Hearing and acoustic orientation in marine animals 4()7 



and Young, 1947). But the sounds of fish and marine mammals are so numerous, 

 so intense, and so diversified, that a review of what is known concerning their bio- 

 logical significance is timely. While only a small minority of the known species of 

 fish emit sounds that are more than mere by-products of other activities (such as the 

 grinding of teeth), many of those that are consistent noisemakers have specialized 

 structures for the generation of sound, such as muscles located in the walls of the 

 swim bladder. It therefore seems likely that these sounds serve some dcfmite purpose 

 in the lives of the fish that make them. Cases are known in which fish are attracted 

 by sound (Westenberg, 1953), but more often they are repelled by artificially generated 

 sounds, while under many circumstances they show no discernible reactions at all. 

 Some species apparently communicate with each other by means of sound, for exam- 

 ple the toadfish Opsanus tau, in which the male calls principally during the breeding 

 season. There is suggestive evidence to be discussed below that orientation may be 

 maintained with respect to the bottom by a process akin to echo-sounding; and it is 

 not beyond the bounds of possibility that fish or marine mammals may employ a 

 sort of natural sonar — an underwater analog of the process of ccholocation so 

 highly developed by bats for use in air. Questions of this type deserve more attention 

 than they have yet received, and the chief purpose of this paper is to review the 

 available evidence of acoustic orientation by marine animals and to suggest pro- 

 mising lines of inquiry for the future. It is particularly appropriate that such a review- 

 should be part of a volume dedicated to Professor Bigelow, for he contributed 

 significantly, in 1904, to the basic biological knowledge that has formed an essential 

 groundwork for these recent developments. 



The sensitivity of hearing in fish 



Fundamental to any consideration of the role of underwater sound in orientation, 

 or in other types of behaviour, is the question of sensory capacities of the animals 

 concerned. No animal can react to a sound it cannot hear, and even the author of 

 an authoritative textbook recently wrote, in introducing the auditory system, 

 " Through the long and complex evolution of the fishes no progress was made, 

 since no auditory receptors exist. . . . Fish do not hear, and there would be little 

 to hear under water . . . Hearing is developed on the assumption of terrestrial hie. 

 hence, is first encountered in the amphibia" (Krieg, 1942). Even when a particu- 

 lar species has been shown, quaUtatively, to be able to react to underwater sound, 

 any careful analysis of its behaviour is Ukely to require quantitative measurement 

 of the range of frequencies and intensities that it can hear. It was to this basic 

 question that Bigelow addressed himself more than fifty years ago. 



There was already observational and anecdotal evidence that certain hsh reacted 

 to sound, usually to sounds generated in air. On the other hand several attempts 

 to demonstrate hearing in fish under controlled conditions had produced negative 

 results, or at best ambiguous evidence. Among the more recent ol these investiga- 

 tions Lad been those of Kre.dl (1895, 1896), who had a few years previously 

 carried out in Sigmund Exner's laboratory in Vienna one of the classic experi- 

 ments o^ comparative physiology (the substitution of iron n ngs - sand gnj.ns 

 in the equiUbrium organs of ceriam shrimps so that he action of tlK o ths 

 could be demonstrated unequivocally by the use ol \;^^-^^^^Z.\^ 

 observed the responses of goldfish (Carassius auntus) to sounds ol nuxkratcK low 



