ELECTRIC AND MAGNETIC SENSES 517 



only the shallow-water, bottom-dwelling sharks, but also the more open- 

 ocean forms off Cap Cod, in particular the blue sharks, Prionace glauca. The 

 Whaler will in addition serve as a nongalvanic working platform for exploring 

 the d.c. and low-frequency electric fields in the animals' natural habitat. 

 Thus, our oceanic endeavors have opened up new avenues expected to lead 

 to a better understanding of the electrical sensory world of the marine 

 elasmobranch fishes. 



Although we conceived our field work mainly with the undisturbed 

 oceanic environment of the sharks in mind, our test results also indicate 

 that attacks on humans and underwater gear may be elicited and guided by 

 electric fields resembling those of regular prey. The human body, especially 

 when the skin is damaged, creates in the water d.c. bioelectric fields that 

 sharks may detect from distances of up to 1 or 2 m (Kalmijn 1971). The 

 galvanic fields of metallic objects on the body are often even stronger. In this 

 connection, the U.S. Navy's antishark screen, which consists of a large 

 polyvinyl bag suspended from an inflatable flotation collar and was designed 

 (by Dr. C. Scott Johnson at the Naval Undersea Research and Development 

 Center, San Diego, California) to visually and olfactorily conceal a mariner in 

 distress, looks promising from an electrical point of view as well. 

 Sidetracking the sharks to alternative sources of electricity that secondarily 

 release a discouraging agent might, under certain circumstances, be another 

 means of warding off electrically evoked shark bites. 



It should be emphasized— whether our interest in the animals is purely 

 academic or more practical— that in behavioral studies of elasmobranch and 

 other electrosensitive fishes the electrical aspect of the environment must be 

 taken into account. In land facilities, the background fields should be well 

 controlled; in the ocean, they should preferably be left undisturbed. Any 

 metallic device lowered into the water is likely to produce fields strong 

 enough to disturb the animals' electromagnetic environment. Fields of 

 biological strength may trigger false responses; stronger fields may first scare 

 the animals, but will soon cause them to ignore electric fields altogether and 

 for the moment deprive them of their electrical sensory abilities (Kalmijn, 

 unpublished). If recognized, these problems can, with proper care, be over- 

 come. Thus, the author's technical efforts have yielded a first insight into the 

 electrical sensory world of the elasmobranch fishes. 



EXPERIMENTAL EVIDENCE OF GEOMAGNETIC 

 ORIENTATION 



With such a marvelous sensory system enabling sharks, skates, and rays to 

 cue in on the bioelectric fields of their prey, one wonders what other 

 underwater voltage gradients elasmobranch fishes might detect and use. Of 

 the various fields in the marine environment, I have more recently been 

 concentrating on those predicted by Faraday (1832) in his historic lecture 

 on electromagnetic induction for they show great potential as a means of 

 open-ocean orientation and navigation. In biological terms, when cruising 



