The Photoreceptors and Visual Pigments of Sharks and Sturgeons

Abstract

With the exception of the stellate sturgeon, which has only cones, all acipenseriform species studied have a duplex retina. The retina is dominated by rods, but about 20% of the photoreceptors are cones. The rods are structurally similar to those of vertebrates in general. Most rods have outer segments that are broad and long, but at least one species has a very rare, very slender rod as well. Cones are also typical in structure, and they too are rather robust. With one exception, the inner segment of each cone houses a colorless oil droplet, the function of which is unknown. Packing density of both rods and cones is relatively low, indicating that both scotopic and photopic acuity is less than that of other nocturnal creatures. The visual pigment in the rods of all species studied has a peak absorbance near 540 nm. All species studied have multiple cone pigments and, therefore, the photoreceptor basis for wavelength discrimination. However, one cannot yet say whether or not any sturgeon or paddlefish has color vision. All visual pigments are based on the vitamin A2 chromophore, and there is no evidence that there is a shift to the vitamin A1 chromophore with change of habitat or with age. At least one species does change its cone pigment complement with age. Larval white sturgeon up to 10 weeks of age have only green-sensitive cones; after 10 weeks, blue-sensitive and red-sensitive cones are also present.