The contents of this cyber-discussion originated as postings to ELASMO-L &, SHARK-L, and later only to ELASMO-L. The contents are presented with the permission of the e-mail's authors.

Here's a question for your intellectual stimulation! I just finished presenting a seminar on the ampullae of Lorenzini and their function as electroreceptors. I was asked if there is any defense against sharks using this sensory capability? It was suggested that things like Torpedo and electric rays and catfish might use their discharge as an electro-repellent (similar to the POD) to repel shark predators. So, does anyone know if there are any sharks that prey on the above fish, and if not could it be due to their electric organ discharges??

Just wondering, Glenn Fines - University of Manitoba

Glenn A. Fines wrote: "Here's a question for your intellectual stimulation! I just finished..."

The amplitude of the electric organ discharge (EOD) of the electric rays is many many orders of magnitude above the sensory threshold of the ampullary system. There is not doubt that this is an effective defense against predators-- elasmobranch and others alike. At close range, this electric field will cause brief tonic contraction in the muscles which must be a surprising and unpleasant sensation to a potential predator. There are anecdotal records of small electric rays (e.g.Narcine) being eaten by sharks... but to my knowledge they do not make up a significant portion of the diet. Predatory success in this interaction needs to be quantified.

It is my understanding that the POD field is relatively weak compared to the electric organ discharge of electric rays, and its reception is probably mediated by the ampullary electroreceptors. Hopefully Chris Lowe, who did his masters research on Torpedo with Dick Bray and Don Nelson, will have something to say about this.

Tim Tricas - Florida Institute of Technology

There is also some anecdotal evidence that the weak electric organs in skates may be used as a defense to startle approaching predatory elasmobranchs. Although the EOD of skates is much weaker than that of Torpedo (the intensity of the field measured across the organ at the skin surface is in tens of millivolts) the ampullae of Lorenzini are so sensitive that a well-timed EOD might startle an attacker, causing it ti hesitate or break off its attack (although an ill-timed one might give its hiding place away!). I don't believe, however, that this hypothesis has been rigorously tested.

John G. New - Loyola University Chicago

Glenn A. Fines wrote: "Here's a question for your intellectual stimulation! I just finished... "

In addition to defense, the powerful electric discharge of Torpedo is used for stunning prey during feeding and the strong discharge of the freshwater catfish is undoubtedly used for defense against all predators. Just try touching one!

Although to my knowledge no experimental work has been done with skates I believe that the discharge from their relatively weak electric organ would be an effective defense against sharks. Although not strong enough to cause a typical electric "shock" it could well over-power the shark's ampullae sensory system and startle the shark much like a blinding flash of light would to our visual system.

The behavioral differences of flatfish, rays and skates when closely approached by a potential predator tends to support this theory. They all rely on camouflage but the responses of flatfish and rays are different. Flatfish and rays tend to take off at high speed at the last minute while skates stay buried unless actually touched.

I have always wanted to set up an experiment using small skates and small flatfish in a very large tank with sharks to see the relative difference in predation and survival. Unfortunately, it never got past the idea stage. It would make a good project, though.

David Powell - Monterey Bay Aquarium

David C. Powell wrote: "... I believe that the discharge from their relatively weak electric organ would be an effective defense against sharks. ... it could well over-power the shark's ampullae sensory system and startle the shark ..."

Just to play the devil's advocate here: The EOD of skates is so weak that it does not even strongly stimulate the skate's *own* electrosensory system (if I may immodestly cite New, JG, 1994, Electric organ discharge and electrosensory reafference in skates, Biological Bulletin 187:64-75). Electrosensory anterior lateral line nerve fibers were modulated only very weakly by an elicited discharge of the skate's own electric organ. Since the animal's own system is only weakly stimulated, it is difficult to believe that it would saturate an approaching shark's system

"...but the responses of flatfish and rays are different. Flatfish and rays tend to take off at high speed at the last minute while skates stay buried unless actually touched."

Yes, but the response of skates to threatening stimuli includes not only "freezing" but a cessation of ventilation with the spiracles closed. Since the standing bioelectric field surrounding the skate is strongest at mucous membranes (including pharynx and gills) closing the spiracle helps minimize the electric field to avoid electrodetection. Perhaps there's a hierarchy of responses, a cessation of ventilation following approach of a threat, and then an EOD as a last resort just before the attack.

John G. New

Although the electric organ discharge of the skate does not greatly stimulate the animal's own ampullary system, it does effectively stimulate the ampullary system of other skates (Sisneros, Tricas and Luer (in press) J. Comp. Physiology, a). thus the EOD potentially has a function in intraspecific communication among skates, especially during their interesting mating behaviors when they are laying on the bottom in groups. This leads to the question of whether the EOD of a discharging skate also advertises its presence to elasmobranch predators. Skates are a very important food item for large coastal sharks in deep water (and is a major bait used by commercial fishermen to capture sandbar sharks). The clearnose skates used by Dr. Carl Luer at Mote Marine Lab move inshore from deep waters during the winter to breed. This species discharges its electric organs routinely during courtship and mating in captivity. Why do the skates move into shallow waters to breed? Fun-in-the-sun or predator refuge?

Thanks for the discussion! - Tim Tricas

Tim Tricas wrote: "...although the electric organ discharge of the skate does not greatly stimulate the animal's own ampullary system, it does effectively stimulate the ampullary system of other skates ..."

I agree that the EOD can be an effective stimulus for the electrosensory system. Even the animal's own weak response is still detectable, and the behavioral thresholds of the electrosense are usually lower than those detectable by physiological means, particularly at the periphery. But, I guess I'm still skeptical about the utility of the EOD in saturating the electrosense of a predator and scaring it off, it seems to me it's more likely to give the skate's position away than frighten off a shark. I've always liked the communication idea better, although that use has yet to be convincingly demonstrated as well

John G. New