VISUAL SYSTEM: STATE OF THE ART 87 



more frequently. The relevance of such studies to visually oriented natural 

 behavior of sharks is questionable. 



An important consideration in the behavior of an animal is how ambient 

 changes in illumination affect its activities. It has often been reported that 

 sharks are strictly nocturnal, remaining quiescent during the day and feeding 

 at night (Walls 1942, Bigelow and Schroeder 1948, Randall 1968). Nelson 

 and Johnson (1970), Nelson (1974), and Finstead and Nelson (1975) have 

 provided experimental evidence in three shark species that favors this view. 

 Laboratory observation of activity (swimming) rates under controlled 

 illumination demonstrated that horn sharks, Heterodontus francisci, and 

 swell sharks, Cephaloscy Ilium ventriosum, are distinctly nocturnal. Both 

 species possess a drifting endogenous rhythm under constant dim illumina- 

 tion and are strongly inhibited from swimming by bright light. Both are 

 immediately active in darkness, and one horn shark remained continously 

 active for 264 h of darkness. These results were confirmed by field observa- 

 tion and experimentation and extended to the angel shark, Squatina califor- 

 nica (Standora 1972), and blue shark, Prionace glauca (Sciarrotta 1974). 



Observations on other species indicate that gray sharks (Carcharhinidae) 

 are crepuscular, feeding primarily at dawn and dusk (Hobson 1968). Still 

 other species are active by day and feed in shallow water on the brightest 

 afternoon (Starck 1968). Again species differences must account for part of 

 the variation in behavior associated with light levels. However, the data on 

 life history and behavior of sharks are so fragmentary that it is difficult to 

 draw a definite conclusion as to whether sharks are nocturnal, as they have 

 been traditionally labeled. Based on the definitive work of Nelson and his 

 colleagues, however, it is clear that some species are truly nocturnal although 

 they will feed during daylight if given the opportunity. 



MEDIAN EYE 



The pineal organ has long been suspected of being a light receptor, and in 

 some lower vertebrates the related parapineal organ is even known as the 

 parietal eye. The elasmobranchs possess a well-developed pineal organ, which 

 was described in the early 20th century. Studnicka (1905) reported the 

 presence of ganglion cells, and Holmgren (1918) described inner and outer 

 segments of sensory cells in the pineal parenchyma of Squalus. In his note- 

 worthy study, Holmgren homologized the pineal receptors with light- 

 sensitive cells of the retina and demonstrated the presence of nerve fibers in 

 the wall of the pineal organ. Contemporary anatomical studies on the 

 median eye have sought to characterize both ultrastructure and function of 

 this organ. 



Anatomy 



Altner (1965) reported on histology and histochemistry of the pineal in 

 Etmopterus, Galeus, and Squalus. He described sensory cells in the pineal of 



