Infrared photography has revealed that European catfish can stalk prey by tracking underwater wakes much as hunters on land follow footprints.
This laboratory demonstration marks the first time that scientists have described a fish following another fish’s wake in the dark, according to project leader Thomas Breithaupt of the University of Konstanz in Germany. Catfish pick up a trail 10 seconds after a guppy waggles by and pursue that trail for up to 55 guppy body-lengths before striking, Breithaupt and his colleagues reported June 5 in the online Proceedings of the National Academy of Sciences.
“It’s a fascinating paper. I loved it,” comments Jeannette Yen of Georgia Institute of Technology in Atlanta. In 1998, her research team made the first detailed report of any underwater wake tracking. Yen found that male copepods, pinhead-size marine crustaceans with only primitive light sensors, chug along the tiny wakes left by females. As it turns out, males often start off in the wrong direction and have to turn around.
For years, Breithaupt has wondered whether fish hunting in the dark follow wakes, but only recently did he find a convenient species to test. The European catfish, Silurus glanis, prowls by night. Examination of its stomach contents indicates that it often manages to catch swift prey.
Breithaupt, his Konstanz colleague Kirsten Pohlmann, and Frank Grasso of the Boston University Marine Program in Woods Hole, Mass., set up an infrared fish-monitoring system in a large laboratory tank. The researchers then placed a 20-centimeter-long European catfish into the tank and added a guppy. They next recorded and statistically compared the pathways that the two fish plied in the dark.
Sometimes the fish merely blundered into each other. But nearly 60 out of 90 catfish attacks on the guppy came after the catfish had twisted and wriggled along the same meandering path that the guppy had just taken.
Breithaupt suspects that the catfish exploit a mix of cues. “When an animal is looking for food, any clue it can get, it will use,” he says.
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The fish-to-fish distances might have been initially too long for the catfish to pick up its prey using electrical-field sensors, Breithaupt says. The catfish’s indirect paths suggest that they couldn’t sense the guppy directly. The darkness eliminated visual cues.
Part of the catfish’s tracking success might be due to the chemical trails that the guppies shed into their wake. Chemoreceptors abound across catfish skin. “These fish are like big tongues swimming through the water,” Breithaupt says.
He also speculates that the predators use water-motion sensors, such as those in the lateral line along each flank. Swimming creatures create vortices that spin in opposite directions on opposite sides of the wake. Yen calculated that copepod vortices didn’t swirl enough for males to detect them, but guppies leave much bigger eddies.
Breithaupt muses that such water motions could tip off a catfish as to which way to go along a wake. “A catfish never goes in the wrong direction,” he says.
Neil Vickers of the University of Utah in Salt Lake City says he now wants to know whether catfish track wakes in the wild. He studies moths, which also follow trails in three dimensions in the dark. They track scents not by working out which way the scent gets stronger, as many organisms do. Instead, they pick up a whiff and then fly upwind. If they blunder out of the scent plume, they cast back and forth across the wind.
For flying insects and swimming fish, Vickers says, “I suspect the challenges are similar.”