Quantitative study of primary sensory neurone populations of three species of elasmobranch fish

In order to assess the ability of sharks and rays to sense pain, the proportion of myelinated versus unmyelinated sensory fibres in the dorsal roots and the diameter spectrum of cells in the dorsal root ganglia of three species of elasmobranch fish were ascertained. Electron micrographs were used to count the numbers of myelinated and unmyelinated fibres in montages of whole dorsal roots of the long-tailed stingray (Himantura sp.), the shovelnose ray (Rhinobatus battilum), and small specimens of the black-tip shark (Carcharhinus melanopterus). The diameters of dorsal root ganglion cells in each species were measured by using the light microscope. Less than 1% of the dorsal root axons in the long-tailed stringray and a large specimen of the shovelnose were unmyelinated, whereas in smaller shovelnose rays and in the small black-tipped sharks, from 14% to 38% of axons were unmyelinated. Unmyelinated fibres differed from those in mammalian nerves in that there was a one-to-one association of the fibre with a Schwann cell. We conclude from these observations that myelination was incomplete in the black-tipped sharks and the smaller specimens of the shovelnose rays. The distribution of the diameter of cells of the dorsal root ganglia of these species was unimodal, resembling the diameter range that has been reported for the somata of myelinated fibres in the cat. We interpret these results as indications that sharks and rays lack the neural apparatus essential for the sensation of pain and we suggest that, to these life forms, the perception of pain might have little relevance to survival.