A backwards shimmy might be all it takes to avoid becoming a snack, according to a team of Japanese researchers who have filmed a surprising strategy juvenile eels use to escape from a predatory fish's guts.
This is the first time this behaviour has been captured from within the digestive tract of a predator using X-rays, and the first time a fish has been confirmed to escape from the stomach of another fish.
The researchers found when eels were swallowed by the fish, they regularly tried to wiggle backwards out of the stomach, and poked their tail out of the fish's gills before making a quick escape.
"Before capturing the first X-ray footage, we never imagined that eels could escape from the stomach of a predatory fish," researchers Yuha Hasegawa and Yuuki Kawabata from Nagasaki University in Japan told the ABC.
"We speculated that eels would escape directly from the predator's mouth to the gill.
"However, witnessing the eels' desperate escape from the predator's stomach to the gills was truly astonishing."
The discovery was published today in the journal Current Biology.
How did the team capture the eels on X-ray?
The team wanted to find out how Japanese eels (Anguilla japonica) behaved when put into a tank with a predator fish called the dark sleeper (Odontobutis obscura).
"After one eel had been captured by the dark sleeper, I was preparing for the next experiment and looked into the tank, only to find that the eel, which should have been eaten, was swimming around," Dr Hasegawa said.
"We kept a close eye on the other eels that had been eaten and, to our astonishment, we discovered that the eel was trying to wriggle its way out via the gill."
While this was a lucky finding, investigating what happened inside the belly of the dark sleeper took much more time and effort for the team.
When they first used an X-ray machine to investigate what was happening inside the stomach of the fish, the thin bones of the eels didn't show up, so they couldn't see anything.
Eventually the team injected barium sulfate as a contrasting agent into the eels' tail and abdominal cavity so that they were visible on X-rays.
There was also the issue of ensuring the dark sleeper would stay in the X-ray's field of view.
"We had to create experimental tanks of a size that could limit the movement of the predatory fish," the two researchers said.
"It took a very long time to get the predatory fish to capture eels in such an uncomfortable environment."
But after 18 months of trial and error, they finally captured the X-ray of the eel escaping — tail first — out the gills.
Interestingly, in follow up experiments, it was found that 28 out of the 32 eels tried to escape. Thirteen managed to get their tails out of the gills of the fish, and nine successfully escaped.
The researchers are now trying to investigate why only some of the eels succeeded.
Do other animals escape like this?
While this might be the first time someone has directly observed an escape as it is happening inside the animal, eels are not the only creature that can wriggle their way out of becoming lunch.
Scientists have documented blind snakes, diving beetles and a ground beetle larvae escaping from toads.
Snails have been known to survive the entire digestive system of a bird, popping out the other side relatively unscathed.
But weirdest of all is the Gordian worm, a parasitic creature that looks a bit like a tape worm, which infects insects and crustaceans turning them into zombies.
Normally Gordian worms — commonly known as horsehair worms — would be eaten by a creature like a cricket, and then the worm would manipulate the cricket's behaviour to make it more likely to be eaten by a predator.
In 2006, a study found that Gordian worms could escape not only the crickets the worms had infected, but also the predators like trout, perch and frogs that ate the crickets.
Similar to the eels, the worms would wriggle out of the mouth, nose or gills of the predators in about 25 per cent of the cases.
