The photoreceptors and visual pigments of two species of Acipenseriformes,the shovelnose sturgeon (Scaphirhynchus platorynchus) and the paddlefish (Polyodon spathula)

Scanning electron microscopy, microspectrophotometry, and spectrophotometry of digitonin extracts were employed to characterize the photoreceptors and visual pigments of two freshwater Acipenseriformes. The retinas of the sho velnose sturgeon, Scaphirhychus platorynchus (Acipenseridae), and the paddl efish, Polyodon spathula (Polyodontidae) are dominated by large rods with l ong, broad outer segments. A second rod, rare and much narrower than the do minant rod, is present in Scaphirhynchus but not seen in Polyodon. The abso rbance maximum of the visual pigment in the rods of Polyodon is near 540 nm ; that of Scaphirhynchus near 534 nm. The retinas of both species contain s ubstantial numbers of large, single cones, about 33% of the photoreceptors in Scaphirhynchus; 37% in Polyodon. Scaphirhynchus cone pigments have absor bance maxima near 610 nm, 521 nm and 470 nm, respectively. Polyodon cone pi gments absorb maximally near 607 nm and 535 nm, respectively. All visual pi gments are based on vitamin A(2). The data are compared to those from other Acipenseriformes and are discussed in terms of lifestyle and behavior.