260 CHEMICAL SENSES 



THE SEARCH FOR CHEMICAL REPELLENTS 



An immediate and practical motivation for much research on the chemo- 

 sensory systems of sharks has been the desire to identify chemical repellents. 

 What may seem to be an obvious objective, even a relatively simple one, 

 turns out to be anything but simple. The key difficulty arises from what is 

 meant by the term "repellent." It may be interpreted, most simply, as any 

 chemical that lessens the probability of a shark's ingesting some normal food 

 substance. On the other hand, the word may be taken to mean a chemical 

 capable of deterring biting, or any other form of attack, by a shark already 

 in a feeding frenzy and behaving aggressively toward virtually anything seen 

 in its immediate vicinity. In the latter case, the repellent would have to be 

 much more than a compound that imparts an unpalatable flavor to a poten- 

 tial food; it would have to incapacitate an attacking shark, through some 

 powerful pharmacological or toxic action. Considerable confusion has arisen 

 from discussion of repellents without specifying what type of repellent 

 action is being studied or sought. 



Early evidence of the simpler types of repellents, decreasing food inges- 

 tion, was obtained from several classic studies, notably that by von Uexkull 

 (1895). He demonstrated that quinine prevented ingestion of fish meat, and 

 that sharks that took quinine-mixed baits into their mouths promptly spit 

 them out; after the quinine was washed out of the bait, the meat was readily 

 ingested. Von Uexkull was also among the first to demonstrate a natural 

 repellent, one produced by the marine gastropod Aplysia. Sharks excited 

 by fish baits were observed to deviate from their usual behavior of ignoring 

 these marine slugs; when taken into the mouth of a shark, the Aplysia was 

 promptly spat out. 



A systematic search for protective shark repellents was begun in the 

 1940s, after many reports of shark attacks on American servicemen from 

 aircraft and ships downed in the tropical Atlantic and Pacific. The first 

 experiments were conducted at the Woods Hole Oceanographic Institution, 

 using the smooth dogfish, Mustelus canis. Unfortunately, the dogfish was 

 not deterred from eating by any of the candidate repellents tested : systemic 

 poisons, chemical warfare gases, ink clouds, etc. Gilbert and Springer (1963), 

 Tester (1963), and Gilbert and Gilbert (1973) have reviewed many of the 

 experiments aimed at the discovery of repellents during that period. 



The following list, incomplete, indicates at least part of the range of com- 

 pounds and extracts tested in the hope that one or another of them might 

 repel sharks: acetylcholine, amino acids, ammonium hydroxide, choline 

 chloride, cortisone acetate, creatinine, diallyl maleate, diallyl phthalate, 

 furfural, histamine, human sweat, human urine, nicotine, and phenylacetate 

 salts. Suffice it to say that none of these gave any evidence of repellent 

 properties. 



The repellent actually developed, and still available, was called Shark 

 Chaser, a mixture of 20% copper acetate and 80% nigrosine dye in a water- 

 soluble wax cake, packaged in a plastic envelope. The rationale for the 

 choices of ingredients has been reviewed by Gilbert and Gilbert (1973), and 

 derives from observations upon the reactions of sharks to breakdown 



