342 



THE BIOLOGY OF MARINE ANIMALS 



The lateral line system of fishes consists of a series of grooves or canals 

 in the head and along the trunk. In some elasmobranchs the lateral canal 

 is represented by an open groove. In teleosts the canals perforate the scales 

 along the lateral line, opening to the exterior at intervals by pores. Within 

 the canals is a series of sensory structures or neuromasts, each containing a 

 group of sensory cells bearing hair-like projections (Fig. 8.27). The system 

 is innervated in the head by branches of the facial, glosso-pharyngeal and 

 vagus nerves, and in the trunk by the ramus lateralis vagi (81). 



Behaviour experiments have shown that fish are very sensitive to under- 

 water sounds. Killifish {Fundulus heteroclitus) respond to sounds (vibrations 

 of a tuning-fork at 128 c/s) by movements of the pectoral fins and an in- 

 crease in respiratory rate, but become insensitive when the auditory nerves 

 are cut. The smooth hound {Mustelus) and squeteague (Cynosciori) show 



Lateral line 

 nerve fibre 

 Fig. 8.27. Transverse Section through Lateral Sensory Canal of 

 Mustelus canis (x 370) (from Johnson, 1917.) 



characteristic reactions to the tapping of the walls of the aquarium; these 

 responses persist after cutting nerves to the skin and the lateral line. 

 Severing the auditory nerves of Mustelus reduces sensitivity but does not 

 abolish it. Following destruction of the utriculus and semicircular canals, 

 Cynoscion loses control of its equilibrium but still responds to sounds. 

 Injury to the sacculus, however, reduces sensitivity to tapping. 



By means of conditioned reflexes, Bull (18) showed that eels (Anguil/a 

 anguilla) are sensitive to a submerged buzzer and to a 128-cycle tuning- 

 fork. Upper limits of sensitivity to sound frequencies (tones) in some fishes 

 are shown in Table 8.1. In behaviour experiments, in which auditory 

 stimuli were associated with feeding, minnows learnt to distinguish two 

 tones differing in frequency by less than an octave (\- \ octave). 



By operating on the ears of minnows (Phoxinus) von Frisch (51) showed 

 that hearing is located in the sacculus and lagena, which are sensitive to 

 frequencies between 32 and 6,000 c/s. Lower frequencies, between 16 and 

 65 c/s, are perceived through tactile skin receptors; at the lower extreme 



