370 MECHANICAL AND ACOUSTICAL SENSES 



fraught with the greatest difficulties and complications. It has not always 

 been appreciated that the absence of a sensory signal may be a major positive 

 trigger to the nervous system, so that dramatic changes may result simply 

 from elimination of an organ. This point is well illustrated by the acoustico- 

 lateralis system; although removal of the lateral-line has no obvious effect, 

 removal of a single labyrinth has an immediate impact on a shark's equilib- 

 rium. 



The notable feature of the tactile reflexes in elasmobranchs is that the 

 response is immediate and local, although in the case of strong stimulation 

 the effect may spread and result in contraction of the white musculature and 

 an overall change in the fish's behaviour. In contrast, the reflexes of the 

 acoustico-lateralis system always affect the whole body and lead to major 

 changes in the fish's orientation. 



Tactile Reflexes in Elasmobranchs 



For the most part the tactile endings are sparsely distributed, but areas of 

 enhanced sensitivity exist on all the fins, particularly the pelvic and caudal 

 fins, and around the head and jaws. It has often been reported that a feeding 

 shark will "bump" possible prey with its snout, presumably to assess texture, 

 and this is done with the sensory endings of nerve V. Nerve V endings are 

 also involved in another aspect of feeding behaviour in which contact with 

 the teeth evokes a fast contraction of the jaw musculature. This is brought 

 about by the stimulation of receptors sited under and around the teeth 

 which discharge along fibres that project to the mesencephalic Vth nucleus 

 and have collaterals connecting monosynaptically with the Vth motor nu- 

 cleus that provokes jaw contraction (Roberts and Witkovsky 1975). 



The reflex responses of the fins to toucjf* ave been described by Lissmann 

 (1946a) and Roberts (1967). Usually touch evokes only a local contraction, 

 but strong stimulation of the base of the fins may affect swimming move- 

 ments, which are inhibited and may not recover spontaneously. Stimulation 

 to the tip of the caudal fin is a very effective stimulus for evoking large- 

 amplitude rapid movements (Gray and Sand 1936) which result from the 

 switch from the red to the white musculature (Bone 1966; Grillner 1974). 

 The level of mechanoreceptive input is very significant, therefore, in switch- 

 ing from one muscle system to the other, although presumably this can also 

 be achieved by descending central pathways. In the electric ray, Torpedo, 

 although electric discharge is given to tactile stimuli applied all over the 

 body, touch to the tail is always followed by a large discharge and by a 

 vigorous turning movement that rotates the ray to face the provoker 

 (Roberts 1969e). 



Reflex responses to touch on the body have been studied in the "spinal 

 preparation" by Le Mare (1936), Gray and Sand (1936), Lissmann (1946a), 

 and Roberts (1967). These studies have shown that a vigorously swimming 

 preparation will respond to a gentle ipsilateral stimulus with a sustained 

 ipsilateral contraction, moving the body away from the stimulus. The ampli- 

 tude of the swimming beat also increases, although if the stimulus persists it 



