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Scientists have captured video evidence of eels wiggling out of the stomachs of bigger fish after being swallowed alive, marking the first such observation of prey behaviour within the digestive tract of any predator.
The study, published on Monday in the journal Current Biology, used X-ray videography to look at juvenile Japanese eels escaping from the guts of their predators.
Researchers found that the eels insert the tips of their tails in the food pipe and the gills of the predator fish before pulling their heads free.
“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,” study author Yuuki Kawabata said.
“This study is the first to observe the behavioural patterns and escape processes of prey within the digestive tract of predators.”
A previous study had shown eels escaping from their predators after being eaten, but the process by which they did so remained a mystery.
For the new study, scientists were able to peer inside the predatory fish Odontobutis obscura using an X-ray videography device.
They injected the eels with a contrast agent to make them more visible under X-ray after they had been eaten.
The videos show all 32 eels eaten by the fish with at least part of their bodies in the predator’s stomach.
All but four tried to escape by going back through the digestive tract towards the food pipe and the gills.
Thirteen eels managed to get their tails out of the fish’s gills and nine effected a successful escape.
It took these eels an average of just under a minute to escape, the study noted.
Scientists initially suspected that the eels escaped directly from the predator’s mouth through the gills. But they were surprised to see the eels escape by going back up the gut, into the stomach, and towards the gills.
They don’t seem to always rely on the same escape route, however. Some appeared to circle along the stomach in search of a way out.
The research is the first to show that a Japanese eel can use a specific behaviour to escape from its predator’s guts after being eaten.
It also shows that X-ray methods can be applied to observe other similar predator-prey behaviours.
