90 VISION 



In general, physical and chemical properties of seawater have profoundly 

 influenced the evolution of the aquatic eye. Differences between water and 

 air with respect to absorption and scattering of light, pressure variations with 

 depth, and frictional forces have led to large differences between the typical 

 aerial and aquatic eye. In aquatic visual systems the eye of elasmobranchs 

 differs considerably from that of bony fishes, for example, in mode of 

 accommodation, pupillary mobility, ocular adnexa, and tapetal structure. It 

 was perhaps Walls (1942) who first realized the peculiar status of the elasmo- 

 branch eye. He was surprised to note that several characteristics such as 

 mobile eyelids, ciliary folds, and a flattened lens drawn back from an 

 arched cornea, so typical of the aerial eye, were also found in the shark eye. 

 He thus wrote "we might conclude, from a cursory examination of the shark 

 eye, that the elasmobranchs must once have lived on land and, like whales, 

 secondarily returned to the ocean. Surely these peculiarities all have explana- 

 tions . . . but we cannot be sure at present that we quite know all the 

 answers" (p 429). This statement is no less true today, 35 years later. In fact, 

 it has been strengthened by newer examples of similarities between the 

 elasmobranch eye and that of "higher" vertebrates, especially mammals. 



Two things we can say with some assurance: (1) these unique ocular 

 features reflect the long and independent phylogenetic history of the carti- 

 laginous fishes and (2) the elasmobranch eye cannot be considered as "primi- 

 tive." Rather, the shark's visual system is housed in a specialized and ap- 

 parently well adapted vertebrate eye. Clearly, however, the adaptive signifi- 

 cance of the ocular components and visual capabilities of elasmobranchs 

 remains unknown. This is partly because knowledge of the natural history, 

 ecology, and behavior of elasmobranchs is confused and fragmentary and 

 partly because it is conceptually difficult to design a direct field study 

 for determining the biological significance of vision. For example, it is not 

 clear whether the typical shark is an active predator, relying on vision to 

 capture prey, or a scavenger. The question of periods of peak activity has not 

 yet been resolved. Is the typical shark nocturnal? To answer these and other 

 questions, studies on ecology and behavior of sharks must be undertaken. 

 Results will certainly aid in interpreting the structural and functional charac- 

 teristics of the shark visual system. 



One final point concerns the relation of vision to the natural behavior of 

 elasmobranchs. Occasionally authors have attempted to correlate visual 

 parameters with ecological and behavioral characteristics of sharks but these 

 have not been very convincing. We now know something of the visual capa- 

 cities of several elasmobranchs from laboratory studies, and speculation on 

 natural visual behavior is possible. For example, the absolute sensitivity of 

 the lemon shark, Negaprion, would permit visual activity under dim moon- 

 light provided hydrographical conditions were favorable. What is unknown is 

 whether these animals actually use that capacity. Thus, a real need exists to 

 study the activities of elasmobranchs under natural or field conditions so 

 that insight might be gained on the importance of the visual system in the 

 lives of the elasmobranchs. 



