248 CHEMICAL SENSES 



The interpretations of results of tests with electrolytes has been discussed 

 in more detail by Hodgson, Mathewson, and Gilbert (1967). It appears un- 

 likely that ions constitute major cues to feeding in sharks, but it is entirely 

 possible that sharks might orient toward or away from hypersaline (or 

 hyposaline) waters, using reactions mediated by their chemoreceptors. 



Carbohydrates— The series of carbohydrates tested has been particu- 

 larly revealing about sugar detection mechanisms of mammals and insects. 

 In both groups, one category of chemoreception can be traced to particular 

 molecular configurations of carbohydrates (Hodgson 1974). Evidently the 

 situation is quite different with sharks, since EEG responses were absent 

 and behavioral responses were considered questionable. 



Amino acids and amines— Protein breakdown products and related 

 compounds have long been known to stimulate chemoreceptors in a variety 

 of marine animals. Among invertebrates, studies that demonstrate this range 

 from experiments on carnivorous coelenterates (Laverack 1968) to experi- 

 ments on many marine arthropods (Case and Gwilliam 1961, Levandowsky 

 and Hodgson 1965). Hara (1973, 1975, 1976) has demonstrated the high 

 stimulatory effectiveness of certain amino acids for olfactory receptors of 

 teleost fishes. Accordingly, five amino acids and three tertiary amines were 

 chosen to determine whether sharks might have some of the same sensi- 

 tivities and responses. 



As shown in Table 1, this was one of the most stimulating groups of pure 

 chemicals tested. Both EEG and behavioral responses were striking and pro- 

 longed. Figures 5 and 6 illustrate EEG responses, and Figures 7, 8, and 9 

 show typical orientation responses by nurse and lemon sharks. 



Glycine and glutamic acid were the most effective of the pure amino 

 acids tested. Betaine, trimethylamine (TMA), and trimethylamine oxide 

 (TMAO), all breakdown products in or from tissues or excreta of fish, elicit 

 particularly strong responses under the test conditions. This parallels many 

 of the results with arthropods and teleost fishes and raises an interesting 

 evolutionary question, for fishes at least. That is, do certain specialized 

 olfactory cells of these animals derive from a single ancestral cell type in 

 evolution? The adaptive value of this sensitivity to protein constituents 

 and breakdown products for predatory carnivores is obvious. 



Lipids and related compounds— Stearic acid was tested as an example 

 of a saturated fatty acid and linoleic acid as an example of an unsaturated 

 fatty acid. Because of its special importance in conjugating with fatty acids 

 to form true fats, glycerol was also tested. (Solubility characteristics dictated 

 that the fatty acids were tested as emulsions.) The polyhydroxyl alcohols 

 (sorbitol, dulcitol, mannitol, and inositol) were tested because they had pro- 

 vided insight regarding mechanisms of other chemosensory systems, and be- 

 cause inositol is useful for identifying chemoreceptors responding to ring- 

 structured, rather than straight chain, molecules. Emulsions of cod and 

 tuna liver oils were used to check the commonly held belief that sharks 

 prefer prey and meat with higher oil concentrations. 



