VISUAL SYSTEM: STATE OF THE ART 25 



retina from the tapetum also caused melanophores to expand at the same 

 rate and to the same extent as in an eye placed in bright light. The pigment 

 of such a preparation does not retreat again when returned to darkness. 

 Though it is not specifically mentioned by Denton and Nicol, this condition 

 signals neuronal or hormonal communication between retina and choroid. 



Perhaps the most noteworthy feature of the elasmobranch tapetum is that 

 its reflection is specular, i.e., more like a mirror than a diffusing screen. 

 Reflection approaches 90% at certain wavelengths, a value far exceeding that 

 of cats, for example (Weale 1953). In addition, reflection from the tapetum 

 is spectral, and eyeshine in different species of sharks varies from blue green 

 to golden. 



As stated, histological observations demonstrated that throughout the eye 

 the tapetal plates are roughly perpendicular to the light that can reach them. 

 This has the important consequence of reflecting light back through the lens 

 into the environment, which optically reduces glare from tapetally scattered 

 light within the eye while maintaining any visual advantages the tapetum 

 might confer. Denton and Nicol (1965) confirmed this anatomical finding by 

 directly measuring the orientation of the tapetal reflecting surfaces in ex- 

 cised and opened eyes. Their measurements were in complete accord with 

 the earlier histological results. The 1965 study was completed with a 

 thought-provoking discussion of the advantages of tapeta. They first suggest 

 a relation between habitat, primarily depth, and pupil mobility. To this is 

 added tapetal occlusability. For example, Scyliorhinus and other nocturnal, 

 bottom-dwelling, littoral sharks appear to have a highly mobile pupil, a 

 permanently bright tapetum, and a black field in the ventral fundus. Denton 

 and Nicol contrast them with the active, diurnal, pelagic sharks such as 

 Prionace and Squalus. These animals have an occlusable tapetum but little 

 iris movement. This point is questionable, however, since, for example, Car- 

 charhinus falciformis and C. longimanus are pelagic, feed in daylight, and 

 have highly mobile pupils. In addition, results from acoustical tagging 

 (Nelson 1974) show that Prionace is very active at night, moving inshore 

 presumably to feed and then moving offshore at dawn. 



Denton and Nicol point out that the usual explanation for possession of a 

 tapetum is that it imparts a twofold increase in sensitivity. But this cannot 

 be the explanation for sharks. The authors determined concentration of 

 visual pigments in the outer segments of various elasmobranch and teleost 

 photoreceptors and found that the bony fishes (without tapeta) have about 

 twice the optical density of photopigment in their rods. Thus, in a given 

 environment the elasmobranch tapetum is not used for greater absorption of 

 light (i.e., higher sensitivity) but rather to give about the same absorption 

 compared to sympatric teleosts. This is because the shark photoreceptors 

 contain only one-half the concentration of visual pigment. Denton and Nicol 

 suggest two possible advantages of low pigment density combined with a 

 tapetum : 



1) The signal-to-noise ratio of the visual system would be improved with 

 less photopigment since there would be less spontaneous bleaching and thus 



