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Biologgers on large sharks show rays wielding a shocking defense strategy in the wild

While more traditional defense methods like camouflage, venom and spines appear ineffective against large sharks and mammals, electric rays are sending out electric discharges to fend off sharks in dangerous waters, said Florida International University Institute of Environment researcher Yannis Papastamatiou. The findings were in Ethology.

"We had an idea that there was something special about the rays because they are very bold," Papastamatiou said. "In Guadalupe Island, Mexico, our cameras on white sharks showed these rays up in the water column, one of the most dangerous places to be. They must be pretty confident."

All electric rays, which include 69 species of four families, can produce electric organ discharge (EOD). But some of the larger species can produce EODs at a maximum voltage of 50 volts. The rays produce this electric discharge using their muscles. It's been shown that these rays in captivity produce EODs for defense. However, its use in the wild remained unknown—until now.

Off the Pacific coast of Mexico in Guadalupe Island, they deployed six Customized Animal Tracking Solutions (CATS) biologgers on white sharks. One showed a shark at a depth of 164 feet approaching an electric ray in midwater. The ray appeared to cup its pectoral fins while the shark approached.

Data showed a sudden increase in the shark's movement as it rapidly ascended to almost 100 feet. Two minutes later, the shark made its return but passed the ray. The ray continued its way with no response to the shark as it continued with no change in activity.

During a baited shark dive in Fuvahmulah, Maldives, a diver participating in the study observed a Gulf torpedo ray buried in the sand. When it began to swim away, it was approached from behind by a tiger shark. But when the shark was about a few inches from the ray, the shark rolled its nictitating membrane—also known as the third eyelid for shielding the eye from debris, dust and injury—and quickly retreated up into the water column.

The researchers compared EOD characteristics from field experiments off Palos Verdes, California from 1989 to 1991. They looked at the differences between predatory and defensive behaviors. The maximum voltage did not vary between the two behaviors, but the initial trains for predatory discharges were longer and more abundant. Their behavior varied when on defense. They moved up into the water column and performed stationary spinning.

"This study adds important insight into the evolution of defensive traits in marine animals," Papastamatiou said.

The increase in the use of animal biologging sets up future studies to measure the foraging and defensive behaviors of these rays.