Cuttlefish, squid and octopus can change colour in milliseconds, rapidly recloaking themselves in the face of danger. A new study, reveals that cuttlefish also respond to predators by freezing, dramatically reducing electrical signals that might give them away.
The paper, published in Proceedings of the Royal Society, builds the case for this new mode of crypsis through a series of clear observations and experiments.
The researchers began by collecting data on the bioelectric field given off by common cuttlefish. At rest, the animals emit electrical potentials through openings in the body. These are tiny, typically between 10 and 30 microvolts, but are still the sort of signals that a honed predator like a shark might detect at close range.
When faced with a predator, however, the cuttlefish freeze. The researchers demonstrated this by exposing cuttlefish to iPad videos. When faced with the looming silhouette of a non-threatening crab, the cuttlefish weren’t particularly bothered and continued their rhythmic pulsing. With the growing outline of a predatory fish or shark, however, the cuttlefish froze in around 80% of trials, flattening their bodies to the bottom of the tank and reducing their rate of ventilation.
So far so expected. But the researchers found that the iPad predators also caused the cuttlefish to close their siphons, funnels and mantles, dramatically reducing the electrical signals leaching out into the surrounding water.
Does this help avoid detection? The scientists asked some sharks, exposing blacktips and bonnetheads to electrical signals of different sizes. The sharks could detect a 35-μV stimulus typical of a resting cuttlefish at a distance of around 20 cm and attacked in 62% of trials. The predators didn’t do so well with an 11-μV potential characteristic of a mid-freeze cuttlefish, needing to get within 15 cm and striking in only around 30% of trials.
If the freezing response fails and the predator keeps coming, the cuttlefish has another trick up its mantle. It can squirt a cloud of ink and jet away from the danger, though the action of jetting generates a give-away electrical signal about four times greater than that given off whilst resting.
“Jetting would only be for use as a last resort,” says Christine Bedore, a biologist at Duke University in North Carolina and the lead author of the study.
Bedore, C.N. et al. (2015) Freezing behaviour facilitates bioelectric crypsis in cuttlefish faced with predation risk. Proceedings of the Royal Society B, 2 December. DOI: 10.1098/rspb.2015.1886
