Visual pigments and photoreceptors in two species of shark,Triakis semifasciata andMustelus henlei

Abstract

The retinal photoreceptors of brown smoothhound sharks (Mustelus henlei) and leopard sharks (Triakis semifasciata) were studied by spectrophotometric analysis of visual pigment extracts, microspectrophotometric examination of single cells, and scanning electron microscopy. Juvenile and adult brown smoothhounds have one, identical visual pigment in their rod photoreceptors. The visual pigment is based on the vitamin A1 chromophore and has a peak absorbance (λmax) (at 496 ± 2 nm. Juvenile and adult leopard sharks also have a single, vitamin A1 based visual pigment, this one with λmax at 502 ± 1 nm. No evidence was found for a shift from a vitamin A2 based visual pigment in the juveniles to a vitamin A1 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 dominating. The rod outer segments of the brown smoothhound are much longer than are those of the leopard shark. The greater length of brown smoothhound rod outer segments and the shorter wavelength λmax of the brown smoothhound's rod pigment support the idea that brown smoothhounds are either more nocturnal than leopard sharks or are active at greater depths. Cones are very rare in both species with only three identified with the scanning electron microscope and none with the microspectrophotometer. The apparent paucity of cones most likely relates to the 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. © 1996 Wiley-Liss, Inc.