What if it is swallowed? The seal said: No problem, no problem

Produced by: Science Popularization China

Author: Su Chengyu

Producer: China Science Expo

What if it is swallowed?

Seal said the question was not funny.

In late September, a whale-watching naturalist in the waters near Anacortes, Washington, captured an unusual photo: a humpback whale swallowing a school of fish, with a bewildered seal mixed in its mouth.

(Photo source: Associated Press)

Fortunately, the seal was fine. Because whales cannot swallow a seal. Large whales like humpback whales have huge mouths that can take in a lot of small fish and water at once through "lunge feeding", but their throats are only the diameter of a grapefruit, which is far from enough to swallow prey much larger than small fish, such as seals.

The reason for being swallowed by mistake is simple. When the whale opens its mouth to catch small fish or krill, seals often feed in the same waters. If the seals get too close at this time, they may be accidentally drawn into the whale's mouth. In the end, because the whale cannot swallow, it eventually spits out the seal from its mouth by repeatedly opening and closing its mouth.

The seals have a good fate, but those that are swallowed have to find another way to survive, such as the Japanese eel (Anguilla japonica) swallowed by the sand snakehead (Odontobutis potamophila).

Eel

(Photo source: veer photo gallery)

How did the sand snakehead meet the Japanese eel?

Japanese eels have a complex life cycle. Their eggs hatch in the ocean and the larvae, called leptocephali , are elongated and almost transparent. These larvae drift westward via the North Equatorial Current and then reach the coastal areas of East Asia with the help of the Kuroshio Current . This process takes about 110 to 160 days, after which the larvae become glass eels and gradually enter estuaries and fresh water.

Glass eels

(Image credit: Joe Reynolds)

When glass eels enter rivers, they stay in freshwater for 5 to 10 years, entering the so-called yellow eel stage . During this stage, the eels gradually grow and adapt to different habitats, mainly feeding on shrimp, insects and small fish. At this time, the eels will migrate to different areas such as estuaries, lakes or upstream rivers as the river environment changes. When the eels reach maturity, they will begin to migrate to the deep Pacific Ocean in the fall to complete their reproduction mission.

eel

(Photo source: global seafood)

During the long journey to the ocean for larval stages (glass and yellow eels), they face a variety of predators and challenges. One of these is the Channa tang, an East Asian freshwater fish that primarily sucks its prey down into its stomach, along with the surrounding water, by rapidly sucking it down. This method of feeding is particularly effective against smaller creatures, such as larval Japanese eels.

Channa

(Image source: Wikipedia)

This seemed like the final moment - for most prey, entering the stomach of a predator means certain death. However, this time, the story did not end as expected.

The escape rate of baby eels is as high as 69%. How did scientists observe this?

In order to observe how young eels escape from the stomach of predators, and to observe how eels can survive and try to escape after being swallowed by the stomach of the sand tang fish, the experiment used laboratory-reared Japanese eel larvae, which had an average length of 68.1 ± 6.0 mm (n = 104). The average length of the predator sand tang fish was 145.4 ± 15.6 mm (n = 11).

The research team injected barium sulfate into Japanese eel larvae raised in the laboratory, so that every move of the eel could be clearly captured under X-rays. Then, the eels were placed in an experimental tank with their predator, the sand tang snakehead, and the story began.

Under X-ray, scientists found that the swallowed eels did not wait for death. They began to move in the opposite direction, trying to swim back through the predator's esophagus and find an exit. At this time, the eels showed their unique escape ability: they were able to pass through the esophagus, put their tails into the predator's gill slits, and gradually exit from the gills.

The eel being swallowed, the white circle is the tail, the black circle is the head

(Image source: Reference 1)

In the experiment, 32 eels were swallowed by predators, of which 13 eels successfully stretched their tails out of their gills, and 9 completed the entire escape process. This process was full of thrills and struggles, like an ultimate life-and-death race. For those eels that successfully stretched their tails out of their gills, their survival rate was as high as 69%.

The eel's tail emerged from the gills of the sand tang snakehead

(Image source: Reference 1)

Scientists marveled at the complexity of this anti-predator strategy. They found that some eels move in circles along the stomach wall after they are completely inside the predator's stomach, as if they are exploring potential escape routes. The secret to escape may lie in the eel's body structure and the order in which it swallows. The sand tang fish usually swallows its prey head first, so the eel's tail often remains near the esophagus, giving them a chance to escape tail first.

X-ray images and schematics

Figure B: The eel's tail has been inserted into the predator's esophagus, and the X-ray image clearly shows the position of the eel in the predator's digestive tract.

C: The eel's tail begins to pass through the predator's gill slits, a crucial process.

D: The eel pulls its head out of the predator's gills, completing its escape.

Figure E: The eel swims around in the stomach, showing an "exploratory" behavior, searching for a possible escape route.

(Image source: Reference 1)

Not only that, the experiment also revealed that eels need to react as quickly as possible in the acidic and oxygen-deficient stomach environment, because once they enter the stomach of a predator, they can only survive for an average of about 211.9 seconds. The clock is ticking, and the boundary between life and death is often decided in just a few minutes.

The eel still needs to struggle to find a way out, but for the bombardier beetle, all it has to do after being eaten by the toad is fart.

Being swallowed by a toad, the yapple beetle chooses to "breathe fire" in its stomach

Similar to the experiment above (without the X-ray), this time the subject is the Pheropsophus kimaniae. Once the beetle is swallowed into the toad's stomach, it does not wait quietly for its fate to come, but launches a brilliant self-rescue operation. The beetle releases a high-temperature chemical spray. This "fire-breathing" effect not only protects itself, but is also enough to cause severe discomfort to the toad.

Yepi Carabidae

(Image source: Wikipedia)

With a faint explosion, Toad began to realize that he had made a mistake. For the next few minutes, Toad tried to endure the irritation in his stomach, but was often forced to make a shocking choice - vomiting.

toad

(Image source: Reference 2)

The success rate of bombardier beetles' life-or-death escapes is not completely random. The study found that the larger the beetle , the more chemicals it released and the higher its chance of escaping successfully. For toads, the smaller they are , the lower their chemical tolerance is, making it easier for them to spit out beetles.

It can be said that this is a battle between big and small: the small toad cannot withstand the beetle's chemical counterattack, while the large beetle has a greater chance of escaping calmly. Don't panic if you are eaten, there is always a way out, provided that you eat it whole and alive...

References:

[1]Hasegawa Y, Mine K, Hirasaka K, et al. How Japanese eels escape from the stomach of a predatory fish[J]. Current Biology, 2024, 34(17): R812-R813.

[2]Sugiura S, Sato T. Successful escape of bombardier beetles from predator digestive systems[J]. Biology Letters, 2018, 14(2): 20170647.