Watch: first video of juvenile eels escaping from fish’s stomach

Slippery elvers move back up the predator’s digestive tract before squirming out through its gills, study finds

 
Baby eels eaten by a fish are able to escape alive by squirming out the stomach and through the predator’s gills, a study has found.
The unusual survival technique has been captured on camera for the first time with x-ray imagery.
Video shows the juvenile Japanese eels escape their fate as a snack by inserting the tips of their tails through the oesophagus and gills before then pulling their heads free.
Prof Yuuki Kawabata, of Nagasaki University in Japan, said: “This study is the first to observe the behavioural patterns and escape processes of prey within the digestive tract of predators.
“They escape from the predator’s stomach by moving back up the digestive tract towards the gills after being captured by the predatory fish.”
In an earlier study, the team had shown that Japanese eels can escape from the gills of predators after being swallowed, but the latest study, published in the journal Current Biology, shows the process in action.
“We had no understanding of their escape routes and behavioural patterns during the escape because it occurred inside the predator’s body,” Yuha Hasegawa, the study’s co-author, said.
The baby eels were injected with a chemical to make them visible on the specialist x-ray video camera before being fed to the fish in a tank.
It took more than a year for the process to be perfected enough to create the clear video footage showing the machinations of the escape.
Some 32 captured eels in their study had at least part of their bodies swallowed into the predators’ stomachs and 28 of them tried to escape by going back up the digestive tract.
Almost half of these (13) managed to get their tails out the fish gill while nine successfully escaped through the gills.
It took the successful ones an average of about 56 seconds to free themselves from the predator’s gills, according to the findings.
Prof Kawabata said: “The most surprising moment in this study was when we observed the first footage of eels escaping by going back up the digestive tract toward the gill of the predatory fish.
“At the beginning of the experiment, we speculated that eels would escape directly from the predator’s mouth to the gill.
“However, contrary to our expectations, witnessing the eels’ desperate escape from the predator’s stomach to the gills was truly astonishing for us.”
Further research found that, despite the similarities, the eels didn’t always rely on the same escape route through the gill cleft.
Prof Kawabata said some of them also circled along the stomach, seemingly in search of a way out.