344 THE BIOLOGY OF MARINE ANIMALS 



frighten fish and warn them of danger, and sound production is used for 

 warning purposes and is part of the social and breeding behaviour of some 

 species. Parker (116) noted three types of responses by fish to sounds. 

 When the side of the container was struck, some species retreated from the 

 source {Tautoga), others were attracted {Prionotus), and still others 

 became quiescent. Male drum fishes (sciaenids) produce sounds by move- 

 ments of the air-bladder, probably as a means of communication during 

 the breeding seasons (p. 406). 



The lateral line organs of fishes are sensitive to pressure waves impinging 

 upon the animal. The delicate hairs on the neuromast sensory cells operate 

 on the principle of microlevers, being deformed by the flow of fluid along 

 the canals. Sand (127, 129), who developed a technique for perfusing the 

 hyomandibular canal, showed that flow in one direction excited some 

 receptors, whereas flow in the opposite direction inhibited them (Fig. 

 8.28). The two directions of flow in the lateral line canal constitute anta- 

 gonistic stimuli; and there is evidence in the neuromast organs for two 

 kinds of receptors which react in opposite ways to flow in the two 

 directions. 



The lateral line nerve shows a constant background of spontaneous 

 activity, on which is superimposed discharge patterns when the lateral line 

 is stimulated by vibratory stimuli up to 350 c/s (Fundulus, Ameiurus). In 

 Fundulus the nervous discharge synchronizes with sonic frequencies up to 

 at least 180 c/s. An increased rate of discharge is also produced by mechan- 

 ical pressure over the lateral line canal, by irregular water currents, ripples 

 in the water and by movements of the fish's trunk. Swimming movements 

 of other fish in the neighbourhood are also detected by the lateral line 

 neuromasts. A microphonic potential can be recorded from the neuromast 

 organ when the pick-up electrode is close to the cupula. In the range 14-48 

 c/s the electrical output is proportional to the amplitude of movement of 

 the cupula over the hair cells (Acerina). 



Some physiologists consider that the lateral line system of fishes partici- 

 pates in sound-perception, but the more probable conclusion is that it is 

 concerned with "distant touch" orientation, thereby supplementing 

 vision. The neuromasts are stimulated by water waves set up by other 

 moving objects, or by waves produced by swimming movements of the 

 fish itself and reflected back from other objects. By this means the fish is 

 able to detect and localize distant objects in turbid waters, or even in the 

 absence of vision (14a, 54, 56, 74, 75, 124, 137). 



Hearing in Cetaceans. There is considerable evidence that whales can 

 hear and that some species produce sounds underwater. During whaling 

 operations whales are visibly affected by noises, and hearing may be 

 responsible for the co-ordinated manner in which they surface, and for the 

 way in which scattered schools join up with one another. Porpoises have 

 been observed to respond to artificial noises produced in the water, and 

 supersonic depth-finders will frighten away schools of these animals. The 

 external auditory meatus is patent in most toothed whales (the cachelot 



