VISUAL PIGMENTS AND PHOTORECEPTORS IN 2 SPECIES OF SHARK, TRIAKIS-SEMIFASCIATA AND MUSTELUS-HENLEI

The retinal photoreceptors of brown smoothhound sharks (Mustelus henle i) and leopard sharks (Triakis semifasciata) were studied by spectroph otometric analysis of visual pigment extracts, microspectrophotometric examination of single cells, and scanning electron microscopy. Juveni le and adult brown smoothhounds have one, identical visual pigment in their rod photoreceptors. The visual pigment is based on the vitamin A (1) chromophore and has a peak absorbance (lambda(max)) at 496 +/- 2 n m. Juvenile and adult leopard sharks also have a single, vitamin A(1) based visual pigment, this one with lambda(max) at 502 +/- 1 nm. No ev idence was found for a shift from a vitamin A(2) based visual pigment in the juveniles to a vitamin A(1) based pigment in the adults, as is known to occur in the lemon shark, Negaprion brevirostris (Cohen et al . [1990] Vision Res., 30:1949-1953). The retinas of brown smoothhounds and leopard sharks contain both rods and cones, with the rods dominat ing. The rod outer segments of the brown smoothhound are much longer t han are those of the leopard shark. The greater length of brown smooth hound rod outer segments and the shorter wavelength lambda(max) of the brown smoothhound's rod pigment support the idea that brown smoothhou nds are either more nocturnal than leopard sharks or are active at gre ater depths. Cones are very rare in both species with only three ident ified with the scanning electron microscope and none with the microspe ctrophotometer. The apparent-paucity of cones most likely relates to t he bottom dwelling lifestyle of these sharks, although the possibility remains that their retinas contain small, specialized regions of high cone density, which were simply missed. (C) 1996 Wiley-Liss, Inc.