AMPULLARY ELECTRORECEPTORS 503 



the canals are quite short, no response has been recorded on the skin surface. 

 Innervation of each ampulla is by several fibers. 



A further similarity between Potamotrygon and freshwater teleosts is in 

 sensitivity to stimuli in the millivolt range. This may represent an adaptation 

 to a greater ambient electrical noise in the freshwater than in the marine 

 environment (Hopkins 1973). The influence of marine and freshwater 

 environments is further attested to by the marine catfish Plotosus (Obara 

 1974, 1976). This fish has developed long canals like those of the marine 

 elasmobranchs. Also, its receptors are very sensitive and generate action 

 potentials as skate receptors do. However, the receptor cells lack a cilium, 

 and the polarity of sensitivity is the same as that in other teleosts and 

 opposite to that in elasmobranchs. Innervation is multiple, as in elasmo- 

 branchs. The comparative data indicate that the marine environment selects 

 for long canals and high sensitivity while the freshwater environment does 

 the opposite. The association of high sensitivity with action potential 

 generation in separately evolved marine fishes supports the hypothesis that 

 greater sensitivity is achieved by an active response mechanism. 



The sturgeon is a primitive, bony fish that also has electroreceptors 

 (Teeter and Bennett 1976). These receptors in the freshwater shovel-nose 

 sturgeon have almost no canals, but they show both apical cilium on their 

 receptor cells (R. B. Szamier, unpublished observations) and the polarity of 

 sensitivity of the elasmobranchs. It seems, therefore, that these receptors are 

 homologous to those of the elasmobranchs and should be classified as 

 ampullae of Lorenzini. The teleost receptors may well have arisen 

 independently in the weakly electric mormyrids, in the gymnotids, and in 

 catfish, but some change in the teleost line appears to have led to their 

 similarity to one another and difference from the elasmobranchs. Investiga- 

 tion of other primitive fishes may further support this hypothesis. 



ACKNOWLEDGMENTS 



This work was supported in part by National Institutes of Health (NIH) 

 Medical Scientist Training grant 5T5 GM 1674, by NIH grants HD-04248, 

 NS-12627, and NS-07512, and by National Science Foundation grant 

 NSF-19120. 



REFERENCES 



Akutsu, Y., and S. Obara. 1974. Calcium dependent receptor potential of 

 the electroreceptor of marine catfish. Proc. Jap. Acad. 50:247-251. 



Bennett, M. V. L. 1971a. Electric organs. Pages 347-491 in Fish physiology, 

 vol. 5. Edited by W. S. Hoar and D. J. Randall. Academic Press, New 

 York. 



Bennett, M. V. L. 1971fr. Electroreception. Pages 443-574 in Fish physiol- 

 ogy, vol. 5. Edited by W. S. Hoar and D. S. Randall. Academic Press, New 

 York. 



