Imagine, for a second, you’re a fish – one that lurks on the murky bottom of the ocean, rendered invisible by your turbid surroundings. Suddenly, the silhouette of a seal flashes over you. You stay completely still and silent, hunkered down in the dirt.
Then snap! Crushed in dog-like jaws, you meet your end as the seal’s dinner. The giveaway? A minuscule current of water passing through your gills, according to Benedikt Niesterok and a team from the Institute for Biosciences in Germany.
In the Journal of Experimental Biology, they report that a harbour seal (Phoca vitulina) boasts sensory whiskers sensitive enough to detect these tiny “breathing currents”.
Seals’ whiskers, or vibrissae, are incredibly sensitive. They can detect vibrations made by fish and follow their hydrodynamic wake for long distances and sometimes several minutes.
But a big part of a harbour seal’s menu is benthic fish – those that live on the sea floor and can stay stock-still. And as seals often hunt at night or in turbid waters, they still need to somehow find and zero in their prey.
It turns out that when fish aren’t moving, they still emit currents passing water through their gills.
To test if harbour seals might use these signals to detect benthic prey, Niesterok and his team created an artificial breathing current apparatus. Eight silent nozzles were set up on a platform in the water and seals were trained to find the one emitting a current.
Even when blindfolded, the seals directly moved their snout towards the target nozzle and were able to find the active opening.
It’s the first study to show that harbour seals can use weak water currents, such as those produced by fish gills, to detect benthic prey.
A telling evolutionary link is that fish can hold their breath for several seconds in response to danger – an ability perhaps adapted in response to the detection skill of seals and other predators.
These results are not unexpected, says Monique Ladds, a marine biologist at Macquarie University in Australia and who was not involved in the study.
But importantly, she adds, the seals’ detection rates were lower when the researchers introduced background noise. And as humans make more noise in the ocean, it might affect their hunting skills.
“Anthropogenic noise in the marine ecosystem has been shown to negatively impact marine life in a number of ways, and we should be curious to see if marine noise affects the ability of seals to find benthic prey in the wild.”