BEHAVIOR AND CNS INTEGRATION 197 



1975, Kritzler and Wood 1961, Nelson 1967). They also respond quickly to 

 standard classical conditioning procedures (Gruber and Schneiderman 1975). 

 Similar breakthroughs have been made in the study of shark neuroanatomy 

 (see Northcutt 1978). As a result, the potential now exists for integrating 

 these related sets of data into a tentative model of the shark brain and 

 behavior relationship. While the purpose of this chapter is not to construct a 

 model, its contents are offered in support of such a project's feasibility. As 

 Aronson (1963) suggested over 10 years ago, the marriange of behavioral 

 analysis and experimental neuroanatomy is long overdue in shark labora- 

 tories. Such a model, if developed, would provide needed insights into the 

 functioning of central neural systems now poorly understood in sharks. It 

 might also heighten our understanding of the behavior of pelagic and other 

 shark species that are poorly suited for laboratory study. 



Traditional Shark Neurology 



Although the recent findings of sensory and behavioral biology are encourag- 

 ing, they raise some difficult questions about brain structure when 

 contrasted with the classic views of comparative neurology. The major 

 problem is the apparent paradox of a complex set of sensory inputs and 

 behavioral outputs being processed by a relatively simple, undifferentiated 

 central nervous system. 



The concept of the shark brain as primitive and dominated by olfactory 

 areas dates back to the early twentieth century. Comparative neurologists, 

 attempting to reconstruct theoretically the evolution of the vertebrate brain, 

 quickly recognized the importance of sharks as "living fossils" and began 

 studying them intensively. Their findings reflected the prevailing views of 

 biologists regarding shark sensory capabilities, particularly those of Parker 

 and Sheldon who had elegantly demonstrated the dogfish's (Mustelus canis) 

 apparent total dependence on smell for locating prey (Parker 1910, Parker 

 and Sheldon 1913, Sheldon 1911). Why else was the telencephalon occupied 

 almost entirely by olfactory areas and pathways? Meanwhile, the functional 

 roles of other known senses, such as vision, appeared to be secondary and 

 less demanding of high-level central integrating mechanisms. The anatomical 

 restriction of visual input to the midbrain optic tectum supported this 

 notion, especially considering the lack of tectal differentiation in sharks as 

 compared to teleosts. 



Even today many authors (Aronson 1963, Lineaweaver and Backus 1973, 

 Nieuwenhuys 1967) state that the shark brain consists of poorly interrelated 

 areas devoted mainly to sensation, especially olfaction. They note that the 

 forebrain appears to lack distinct nuclear groups and to be devoid of any 

 neural basis for associative processes (Voronin et al. 1968), a finding con- 

 sistent with the view that sharks are primitive fish whose behavior is often 

 "clumsy" and is based mainly on instinctive stereotyped response patterns 

 (Aronson 1963). It is therefore not surprising that no attempt has been 

 made to search for a neural substrate underlying behavioral flexibility in 

 these fish. 



