Henry the seal preparing to test the sensitivity of his whiskers to jets of water. Photo credit: Marine Science Center Rostock.
Plunging through gentle swell in the northern oceans in search of food, harbour seals are guided by a cryptic superpower that few fish can evade. ‘Fish produce lots of water flow that remains in the water, even after the fish has gone’, says Wolf Hanke, from the University of Rostock, Germany, and it is these swirling wakes that betray the presence of fish dinners. A hungry seal can sense the tell-tale turbulence left by a passing fish with flow-sensitive whiskers on its snout, before pursuing the hapless victim's trail. However, seals are not limited to hunting in open water. Explaining that it was apparent from crittercam movies that the diving mammals also stalk seafloor-dwelling species, Hanke and his colleagues were curious to find out how seals pinpoint well-camouflaged static flatfish that would evade even the sharpest eyes. Could the predators be honing in on the gentle jet of water produced by the concealed fish's gills with their super-sensitive whiskers?
Having studied the seals’ extraordinarily sensitive flow sensation for more than a decade, Guido Dehnhardt's team a the University of Rostock had three willing harbour seal colleagues – Henry, Filou and Luca – who were well prepared to help tackle this question. And despite living in captivity all of their lives, the animals were adept at catching flatfish on the bottom of their outdoor enclosure: ‘The fish enter the netting of the semi-natural enclosure and the seals hunt them’, says Hanke. However, working in the semi-natural setting has its drawbacks, as it was difficult to measure the speed of the jets of water encountered by the seals against the background of the sea's natural turbulence.
As camouflaged flatfish angle pulses of water upward at about 45 deg out of their gills, Hanke and Benedikt Niesterok constructed a platform 1 m below the surface of the enclosure with eight angled nozzles that could be independently activated by a pump to simulate a flounder exhaling at ∼25 cm s−1. Then, having trained the seals to swim counter-clockwise around the platform with and without a blindfold, Niesterok, Hanke, Yvonne Krüger and Sven Wieskotten filmed the animals’ responses and were impressed to see that the blindfolded seals could sense a continuous jet of water, regardless of the direction from which they approached. And when the team stepped up the challenge by pulsing the simulated breath jet to reproduce the exhalation pattern of a flounder, both of the animals that participated in the test successfully picked out the active nozzle with their whiskers. However, high-speed approaches impacted on their success, with the animals overshooting the fake flounder at speeds of 1.3 m s−1, while they recorded the most success at speeds ranging from 0.4 to 0.8 m s−1. Niesterok also noticed that one of the seals retracted its head towards its shoulders as it closed in on the jet of water; ‘It can move its head in a way that slows down the sensory system in the water, using the head movement and not the swimming itself’, Hanke says.
Having confirmed that the seals’ sensitive whiskers are capable of detecting the breathing currents of flatfish submerged beneath the sand and knowing that many fish species are capable of holding their breath, Hanke speculates that flatfish may stop breathing deliberately to avoid revealing their location as seals sail past. ‘It seems conceivable that the detection of breathing currents by predators is one of the evolutionary drivers for this respiratory suppression in fish, he says’.
