Hasegawa et al./Current Biology
Imagine you’re a Japanese eel, swimming around doing its thing, and then — bam! A predatory fish swallows you whole, and you have just minutes to escape before you’re certain to die. What’s an eel to do? According to a new paper published in the journal Current Biology , Japanese eels choose to push their way out of the digestive tract tail first, through the esophagus, and out through the gills of the predatory fish.
According to the authors, this is the first study of its kind to observe the behavioural patterns and escape processes of prey in the digestive tract of predators. “At present, the Japanese eel is the only fish species confirmed to be able to escape from the digestive tract of predatory fish after it has been caught,” co-author Yuha Hasegawa of Nagasaki University in Japan told New Scientist.
There are several strategies in nature to escape from predators after being swallowed. For example, a parasitic worm called Paragordius tricuspidatus can force its way out of a predator's system when the host organism is eaten. There was also a fascinating study in 2020 by Japanese scientists about the water beetle's unusual survival strategy Regimbartia attenuata. They fed some of the beetles to a pond frog (Pelophylax nigromaculatus) under laboratory conditions, expecting the frog to spit out the beetle. This is what happened in earlier experiments with bombardier beetles (Pheropsophus jessoensis), which spray toxic chemicals (described as an audible “chemical explosion”) when they enter a toad's intestines, causing the toad to turn its stomach and regurgitate the chemicals.
But R. to weaken essentially passes through the digestive tract and escapes through the frog's anus after being swallowed alive. It proved to be a successful escape route. In the case of the bombardier beetles, an average of between 35 and 57 percent of the toads vomited within 50 minutes, ensuring the survival of the regurgitated beetles. R. to weaken's survival rate was a whopping 93 percent. In fact, 19 of the 20 walked out of the frog unharmed within an hour, although one industrious beetle bolted out within five minutes. Granted, the beetles often emerged covered in fecal pellets, which can't have been pleasant. But that didn't stop them from resuming their little beetle lives; all survived for at least two weeks after being swallowed.
Hasegawa had co-authored a previous study in which they had seen Japanese eels emerge from the gills of a predator after being swallowed, so they knew this unique strategy was possible. However, they didn't know the details of what was happening in the digestive tract that allowed the eels to perform this feat. So the team decided to use X-ray videography to peer inside predatory fish (Odontobutis obscura) after the eels had been eaten. They injected barium sulfate into the abdominal cavity and tail of the Japanese eels as a contrast agent and then introduced each eel into a tank with one O. obscuraThe X-ray video system captured the interactions after an eel was swallowed.
Out through the gills
The escape behavior of a Japanese eel. Credit: Hasegawa et al./Current Biology
O. obscura swallow their prey whole, along with the surrounding water, and a swallowed eel quickly enters the digestive tract, a highly acidic and oxygen-poor environment that kills the eels in 211.9 seconds (a little over three minutes). Thirty-two of the eels were eaten, and of those, 13 (or 40.6 percent) managed to insert at least their tails through their predator's gills. Of those 13, nine (69.2 percent) escaped completely within an average of 56 seconds, suggesting “that the period until the tails emerge from the predator's gills is particularly crucial for successful escape,” the authors wrote. The final push toward freedom involved coiling their bodies to remove their heads from the gills.
It helps to be swallowed head first. The researchers found that most captured eels tried to escape by swimming back through the digestive tract to the esophagus and gills, tail first in cases where escape was successful. However, eleven eels ended up completely in the stomach and began swimming in circles, probably looking for a possible escape route. Five of them managed to insert their tails correctly into the esophagus, while two died because they pointed their tails in the wrong direction.
“The most surprising moment in this study was when we saw the first images of eels escaping by going back through the digestive tract to the gills of the predatory fish,” said co-author Yuuki Kawabata, also of Nagasaki University. “At the beginning of the experiment, we speculated that eels would escape directly from the predatory fish's mouth to the gills. But contrary to our expectations, it was truly astonishing for us to witness the eels' desperate escape from the predatory fish's stomach to the gills.”
Current Biology, 2024. DOI: 10.1016/j.cub.2024.07.023 (About DOIs).
