VISUAL SYSTEM: STATE OF THE ART 



23 



the arthropods and vertebrates, including elasmobranchs, whose tapetum 

 consists of a series of reflecting plates in the choroidal layer behind the 

 retina. These plates and associated melanocytes, known collectively as the 

 tapetum lucidum choroidale (Figure 4), have been well studied over the last 

 10 years. Gilbert (1963) presented a rather complete review of the elasmo- 

 branch tapetum and suggested a need for further inquiry into mechanisms of 

 tapetal occlusion. In a series of independent investigations, both Nicol and 

 Kuchnow addressed themselves to some of the questions Gilbert raised. 



One of the most unusual features of the elasmobranch's tapetum is its 

 ability to darken. This so-called occlusability, described by Franz (1931), 

 was never subjected to experimental verification. During occlusion, light- 

 screening pigments are said to migrate over the reflecting plates, darkening 

 the tapetum. 



Anatomy and Function— To confirm this proposed mechanism Nicol 

 (1961) investigated structure and occlusability of the tapetum of Scyl- 

 iorhinus. The eyes of experimental animals held under various light regimes 

 were removed for gross observation and histological study. Experimental 

 results disagreed with Franz' (1931) original proposal. Nicol's data clearly 

 demonstrated that the tapetum of Scyliorhinus does not darken, i.e., is not 

 occlusable. Histochemical tests on tapetal pigments suggested that the light- 

 screening material was melanin; the reflecting pigments were thought to be 

 guanine, but chemical tests were inconclusive. One final observation in the 

 1961 paper was that photoreceptors underlying the permanently bright 

 tapetal stripe were twice as long as elsewhere in the retina. 



The failure of Franz' occlusion theory for Scyliorhinus prompted Nicol to 

 continue this work, and he eventually published observations on some 20 

 elasmobranch species. Quantitative results on percent reflection and kinet- 

 ics of occlusion in several species, most notably Squalus acanthias, were 



Figure 4 The tapetum lucidum cho- 

 roidale in Squalus acanthias. Environ- 

 mental light entering the eye and passing 

 through the retina continues through the 

 retinal epithelium (RE), which unlike 

 that of most vertebrates, is devoid of 

 screening pigment. Light then passes 

 through the nutritive choriocapillaris 

 (CC) to impinge on the tapetal plates 

 (TP). If, as in darkness, the melanin pig- 

 ment (M) is withdrawn into the pigment 

 cells (PC), light will reflect from the re- 

 flecting crystals (RC) back through the 

 retina. This arrangement produces eye- 

 shine in sharks. (Modified from Best and 

 Nicol (1967). (Reproduced by kind per- 

 mission from the authors and the editor 

 of Contributions in Marine Science.) 



light 



