When a predator is lurking nearby, holding perfectly still might be a life-saving strategy. But what if the predator can sense electrical signals generated by breathing? According to new findings published in Proceedings of the Royal Society B, by freezing in place and essentially holding its breath, a cuttlefish can cloak its electrical cues to prevent predation by hungry sharks.
Soft, stealthy cephalopods are masters of camouflage, but their predators include sharks with sensor-studded snouts for detecting electrical fields. With eyes on the sides of the head, sharks can’t see much straight ahead of them. Instead, they rely on electroreception; they bite when they sense the faint currents that emanate from a cuttlefish’s gills and various gaps in their bodies. All marine organisms produce weak bioelectric fields when they respire. It’s the result of ion exchange during metabolic processes within electrically conductive environments, like seawater.
The common cuttlefish (Sepia officinalis) may have figured out a way to protect itself in the electrical spectrum. It’s a tactic called bioelectric crypsis, and it supplements visual camouflage. Duke University’s Christine Bedore measured the voltage and frequency of tiny electrical fields generated by captive-reared cuttlefish as they rested on the floor of a tank, and then again as they reacted to videos on an iPad next to the tank. These depicted dark, growing silhouettes of a looming shark, fish, or crab. You can see these experiments in the video below.
When they saw the fish or the shark, the cuttlefish almost always froze, flattened their body against the bottom of the tank, slowed their breathing, and covered their body openings with their arms. Having their arms over their ion-leaking, gill-associated cavities reduced their bioelectric field dramatically. A cuttlefish at rest has a bioelectric potential of 10 to 30 microvolts – which is around 75,000 times weaker than an AAA battery – but the voltage at the openings of alarmed cuttlefish drops to about six microvolts.
And this strategy works too. The team conducted a separate experiment with nine juvenile blacktip sharks from Tampa Bay and seven adult bonnethead sharks from the Florida Keys. When electrodes were used to simulate a cuttlefish at rest, both shark species responded by biting the electronic equipment. However, when the voltage was dropped down to the level of a cuttlefish holding its breath, the sharks bit down about half as much, and they had to be a lot closer.
If and when the bioelectric crypsis strategy fails, the cuttlefish resorted to squirting a cloud of ink while jetting away. But these actions created a fourfold increase in voltage compared to their resting condition. Not only did the sharks pick up on this easily and from a distance, they also seem to be attracted to (and readily consumed) the ink – making this last-ditch defense a risky option, with sharks at least.
