SHARKS AND UNDERWATER SOUND 409 



orientation in fishes. In that theory, he maintained that (1) orientation to a 

 sound source occurs only when a fish is extremely close to that source (i.e., 

 within the acoustic near-field (see page 392) and (2) oriented responses are 

 mediated only through the lateral line system, the labyrinths being precluded 

 from that function. 



This theory was of concern to naturalists and other functional biologists 

 interested in the adaptive value of sound detection in fishes. Although it was 

 clear that sharks and teleosts could detect low-frequency sounds, serious 

 questions could be raised as to the importance of any such detection when 

 sound sources had to be localized through random movement. Initial studies 

 indicated that fishes followed the rules, the single exception being the sharks. 

 Early tests of acoustic orientation in teleosts were invariably carried out 

 under the acoustically complex conditions of the laboratory setting (for a 

 critique, see Chapman and Hawkins 1973, Sand and Enger 1974). The only 

 studies conducted in open waters were those centering on sharks, and these 

 went counter to the rules. This suggested that some important factor or 

 factors had been overlooked by van Bergeijk and subsequently by the ad- 

 herents of his theory. If so, at least some teleosts might well also demonstrate 

 far-field orientation under appropriate conditions. 



These conditions were recently provided at various locations, with experi- 

 ments being carried out at sea (Chapman 1973, Olsen 1969 in Sand and 

 Enger 1974, Sand and Enger 1974, Schuijf 1974, Schuijf and Siemelink 1974, 

 Schuijf et al. 1972) as well as in the laboratory (the latter using electro- 

 physiological techniques— Enger et al. 1973, Sand 1974). These studies 

 showed that selected teleosts can orient to sound in the far-field and in a 

 number of instances that orientation is effected through the labyrinth organs. 



Interest in spatial orientation has now shifted to the fascinating problem 

 of its precise underlying mechanisms. Recently, Schuijf (1974) has proposed 

 an elegant model to explain directional hearing in sharks and in teleosts with- 

 out swimbladders. First, it states that directional hearing is not limited by 

 distance, so long as the signal level exceeds the threshold for directional hear- 

 ing at the existing noise level. Also, such animals can localize a sound source 

 only if, in addition to the radial component of the particle displacement at 

 the position of the fish, there is also a tangential component in the vertical 

 plane through the source and the fish's position. The importance of the ver- 

 tical plane to this model corresponds well with evidence that fishes are sensi- 

 tive to displacements in both the horizontal and vertical planes. 



The carefully reasoned theory certainly can explain instantaneous resolu- 

 tion of the 180° ambiguity within certain depth restraints. Once that 

 ambiguity is resolved, even slight head movements of a moving shark should 

 provide accurate information relative to a right-left decision if there is ade- 

 quate detection by both ears and the respective axes of maximum sensitivity 

 within the appropriate maculae are not parallel (see Vilstrup 1951, Figs. 16, 

 17, and 18). Differences in the amplitude of the microphonic potentials 

 from the two labyrinths of the perch, Perca fluuiatilis, have been shown to 

 depend on the direction of vibration (Sand 1974; see discussion in Sand and 

 Enger, 1974). 



