262 CHEMICAL SENSES 



this group of chemicals was recognized as an important category of natural 

 products. Hodgson, Mathewson, and Gilbert (1967) reported that holothurin 

 (from Cuvier's gland of the sea cucumber, Actinopyga agasszi) elicited 

 avoidance behavior by lemon, nurse, and bonnet sharks tested in the hydro- 

 dynamic tunnel. Backus (1973) noted that "... holothurin is a very effective 

 deterrent against predation" and found that fishes seldom even attempt to 

 mouth holothurians, and that holothurians are only rarely eaten by sharks. 



Cameron and Endean (1973) and Cameron (1974) have theorized that 

 naturally occurring chemical defenses, repellents, or toxins active against 

 potential predators may be particularly prevalent among the relatively im- 

 mobile or stationary organisms in marine environments. This is especially 

 true in coral reef ecosystems where great adaptations to stationariness and 

 intensive interspecific predation take place. 



An interesting example of a relatively immobile teleost fish that appears 

 to produce a chemical shark repellant was reported by Clark and Chao 

 (1972). Pardachirus marmoratus, the "Moses sole" from the Red Sea, 

 secretes a milky substance that, at certain concentrations, is extremely 

 aversive to sharks. It has been demonstrated that sharks will approach 

 Pardachirus, but fail to close their jaws on it. Under experimental condi- 

 tions, the fish's secretion affords protection to other kinds of fish that 

 sharks otherwise eat (Clark 1974). 



The source of the secretion in Pardachirus appears to be enlarged dermal 

 mucus glands or modifications of muscles controlling movements of dorsal 

 and anal fins. Clark and Chao consider that the toxin is "probably a pro- 

 tein" since it is ineffective after boiling, and it is not affected by freezing. 

 Dr. Eliahu Zlotkin (cited by Clark 1974) found that the exudate has both 

 neurotoxic and hemotoxic effects, and it can be separated into three pro- 

 tein components. A natural inhibitor of the hemotoxin was found, pos- 

 sibly protecting Pardachirus from its own poison. 



It would be interesting to compare the finding on P. marmoratus with 

 the other species of Pardachirus inhabiting the littoral waters of the Indian 

 and Pacific Oceans. A closely related genus, Aseraggodes, which has no 

 open pores above the base of the dorsal and anal fin rays (Marshall 1964), 

 would also make an interesting comparison. 



In addition to the echinoderms cited above, the invertebrate animals 

 offer a potentially rich field for exploration of naturally occurring repel- 

 lents and toxins. The Proceedings of Recent International Coral Reef 

 Symposia (Cameron et al. 1974, Taylor 1977) provide numerous examples 

 of potentially valuable sources of natural repellents and toxins. One exam- 

 ple is the highly toxic "palytoxin" produced by the zoanthid Palythoa; 

 paly toxin is one of the most highly toxic nonprotein substances known. 

 It is a vasoconstrictor affecting smooth, striated, and cardiac muscle, but 

 repellent properties are as yet untested (Attaway and Ciereszko 1974). 

 Among the arthropods, several highly poisonous species of crabs have been 

 known since the late eighteenth or early nineteenth centuries. Recent 

 studies (Garth and Alcala 1977) indicate that several species produce a 

 heat-stable, water-soluble, and highly toxic substance that quickly affects 



