The photoreceptors and visual pigments of two species of Acipenseriformes, the shovelnose sturgeon (Scaphirhynchus plato
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Ó Springer-Verlag 1999
ORIGINAL PAPER
A. J. Sillman á C. J. O'Leary C. D. Tarantino á E. R. Loew
The photoreceptors and visual pigments of two species of Acipenseriformes, the shovelnose sturgeon (Scaphirhynchus platorynchus ) and the paddle®sh (Polyodon spathula ) Accepted: 7 October 1998
Abstract 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 shovelnose sturgeon, Scaphirhynchus platorynchus (Acipenseridae), and the paddle®sh, Polyodon spathula (Polyodontidae) are dominated by large rods with long, broad outer segments. A second rod, rare and much narrower than the dominant rod, is present in Scaphirhynchus but not seen in Polyodon. The absorbance 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 substantial numbers of large, single cones, about 33% of the photoreceptors in Scaphirhynchus; 37% in Polyodon. Scaphirhynchus cone pigments have absorbance maxima near 610 nm, 521 nm and 470 nm, respectively. Polyodon cone pigments absorb maximally near 607 nm and 535 nm, respectively. All visual pigments are based on vitamin A2. The data are compared to those from other Acipenseriformes and are discussed in terms of lifestyle and behavior. Key words Fish á Microspectrophotometry á Retina á Scanning electron microscopy á Vision
A.J. Sillman (&) á C.J. O'Leary á C.D. Tarantino Section of Neurobiology, Physiology and Behavior, Division of Biological Sciences, University of California, 1 Shields Ave., Davis, CA 95616, USA e-mail: [email protected] Tel.: +1-530-7523207; Fax: +1-530-7525582 E.R. Loew Section of Physiology, Division of Biological Sciences, Department of Biomedical Sciences, School of Veterinary Medicine, Cornell University, Ithaca, NY 14853, USA
Introduction The Acipenseriformes are an order of primitive chondostreans whose ancestors emerged some 250 million years ago. The 25 living species in the order are divided into two families, the Acipenseridae, or sturgeons, and the Polyodontidae, or paddle®shes. Three obvious morphological characteristics distinguish the living polyodontids from the acipenserids. Firstly, they lack the bony scutes arranged in rows along the body of acipenserids; secondly, they are missing the barbels found anterior to the mouth in the acipenserids; and thirdly, although members of both families are characterized by a well-developed rostrum, the rostrum of the polyodontids is exceptionally long and ¯attened so as to resemble a paddle (Sokolov and Berdichevskii 1989). The two families diverged from a common evolutionary line early on and are already recognizable as distinct groups by the late Cretacious (Doroshov 1985; Carroll 1988; Conte et al. 1988). All species spawn in fresh water, generally the fastmoving water of rivers and streams. Some species, such as the Atlantic sturgeon, Acipenser oxyrhynchus, and the gr
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