ratio of cone types in the ostrich and rhea, are remarkably similar to those found in many groups of neognathous birds.