buried under the skin. Obara and Bennett (1972) recently presented a 

 novel theory and preliminary experimental evidence regarding the receptor 

 mechanism of the ampullae of Lorenzini in an effort to explain their high 

 electrical sensitivity and the anomalous polarity of response. 



It seems now generally agreed that the ampullae of Lorenzini function 

 as electroreceptors. It has been shown by Dijkgraaf and Kalmijn (1962, 

 1963) that sharks and rays respond to very weak electric fields of the 

 order of 0.1 uV/cm, and that their high electrical sensitivity is due to 

 the ampullae of Lorenzini. Subsequent studies (Kalmijn, 1966) lowered this 

 threshold to 0.01 uV/cm, representing the greatest electrical sensitivity 

 known in the animal kingdom. Further behavioral experiments by Kalmijn 

 (1971) showed that both Scyliorhinus and Raja are able to detect flatfish 

 buried in the sand by virtue of the bioelectric potentials that the prey 

 produce. The predators cued in on the flatfish's bioelectric fields 

 from distances up to 10-15 cm. Because of the biological significance of 

 this response, Kalmijn concluded that sharks and rays are endowed with an 

 acute electric sense and that the ampullae of Lorenzini are true electro- 

 receptors. 



The high electrical sensitivity of sharks and rays allows for some 

 interesting speculations. If the animals are capable of interpreting the 

 large-scale electric fields induced by open-ocean streams flowing through 

 the earth's magnetic field, they may sense upstream and downstream 

 directions, e.g. for compensating unwanted drift or for gaining passive 

 transport. If the animals are capable of appreciating the electric fields 

 that they produce when swimming through the earth's magnetic field, they 

 may even sense the actual compass directions. The feasibility of orienting 

 with respect to uniform electric fields has recently been demonstrated in 

 successful training experiments on catfish and weakly electric fish. More- 

 over the shark Triakis was found to respond in a frightened manner to local, 

 experimentally induced non-uniformities in the earth's magnetic field 

 (Kalmijn, 1973, 1974). Since understanding the principles of shark navi- 

 gation and open-ocean orientation in general is of utmost importance, these 

 advanced studies should be pursued tenaciously despite their inherent bio- 

 logical, physical, and technological difficulties. 



Although there is good evidence that the ampullae of Lorenzini serve 

 an electroreceptive function, this does not necessarily rule out multi- 

 functional performance, e.g. the ampullae may serve as mechanoreceptors to 

 enable sharks to distinguish between the texture of objects in "head 

 bumping", though the ordinary lateral-line organs and the free nerve 

 endings could be involved as well. The responses of the ampullae of 

 Lorenzini to stimuli of various modalities should be critically examined in 

 other sharks and rays, particularly the pelagic species, using both electro- 

 physiological and behavioral techniques. A fruitful approach in the invest- 

 igation of the ampullae of Lorenzini may be through the use of recently 

 developed locomotory monitoring techniques under controlled conditions 

 (Kleerekoper, 1969). 



d. The Ear 



The gross morphology and histology of the elasmobranch ear has been 

 known since the classical work of Retzius in 1881, and has been updated for 

 Squalus acanthias by Quiring (1930), for Raja clavata by Lowenstein, 



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