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Juvenile Japanese eels can escape from the stomach of a predator and exit through the gill openings. Image © Yuha Hasegawa and Yuuki Kawabata, Nagasaki University.
When Pinocchio and his father, Geppetto, were swallowed by a whale, they started a fire to make the whale sneeze them out. That’s not exactly the strategy that juvenile Japanese eels use, but the eels have managed to find a way to escape the stomachs of predators that swallow them—and biologists have captured their Houdini-like feats on video.
Japanese eels split their lives between freshwater and saltwater. After hatching in the oceanic waters of the western North Pacific, they make their way to the estuaries, rivers and lakes of Japan, Korea, Taiwan, China, Vietnam and the Philippines. At breeding time, adults trek back to the ocean to spawn, and then they die. Researchers previously observed young eels escaping through a predator’s gill openings, but they didn’t know how the eels did it. They suspected the eels reached the gills directly from the mouth, but an experiment using the dark sleeper fish, a likely predator of the eels, revealed a surprisingly different path.
Scientists injected a contrast agent into a dark sleeper so they could use X-ray videography after the fish captured the eels. They discovered that the eels had all been swallowed and were at least partially in the sleeper’s stomach. Most of the 32 eels they observed tried to escape. They did so by inserting the tips of their tails into the fish’s esophagus and pulling themselves from the stomach, through the esophagus, and toward the gills. Then, they extended their heads past the gills and swam away before they could be swallowed again. Of the eels that attempted to escape, 13 made it far enough that their tail exited the gill opening, and nine ultimately got away. This resulted in a 28% escape rate for the prey, and it only took an average of 56 seconds—not quite a full minute—to escape being digested. Even Houdini would be impressed!
