FISHERY BULLETIN: VOL. 72, NO. 1 



based on chemical agents. These interactions 

 and the characterization of the chemical agents 

 involved are the subject of the newly developing 

 field of chemical ecology (Sondheimer and 

 Simeone, 1970). Chemicals that are syntheized 

 and released by one individual of a species to 

 alter the behavior of other members of the 

 species are termed pheromones. These signals 

 range in their function from trail markers and 

 territorial markers through alarm and defense 

 signals to those which control caste structure in 

 social insects and the sex pheromones that are 

 calling signals and aphrodisiacs. Chemicals also 

 have a wide range of interspecific interactions. 

 A substance produced by one organism may 

 influence the behavior of members of other 

 species. A flower scent that enhances pollina- 

 tion is a well-known example. This field of 

 chemical ecology has been termed allelo- 

 chemics, and the chemical agents have been 

 subdivided on the basis of function into allo- 

 mones, which give adaptive advantage to the 

 producing organism, and kairomones, which 

 give adaptive advantage to the receiving 

 organism (Whittaker and Feeny, 1971). The 

 allomones include the repellents produced by 

 many plants and animals, suppressants which 

 inhibit competitors (e.g., fungal antibiotics), 

 venoms, inductants (e.g., gall producing agents), 

 and attractants (e.g., chemical lures). The kairo- 

 mones include attractants (e.g., the scent of a 

 prey), inductants (e.g., the factor that stimulates 

 hyphal loop development in nematode-trapping 

 fungi), danger signals (e.g., predator scents, 

 secondary plant substances indicating toxicity), 

 and stimulants (e.g., hormones that induce 

 growth in the receiving organism). 



The diverse natural products, coumarins, 

 quinones, flavonoids, acetylenes, terpenoids, 

 saponins, cardiac glycosides, alkaloids, thiols, 

 and cyanogenic glycosides, which were long 

 considered metabolic waste products, are now 

 recognized to be allelochemic agents. Examina- 

 tion of the function of these natural products 

 provides some insight into their evolution. Some 

 of these compounds are toxic, some are 

 chemical lures, others inhibit the growth of 

 competitive plant species, but the bulk of these 

 compounds probably function as "feeding inhi- 

 bitors" of herbivores (Gilbert, Baker, and Norris, 

 1967; Munakata, 1970). The coevolution of 

 butterflies and plants is considered by Ehrlich 

 and Raven (1964). They emphasize the role 



of reciprocal selective responses during this 

 evolution and conclude that "the plant-her- 

 bivore interface may be the major zone of inter- 

 action responsible for generating terrestrial 

 organic diversity." The "accidental" evolution 

 of a metabolic sequence resulting in the produc- 

 tion of a noxious substance by a plant provided 

 a selective survival advantage in the clone 

 carrying this capability. Decreased predation by 

 herbivores on those individuals containing 

 the highest concentrations of the new sub- 

 stance resulted in genetic selection for increased 

 synthesis and storage of the noxious substance. 

 Such "protected" species experience an 

 explosive increase because of their protection 

 from contemporary phytophagous organisms. 

 The first evolutionary response of the her- 

 bivores must have been the development of the 

 capability to detect the compound, i.e. sensitive 

 external chemoreceptors. Later evolutionary 

 events led to the development in some indi- 

 viduals of a tolerance for the noxious substance. 

 The herbivores which developed this tolerance 

 then had access to a large food supply for which 

 there was no competition. The ability to detect 

 the substance then had an altered function, the 

 feeding inhibitor was now a feeding stimulant. 

 The present evidence of the repeated occur- 

 rence of this cycle is the existence of tightly 

 coupled herbivorous insects and their host 

 plants, presumably arising through coad- 

 aptation. 



In 1955 Haldane, in a consideration of 

 chemical communication and visual signals, 

 wondered if cryptic odors had ever evolved. 

 While most of the feeding inhibitors that have 

 evolved are probably irritants, many may be 

 cryptic odors. It is likely that the two activities 

 may only differ in the membrane affected. The 

 term irritant implies membranic sensitivity 

 and, of those membranes of an organism in 

 immediate contact with the environment, the 

 chemosensory membranes are likely the most 

 sensitive to chemical irritation. In an environ- 

 ment in which a major fraction of the informa- 

 tion flow is chemical, any agent capable of 

 disrupting the chemosensory organs of a preda- 

 tor would provide an ideal mechanism for "hid- 

 ing" from that predator. Cryptic odors may be 

 either "negative odors" altering, for protracted 

 periods, the membrane potential of the dendrites 

 and blocking their normal generator potential, 

 or they may be the chemical equivalent of a 



