Double-sided. When a male cuttlefish (right) is courting a female (left), he displays two different sides: a male pattern facing the female, and a female pattern facing away, to trick other males.

Culum Brown

When cuttlefish hold their breath, they become nearly invisible to sharks

Scientists have long known that cuttlefish (Sepia officinalis, shown) and some of their kin stop breathing when threatened by predators, but they supposed that freezing in place merely aided the creature’s visual camouflage. Turns out, it serves another purpose: A lack of water flowing over the gills decreases electrical activity there that can betray the cuttlefish’s presence to foraging sharks, a new study suggests. In the lab, researchers measured the electrical signals generated by cuttlefish at rest on the floor of their tank. Then, they measured those generated by cuttlefish startled by videos of a looming predator. When the frightened creatures froze in place and covered the cavity leading to the gills with their tentacles, the voltage in the water nearby dropped by about 80%, the researchers report online today in the Proceedings of the Royal Society B. Subsequent lab tests on two types of sharks revealed that the tactic works beautifully: When the researchers generated voltages to simulate the presence of a resting cuttlefish, the sharks could detect the electronic equipment from a distance of 20 centimeters and struck at it 62% of the time. But when they simulated the presence of a cuttlefish holding its breath, the sharks had to get 5 cm closer to the equipment to notice it—and even then they struck at it only 30% of the time. On the other hand, when the researchers simulated a fleeing cuttlefish, sharks detected the equipment a whopping 94% of the time and from distances up to 38 cm, rendering this tactic the worst option of all. Covering the cavity holding their gills may help a stock-still cuttlefish in another way too, the researchers suggest: It may stifle water flow in and out of that recess, thereby reducing the size of tiny pressure waves that could alert predators to the creature’s presence.