Eels escape predatory fish even after being swallowed, study reveals

When you’re a predatory fish, sometimes the lunch you’ve swallowed doesn’t do you any good. Instead of accepting its fate, the still-living meal escapes your stomach and escapes through the nearest orifice.

The species Odontobutis obscura can swallow young Japanese eels (Japanese eel) whole, but swallowed eels can wriggle back through the digestive tract and out of the stomach, swimming to freedom through the gills of the larger fish, according to recent findings by a group of scientists.

Such a feat may be hard to imagine, but researchers captured the eels’ daring escapes using an X-ray video system and described the discovery in a study published last week in the journal Current Biology.

“Before capturing the first X-ray footage, we never imagined that eels could escape from the stomach of a predatory fish,” said study lead author Yuha Hasegawa, an assistant professor in the Graduate School of Fisheries and Environmental Sciences at Nagasaki University in Japan.

“To witness the desperate escape of the eels from the predator’s stomach through the gills was really surprising to us.”

It’s generally unknown what happens when a fish is swallowed whole by another fish, which makes the revelation “super surprising and informative,” says ichthyologist Kory Evans, an assistant professor of biosciences at Rice University in Houston.

“We know that many fish have a second set of jaws in their throats that they use to crush, tear or grind prey before swallowing. So the idea of ​​anything escaping from those jaws not only intact, but alive is pretty impressive,” Evans said in an email.

Escape routes

Hasegawa and two of his co-authors — Kazuki Yokouchi, a researcher at the Japan Fisheries Research and Education Agency, and Yuuki Kawabata, an associate professor at Nagasaki University — previously described this behavior in a December 2021 study.

But while they watched swallowed eels writhe tail-first out of the gills of larger fish, “we had no knowledge of their escape routes and behavioral patterns during escape, because this occurred inside the predator’s body,” Hasegawa added in an email.

Scientists hypothesized that perhaps the eels accessed the gills directly from the predators’ mouths. There was only one way to be sure: using an X-ray video to peek inside a fish that had just eaten an eel.

To do this, the researchers gathered fish in laboratory aquariums. The predators measured about 14.5cm long — about twice the length of eels A. japonica juveniles.

To make the eels more visible to X-rays, scientists injected them with a harmless high-contrast medium called barium sulfate. Once a predator swallowed an eel, the sated fish was quickly moved to a special tank where the X-ray video camera was ready to roll.

“Due to the very narrow field of view of the X-ray video camera, we had to create experimental tanks of a size that could limit the movement of predatory fish,” Hasegawa said.

After about six months of trial and error, they captured the first glimpses of what the eels inside the predators were doing. To escape a fish’s gut, an eel would insert its tail into the esophagus and then wriggle backward.

Once free of the stomach, the eel would stick its tail out of its gills and undulate until the rest of its body emerged. On average, it took about three and a half minutes for a swallowed eel to resurface and swim away.

“The X-ray video of the eels circling around in the stomach, looking for a way out, is particularly impressive and shows that for some prey animals, the fight for survival doesn’t end after they’ve been eaten. It’s kind of inspiring, actually,” Evans said.

Japanese eels find a way out

From 32 eels that were eaten, 13 managed to reach the gills of their captors and put their tails out, with only nine eels coming out completely .

Fish that swallow their prey whole tend to devour it head first, and for the sinuous A. japonicaas the head and upper body slide into a predator’s stomach, its tail often remains in the esophagus. This maneuver may play a role in helping certain eels wriggle back out, according to the study.

For Prosanta Chakrabarty, a professor at Louisiana State University (LSU) and curator of fishes at the LSU Museum of Natural Sciences, the images of eels escaping through their gills bring to mind the terrifying appearance of the xenomorphs in the “Alien” movies. Chakrabarty was not involved in the new study.

While this escape behavior in fish has so far been documented only in Japanese eels, the discovery could shed light on the peculiar habits of another eel species the LSU professor has studied: the snub-nosed eel (Simenchelys parasitica), which can inhabit “and presumably escape from the hearts of living sharks,” Chakrabarty wrote in an email.

Eels A. japonica Larger, more muscular ones that can tolerate the acidic, oxygen-poor environment of a stomach may have a better survival rate after being swallowed, but more research would be needed to know for sure, the study authors said.

“More experiments involving predator-prey interactions with various sizes of eels, while measuring their locomotor performance and tolerance to harsh environments, are needed to identify the specific factors that enable successful escape,” Hasegawa said.

* Mindy Weisberger is a science writer and media producer whose work has appeared in Live Science, Scientific American, and How It Works magazine.

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This content was originally published in Eels escape predatory fish even after being swallowed, study reveals on the CNN Brasil website.

Source: CNN Brasil