An Australian navy diver was attacked in Sydney Harbor on Thursday by a shark that partially severed his right hand. According to an Australian defense spokeswoman, the diver, Able Seaman Paul Degelder, was not wearing a shark-repelling device at the time because the navy thought the waters were safe. How do shark repellants work?
Shock treatment. Most modern shark-repelling devices are battery-powered electronic units that clip onto divers' fins or surfboards and emit electronic pulses that irritate the shark. Sharks have sensory organs called the ampullae of Lorenzini—tiny receptors clustered around the head that are hypersensitive to electric fields. These help the shark detect nearby prey (as well as mates) and facilitate navigation by functioning as a sort of built-in compass. Shark-repelling gadgets are designed to overwhelm the animal's receptors and drive them away—the squaline equivalent of painfully loud music.
Several other shark repellants have been developed over the years. During World War II, the military gave sailors a mixture of copper acetate and black dye, which created a chemical cloud in the water meant to smell like dead shark. It didn't work. In the 1970s, Eugenie Clark discovered that a fish found in the Red Sea, the Moses sole, secretes a natural shark repellant. Researchers later developed a synthetic version, but it worked only when squirted directly into a shark's mouth—by which point it's usually too late. Around 2004, a group of New Jersey researchers created a liquid repellant derived from ground-up shark but only tested it on relatively harmless species of shark. Other shark protection technologies include chain mail designed to obstruct shark bites and "bubble screens" that blow air through a hose and, at least in theory, deter sharks.
So, do these repellants work? Hard to say, since testing them out is so difficult. For one thing, there's a distinction between deterring curious sharks and preventing an ambush-style attack by a 2,000-pound great white traveling at 20 knots. It's easier to create a test situation that replicates the former than the latter. But, even then, it's extremely costly and time-consuming—you have to rent a tank, bring in sharks, and prove that they're physically repelled by the substance and not just bored.
Shock repellants have failed to prevent attacks on at least two notable occasions. In 2003, a diver looking for scallops close to the Australian coast turned off his electronic device once he reached the sea floor and was killed. (A company representative said he should have left it on for the entire dive.) In 2008, a shark actually ate a repelling device during testing off the coast of South Africa.
Bonus explainer: After the attack, Rear Admiral Nigel Coates assured reporters that Able Seaman Paul Degelder was going to be OK. What's with the weird navy titles? They're British. In the 17th century, the Royal Navy used the rank of "able seaman" to distinguish sailors with more experience from "ordinary seamen," who got paid less. The convention survived. Nowadays, in the Canadian navy, able seamen is the second-lowest noncommissioned rank, between ordinary seaman and leading seaman. (The U.S. Navy doesn't use the rank, opting instead for plain old "seaman.") The title "rear admiral" dates at least as far back as the 16th century, when rear admirals commanded the rear portion of the British fleet and served as deputies to the vice admirals. ("Vice vice admiral" doesn't have the same ring to it.) The U.S. Navy uses the term for the position between captain and vice admiral.
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Explainer thanks Kim Holland of University of Hawaii; Chris Lowe of California State University, Long Beach; Frank Schwartz of University of North Carolina; and John Sherwood of Naval History and Heritage Command.