Cuttlefish number among the ocean's ultimate masters of disguise, able to change the color, pattern and even the texture of their skin in an instant to blend into their surroundings. But according to a new study, these chameleons of the sea don't just know how to hide their visible selves — they also can mask their bodies' electrical fields.
The findings, published in the Proceedings of the Royal Society B and funded by a grant from the U.S. Office of Naval Research, shed light on the lengths to which these animals must go to hide themselves from predators.
Cuttlefish are cephalopods, in the same branch of the animal family tree as the octopus and the squid. These tentacled critters don't have any armor to protect their soft bodies, making them a quick and easy meal for many ocean-dwellers, particularly larger fish and sharks. Cuttlefish have few defenses against attack; they essentially just squirt ink and suddenly shoot backward through the water. Instead, they have evolved impressive adaptive camouflage that allows them to avoid detection in the first place.
But plenty of fish don't rely only on visual cues to find a tasty morsel, said lead author Christine Bedore, a sensory ecologist at Georgia Southern University. Sharks and rays, for example, can sense the electric fields emanating from other animals' bodies, and so would not necessarily be fooled by visual trickery. Did cephalopods have any defense against that?
"I wanted to take a look and see if they could be using any other alternative mechanism that might add to that visual camouflage," Bedore said.
To find out, the researchers set up a series of experiments to test the cuttlefish's reactions to a looming predator. They put single cuttlefish in a tank and then showed them videos that simulated different marine denizens: crabs, large fish and sharks. When the sharks and fish loomed in their vision, the cuttlefish actually employed a freezing behavior; they flatten their bodies and squeeze shut their orifices — their funnel, siphons and the gap in the mantle behind the head. These just happen to be the spots where the body's electric field could easily leach out. (With crabs, the cuttlefish employed this behavior far less.)
To see if there was an actual effect on the cuttlefish's electric field, the scientists used electrodes to measure the field generated by the cuttlefish's breathing during, and outside of, these staged encounters. They found that while the animals generated an average voltage of about 15.3 microvolts when they were at rest, their freezing-and-squeezing behavior cut that down to a mere 6 microvolts when under threat.
The researchers then took those measurements and used a device powered by a 9-volt battery to generate differing electric fields, to see how bonnethead and blacktip sharks would react to a normal cuttlefish electric field and the minimized one produced under threat. The reduced electric fields seemed to throw the odds in the cuttlefish's favor: Under the "resting" electric field, sharks attacked 62% of the time, but with the reduced "threat mode" electric field, the sharks struck only 30% of the time — about half of the original strike rate.
"I was a little bit surprised because cuttlefish themselves can't detect this electric field," Bedore said.
The next step is to see how these two different types of camouflage — visual and electric-masking — might work together in the presence of a shark, she said.