332 MECHANICAL AND ACOUSTICAL SENSES 



INTRODUCTION 



Because most sharks are too large to be kept easily in public aquaria much of 

 our knowledge of their behaviour comes from encounters with sharks in the 

 wild. Although much first-hand information has been acquired in this way, it 

 is seldom objective. These experiences have produced two popular but op- 

 posing views about sharks. On the one hand, because of its size, strength, and 

 voracity the shark, more than any other creature, is regarded as an efficient 

 hunting machine. On the other hand, there is the impression that each 

 shark's behaviour is unique and artfully unpredictable. Much of this un- 

 certainty stems from the failure to recognise the environmental features that 

 excite a shark, for what goes on in the brain of a shark, to a large extent at 

 least, is controlled by what is sent into the brain by the sense organs. 



Because of the properties of water, mechanoreceptors are an important 

 source of information about the external world for any marine animal.. In 

 pelagic sharks in open water tactile endings of the skin and the vibration 

 sensors of the labyrinth and the lateral line give important environmental 

 information, while the labyrinth and body proprioceptors meet the demands 

 of equilibrium and balance during movement. 



This chapter is concerned with summarising current knowledge about 

 these receptors and their importance in elasmobranch behaviour; our first 

 task is to compile a catalogue of the known mechanoreceptors. 



A CATALOGUE OF ELASMOBRANCH MECHANORECEPTORS 



The sense organs of the skin of fishes, which are used by them to obtain 

 information about the external world, were arranged by Herrick (1903) in 

 three main groups: 



1) The taste buds and related chemoreceptors of the communis system 



2) The general cutaneous system, which includes the tactile endings of 

 nerve fibres contained in the spinal nerves of the body and the cranial 

 nerves of the head 



3) The special cutaneous system, which comprises the sense organs of the 

 lateral line and its derivatives, and which has its own special innervation. 



The mechanoreceptors that are the subject of this review are in the last two 

 groups; as well as providing information about the external world, some of 

 them play a significant role in proprioception. The general cutaneous system 

 of elasmobranch fishes contains three types of receptor that may be phylo- 

 genetically related, since they are all derivatives of nerve endings. In contrast, 

 the sense organs of the acoustico-lateralis organs are all modifications of 

 ciliated epithelial cells. 



The General Cutaneous System 



The 'Free' Nerve Ending— The simplest form of mechanoreceptor in 

 fishes, the 'free-ending' termination of sensory nerve fibres, provides tactile, 



