From camouflage to poison, animals have developed dozens of ingenious ways of avoiding being eaten.
However, researchers have now found that the Japanese eel has an ability that takes slipping the jaws of death to a whole new level.
Incredible video shows the shocking moment an eel ‘reverses’ back out of its predator’s stomach after being eaten.
Scientists from Nagasaki University developed a new X-ray video method to capture the first-ever footage showing how eels force their way to freedom by slipping through a fish’s gills.
Lead researcher Professor Yuuki Kawabata told MailOnline: ‘Contrary to our expectations, witnessing the eels’ desperate escape from the predator’s stomach to the gills was truly astonishing for us.’
Researchers from Nagasaki University have captured a bizarre X-ray video showing how juvenile eels escape from inside their predator’s stomachs
In a previous study, Professor Kawabata and her colleagues had already observed that some juvenile Japanese eels were able to escape from the stomachs of other fish.
However, until now they had no idea how the eels were able to achieve this bizarre feat of escapology.
The researchers spent a year developing a new method of X-ray videography to see what was actually happening to the eel inside the fish’s stomach.
Eels were injected with a contrasting agent called barium sulphate, allowing their thin bones to show up on an X-ray of the fish’s insides.
During the experiment, 32 juvenile eels were injected with this chemical before being fed to a native predatory fish called a ‘dark sleeper’, or odontobutis obscura.
Like many predatory fish, the dark sleep swallows its prey whole along with the surrounding water by rapidly opening its mouth.
The researchers had observed that Japanese eels (pictured) were able to escape from their predators’ stomachs, but until now they did not know how this was possible
Once inside, prey are deposited in the stomach where they are killed by the acidic and oxygenless environment in 211 seconds on average.
However, when the footage taken inside the fish’s stomach showed that this was not the case.
Before the experiment, Professor Kawabata says they had expected the eels to try and escape directly from the predator’s mouth into their gills.
Professor Kawabata says: ‘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.’
Instead of making their way into the mouth, the researchers observed that the eels would force their tails through the oesophagus and directly into the gills.
From there, the eels coiled their bodies to pull their heads loose and swim away.
The researchers observed that some eels would circle the fish’s stomach. Of the eels tested 13 were able to push their tails through the gills and nine were able to escape completely
‘We have discovered a unique defensive tactic of juvenile Japanese eels using an X-ray video system: 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 their paper, published in Current Biology, the researchers suggest that the long body of the eels makes it more likely that their tails remain poking out into the oesophagus.
Of the 32 eels tested, all but four attempted to force their way out of the fish’s digestive tract with 13 managing to get their tails loose and nine managing to escape fully.
On average, the eels were able to escape from the dark sleeper’s gills just 56 seconds after being swallowed.
A number of eels also displayed ‘circling’ behaviour during which they raced around the stomach of the predator fish as if looking for a way out.
32 eels were injected with barium sulphate to make them show up under X-ray and were then fed to a native predatory fish called a ‘dark sleeper’ (pictured)
However, only those eels which transitioned to inserting their tails through the oesophagus were actually able to avoid death.
The researchers note that the fish seem to be aware of that their dinner is attempting to escape and will often try to fight back
Co-author Yuha Hasegawa told MailOnline: ‘Many predatory fish exhibit resistance behavior by swallowing the escaping eel again, during which they draw water into their mouths and expel it through their gills.
‘The eels might take advantage of this water flow to successfully escape via the predator’s gills.’
However, the predatory fish do not seem to be hurt by the escape attempt.
Using an X-ray camera, the researchers found that the eel would force its tail into the oesophagus of the fish before reversing out of the gills
Due to the fact that larger eels were able to escape more often, the researchers think that it might be critical for young eels to rapidly develop the strength and locomotor skills they need to escape.
Dr Hasegawa says: ‘This discovery has provided us with new insights: muscle strength and tolerance to highly acidic and anaerobic environments, as well as their elongated and slippery morphology, are necessary for eels to quickly escape from the digestive tract before being digested.’
In the future, the researchers hope that this X-ray technology will be useful for more research into how prey are able to escape after being eaten.
