Bioelectric Fields in Sea Water and the Function of the Ampullae of Lorenzini in Elasmobranch Fishes

The ampullae of Lorenzini, so characteristic of sharks and rays, have been examined by electrophysiological techniques, and were found not only very sensitive to thermal stimuli, but also remarkably responsive to weak mechanical and electrical stimuli. With these results, the ancient question about the function of these curious sense organs remained, however, unanswered. Subsequently, Dukgraaf and Kalmijn made a behavioral study of the electrical sensitivity of sharks and rays. They found 1. that the shark Scyliorhinus canicula and the ray Raja clavata react reflexly to very weak electric fields in the surrounding sea water, and 2. that the ampullae of Lorenzini are the sense organs by which these reflex actions are mediated. In the following years, Kalmjjn investigated especially the biological significance of the electrical sensitivity. It was demonstrated that 1, the plaice Pleuronectes platessa produces electric fields in the surrounding sea water that are sufficiently strong to be detected by sharks and rays, and 2. that the sharks and rays do not only respond reflexly to these fields, but are also capable of using them in localizing the plaice, even if it has burrowed into the sand. To what extent do sharks and rays in their natural habitat employ their electrical sensitivity? In order to answer this question it was necessary 1. to perform a more extensive study of the electric fields occurring in sea water, and 2, to determine the role these fields play in the life of sharks and rays. The first half of the three month stay at Banyuls was spent in collecting data on these two topics. What is the biological significance of the thermal and mechanical sensitivities of the ampullae of Lorenzini? Sand and Murray demonstrated the thermal and mechanical sensitivities in freshly killed specimens of which the ampullae of Lorenzini were partly or even completely severed from the rest of the animals. Moreover, they applied rather unnatural test stimuli. However, how do these sense organs respond in living animals to more natural stimuli? To examine this aspect, a method was developed by which the activity of the ampullary nerves could be recorded from live, free-swimming sharks. The preliminary results were promising, but, due to lack of experimental animals, this method could not be applied extensively in Utrecht, Therefore, the second half of the time available at Banyuls was reserved for these experiments,