orientation through sound in teleosts have been inconclusive and the 

 physical basis for an orientation mechanism using acoustical cues seems 

 difficult to formulate. 



A physical theory of electrical orientation with respect to open-ocean 

 streams and to the earth's magnetic field has recently been developed 

 (Kalmijn, 1973, 1974). Now that we know the high electrical sensitivity 

 of the animals and the strengths of the pertinent stimulus fields, it 

 seems quite feasible that electro-orientation aids short- and long-range 

 navigation in sharks and rays. 



The possible significance of other sensory cues for orientation in 

 sharks has hardly been considered experimentally. Although the lateralis 

 system has been studied to a considerable extent in these animals, its 

 role in orientation is unknown. Orientation through temperature perception 

 and vision has not been investigated. Particularly, the possibility of 

 perception of and oriented response to polarized light deserves early 

 investigation in view of recently demonstrated capabilities of some 

 teleosts in response to the e-vector of a polarized laboratory "sky" 

 (Kleerekoper e_t al_. , 1973) and in field experiments (Waterman, 1973) , In par- 

 ticular, the interaction of two or more cues in locomotor orientation in 

 sharks merits urgent attention. Apart from the recent experiments on 

 flow-chemical and electrical-chemical cue interaction, no work has been 

 done in this area. Many of these fields of research will have to be 

 attacked initially in the laboratory so that quantitative approaches can 

 be used. Recent monitoring and statistical techniques would enhance the 

 chances of success. New telemetric techniques now offer possibilities in 

 the field for the coarser tracking of movements of sharks and for the 

 design of meaningful experimentation. Whether in the laboratory or in the 

 field, it should be emphasized that short term experiments on locomotor 

 orientation do not inspire confidence in view of the great variability in 

 this behavior over time. This variability can be statistically accounted 

 for in stochastic models of locomotor behavior extending in time. 



c. Osmoregulation 



The biology of the shark's nervous system, both motor and sensory, is 

 of obvious importance to its efficiency as a predator and its threat to 

 man. Perhaps less apparent, yet a most important aspect of the shark's 

 physiology, is the way it maintains water and solute balance under con- 

 ditions which tend to disrupt it, i.e., its osmoregulatory mechanisms. 



Earlier work on elasmobranch osmoregulation has been reviewed, and 

 much of the recent work reported, in two symposium volumes: Gilbert, 

 Mathewson, and Ralls (1967); and Goldstein (1972). 



In broad outline the major features of the osmoregulation of marine 

 elasmobranchs have been fairly well worked out. Instead of regulating 

 the solute content of the body fluids well below that of the sea water, 

 with the expenditures of much energy, as is done by marine teleosts, 

 elasmobranchs accumulate large quantities of organic substances, especially 

 urea, and to a lesser extent trimethylamine oxide. These are retained in 

 quantities sufficient to make the body fluids hyperosmotic to sea water 

 and thus provide water for a limited quantity of urine, relatively con- 

 centrated, but still hypotonic to the serum. Excess salt is excreted by 



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