Clever Cuttlefish Muffle Electrical Signals to Hide from Sharks

Hans Hillewaert, Wikimedia Commons // CC-BY-SA 4.0
Hans Hillewaert, Wikimedia Commons // CC-BY-SA 4.0 / Hans Hillewaert, Wikimedia Commons // CC-BY-SA 4.0

At first glance, the cuttlefish looks like a pretty easy meal. It’s squishy, stubby, and seemingly defenseless. But the cuttlefish doesn't give up so easily.

Like its cousins the octopus and the squid, the cuttlefish has no external shell, and has therefore been forced to get creative in its defenses. Biologists and material scientists alike are captivated by the cuttlefish’s talent for subterfuge. By compressing, stretching, or relaxing pigment and light-reflecting cells, the cuttlefish can change color and even create patterns on its skin. As if that’s not enough, they’ve become shape-shifters, too. Any cuttlefish with a half-second’s head start can vanish into its surroundings.

But all this razzle-dazzle will only thwart predators who hunt by sight. Those that rely on scent or touch won’t be fooled, nor will those hunters who locate prey via electrosensing.

It’s more common than you’d think. Every living thing gives off at least some electricity. Some, like the electric eel, carry a significant charge. But most of us just trundle along, emitting a faint electrical aura as we go about our daily business.

Scientists’ list of animals that can sense other animals’ electrical fields is growing all the time, and a lot of those animals are aquatic. At the top of the list are sharks, with electrosensing skills 10,000 times more powerful than those of any other animal.  

And what do sharks like to eat? A lot of things, including—you guessed it—the cuttlefish. Once again, it looks like the cuttlefish doesn’t stand a chance—and once again, looks are deceiving. Researchers recently discovered that cuttlefish can actually muffle their electrical fields, rendering themselves nearly invisible.

This insight comes courtesy of Duke University biologist Christine Bedore, who has made it her business to study electrosensing in sea critters. Bedore found that the electric field given off by the common cuttlefish (Sepia officinalis) is quite weak, about 75,000 times weaker than a single AAA battery. But even a weak field is still recognizable to a hungry shark.

To find out how a cuttlefish responds to the sight of a shark, Bedore set an iPad screen against the wall of a cuttlefish tank. She then played the cuttlefish videos of what looked like the silhouettes of approaching crabs, sharks, and groupers (another cuttlefish predator).

The crab silhouette, which presented no threat, didn’t inspire any changes in the cuttlefish’s behavior. But each time the silhouette of a shark or grouper approached, the cuttlefish in the tank froze. Its breathing slowed, and it seemed to be covering parts of its body with its little arms.

Throughout the experiment, Bedore was tracking the cuttlefish’s electrical output. Watch for yourself:

The cuttlefish’s tactic had a huge effect on the strength of its electrical field. By covering its siphons with its arms, the cuttlefish masked its electrical output by as much as 89 percent.

For the second phase of the study, Bedore and her colleagues offered real sharks access to a simulated cuttlefish in two positions: relaxed and frozen. The “cuttlefish” at rest was not only visible, but it was irresistible; the sharks started biting the equipment. The muted electrical field of a frozen cuttlefish, on the other hand, attracted only half as many shark bites.

Bedore and her colleagues published their findings in the December 2 issue of the Proceedings of the Royal Society B.