When prey enters the predator's digestive tract, it is either dead to begin with or dies quickly in the aggressive digestive environment. In either case, it has no chance of survival, unless it is a Japanese river eel (Anguilla japonica). A recent study has shown that this fish is capable of performing tricks no worse than the famous illusionist Houdini, and escaping from the body of a predator. Currently, it is the only fish species known to science that is able to survive and escape after being swallowed.
Japanese River Eels Escape from Predator's Stomach. Photo source: joelsartore.com
Contents
- 1 Japanese River Eel – A Meter-Long Predator
- 2 How Eels Escape from Predator's Stomach
- 3 Why Eels Manage to Survive
- 4 Why Eels Don't Die in Predator's Stomach
Japanese River Eel – A Meter-Long Predator
The Japanese river eel is itself a predator. It is a species of the eel family that reaches 40 centimeters in length, but sometimes grows up to 1 meter. The fish is found off the coast of Japan and in the East China Sea, as well as off the coast of Taiwan, Korea, China, and the northern Philippines.
Although the eel is called a river eel, in fact it can live not only in fresh water, but also in salty sea water. Moreover, it lives at a depth of up to 400 meters. However, during the day, this fish can rise to the surface and even move on land. An interesting feature of these fish is the ability to fluoresce, that is, it glows under the influence of an external source of radiation. By the way, many animals are able to glow under the influence of ultraviolet light, although not as intensely as the eel.
Scientists have discovered that eel fry escape from the predator's body through its gills. Photo source: sciencealert.com
How eels escape from the stomachs of predators
Employees of the Japan Agency for Fisheries Research and Education discovered several years ago that eels are able to leave the digestive tract of predators that swallow them. However, we are not talking about adult specimens, but about fish fry.
As it turned out, they leave the body of the predator that swallowed them not through the mouth, and not even the anus, as many might think, but through the gills. However, until the last moment it was unclear how the escape occurs. It must be said that eels are not the only living creatures that are able to escape from a predator after being swallowed. But this ability is incredibly rare in the animal world, and is found mainly in invertebrates.
For example, scientists have previously observed a beetle that emerged unharmed from a frog's anus, as well as a parasitic worm that escaped from a predator that had eaten its host. Moreover, the worm, as it turned out, uses any openings of the predator that it can find to escape. We previously talked about eels that live in Australia. They literally rip the stomach of the predator that swallowed them. However, unlike Japanese eels, they still cannot escape from the body.
Eels manage to escape from the bodies of predators thanks to several of their features. Photo source: biolib.cz
Why Eels Manage to Survive
As mentioned above, it remained a mystery to scientists how the Japanese eel managed to avoid digestion. In addition, it was not entirely certain that the escape strategy was purposeful. Perhaps the fry managed to escape the predator's body by accident. To find out, the researchers conducted an experiment using X-rays. Thanks to them, the team was able to observe what was happening inside the predatory fish after the eel fry disappeared into its mouth.
To make the fry clearly visible on X-rays, the researchers first injected them with barium sulfate, a contrast agent. The eels were then placed in a tank containing a black-mouthed rotan (Odontobutis obscura) – a predatory fish for which eels are a delicacy.
Initially, scientists assumed that the eel fry entered the gills directly from the mouth. However, studies have shown that in the “escape trick” everything happens honestly – first the fish go into the stomach, but then leave it through the esophagus. This is done by wriggling and moving the tail. However, not all eels manage to escape and survive.
As the researchers report in the journal Current Biology, of the 32 eels swallowed by a predator, only 13 were able to reach the gills. And only 9 fish managed to complete the escape and break free.
Why eels don't die in the stomach of a predator
The results of the study indicate that the key to the success of eels is their muscle strength, elongated shape and slippery body. Thanks to this, the fish can move through the esophagus. In addition, they are highly resistant to the aggressive environment in the stomach of a predator. Those fish that were unable to get into the esophagus continued to circle in the stomach for some time, as if trying to find a way out.
A diagram of how eels escape from a predator's body. Image source: sciencealert.com
The average time each fish was active in the stomach was more than three minutes. Only after that did they succumb to the aggressive digestive environment and die. Of course, despite all these abilities, the success of eels is far from 100%, however, no other species of fish is capable of surviving in such a situation.
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There is no doubt that the ability to escape from the stomach is extremely rare in the animal kingdom. However, it is quite possible that in fact there are other creatures that are also capable of this, but they are simply unknown to science. We wouldn't even know anything about eels now if scientists hadn't accidentally noticed how they break free through the gills of a predatory fish.