MECHANORECEPTORS AND BEHAVIOR 377 



closer than 0.2 wavelengths from the vibrating source, whereas at greater 

 .distances only the far field would be important. By applying this idea, and 

 recording the microphonic potential from the canals of Fundulus, Harris and 

 van Bergeijk (1962) neatly demonstrated that the lateral line responds only 

 to water displacement. 



This does not mean, as has often been thought, that the lateral line detects 

 only objects close to the fish for of course, as van Bergeijk (1964) em- 

 phasised, displacements occur at all distances between the source and the 

 receptor. When the extreme sensitivity of the hair cell is taken into account, 

 it is probable that the range of these sense organs is in fact much greater than 

 is usually appreciated. Furthermore, the overall sensitivity of the system is 

 certainly greater than that of individual organs because the signal-to-noise 

 ratio can be improved greatly by central averaging. In the electric fishes, for 

 example, the behavioural measure of threshold is about 200 times better 

 than the thresholds, determined electrophysiologically for individual organs 

 (Machin 1962). 



Because the lateral line and the ear are of very distinct design, their 

 operational capabilities and individual responsiveness to pressure and dis- 

 placement would be expected to differ. It is very unfortunate that, although 

 the mechanics of the semicircular canals have been well studied, the hydro- 

 dynamic properties of the lateral-line system have not yet been examined. 

 The lateral line of sharks, though it involves a canal system, is open to the 

 sea, and its endolymph probably has physical properties similar if not identi- 

 cal to those of seawater. Most biologists believe that a displacement in the 

 seawater directly disturbs the canal fluids, although how this takes place in 

 fishes with closed canals is unclear. However, except for the endolymphatic 

 canal of elasmobranchs, the ear is closed to the seawater and the internal 

 fluids cannot be agitated directly by an external displacement. Instead, it is 

 assumed that the whole fish vibrates slightly to a sound source and the hair 

 cells are stimulated because of the difference in density between the otolith 

 organs and the fish. 



The displacement of the labyrinthine hair cells will certainly be out of 

 phase with the external signal, and the response of the hair cells will be 

 governed by the "stiffness" and "damping" of the system, characteristics 

 which in turn are very dependent on the mass of the otolith. It is perhaps 

 significant therefore that the otoconia of sharks vary in size even for one 

 neuromast organ; perhaps in this way some "tuning" of the sense organs 

 would be established which is essential in these fishes, where it is improbable 

 that any nerve fibre can carry impulses faster than 300-400 Hz, but in which 

 behavioural responses to frequencies higher than this have been observed. 



An obvious pressure transducer is to be found in those bony fishes in 

 which the swim bladder is coupled to the ear, but no comparable structures 

 are found in elasmobranchs and it is uncertain whether they can respond to 

 the pressure component of a sound. A recent finding of Fay and Popper 

 (1974), if applicable to sharks, would suggest that the ear alone responds 

 only to displacement. They were able to show by recording labyrinthine 

 microphonic potentials in goldfish that in fish with the swim bladder 



