It sounds like the plot of a horror movie – a predator swallows its prey only for the creature to burst out of its captive’s body. But it turns out that Japanese eels do just that.
Scientists in Japan have discovered that when swallowed by a dark sleeper fish, the eels can escape.
In a maneuver reminiscent of the Fosbury flop high-jump technique, the eels retract the digestive tract of the predatory fish into its esophagus, insert their tail through its gills, and complete their attempt at freedom by pulling their head free.
The researchers say they initially assumed that the eels escaped the predator via its mouth.
“However, contrary to our expectations, it was truly amazing for us to witness the eels’ desperate escape from the predator’s stomach to the gills,” said Yuha Hasegawa, first author of the research from Nagasaki University.
Write in the journal Current Biologythe team reports how it started with 104 Japanese eels. The researchers placed one eel at a time in a tank that also contained one dark sleeperfish from a collection of 11.
The eels had barium sulfate injected into their abdominal cavity and tail, which allowed a swallowed eel to be detected with an X-ray video system.
The team recorded 32 eels caught and swallowed by the sleeperfish. Some have completely penetrated the predator’s stomach and have been observed circling, apparently looking for a way out.
Nine swallowed eels managed to break out, using a tail-first approach.
“The predatory fish were not injured. However, the eels that managed to escape sometimes showed signs of abrasions,” Hasegawa said.
Not all the eels were successful. Four got their tails out of the predator’s gills but did not complete their exit, while two failed because they stuck their tails in the direction of the predator’s vent rather than its esophagus.
The team says its findings are a first. While previous studies have observed other species of eel dying while trying to escape from predators’ stomachs, the latest study shows the Japanese eels escaped alive.
“At this point, the Japanese eel is the only fish species that has been confirmed to be able to escape from the digestive tract of the predatory fish after being caught,” said Yuuki Kawabata, another author of the research.
The researchers add that the elongated shape of the eels may have aided escape by increasing the chance that their tail would remain in the predator’s esophagus when swallowed head first. They now plan to investigate specific factors that may be involved in a successful escape, with experiments involving other eels and fish with a similar body shape.
“Before we captured the first X-ray footage, we never thought that eels could escape from the stomach of a predatory fish,” said Hasegawa.
