Eels really are slippery customers. Juvenile eels captured by fish can escape by wriggling backwards through the predators鈥� gills. This unique behaviour, never seen before, has been filmed by 聽at Nagasaki University in Japan and his colleagues.
The researchers originally planned to study the predator escape behaviour of juvenile Japanese eels (Anguilla japonica). However, in the initial experiments, team member Yuha Hasegawa noticed that juvenile eels he had filmed being captured by dark sleeper fish (Odontobutis obscura) were somehow swimming around the aquarium again.
So on the next occasions, the team kept the camera running after the eels were captured. The videos reveal the tails of the eels appearing through the gills of the fish. At this point, the fish often respond by swimming vigorously or rubbing against the aquarium wall. Soon afterwards, the rest of the eels鈥� bodies emerge backwards through the gills.
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Of 54 eels observed being captured, 28 escaped in this way, and almost all appeared completely unharmed.
It isn鈥檛 clear whether or not these escapes are fortuitous. Some eels swim backwards rapidly when threatened, and this simple response may allow the young eels to escape. But it could also be a specialised tactic that the eels have evolved, says Kawabata. 鈥淣atural selection would favour this behaviour,鈥� he says.
Many animals have evolved extraordinary tactics for escaping predators, from producing slime and squirting blood to jumping out of their skins.聽 As far as Kawabata is aware, this is the first time that any prey animal has ever been recorded escaping through the gills of a fish. 鈥淣o one has reported this behaviour in the past,鈥� he says.
In 2006, , now at Macquarie University in Australia, and her colleagues reported that the parasitic gordian worm (Paragordius tricuspidatus) that eat insects infested with the worms. But this situation is different as the prey of the fish is the insect, not the parasitic worm inside it. Ponton says she hasn鈥檛 heard of any other animals escaping predators via their gills.
There are some other prey animals that can escape after being captured, however. The aquatic beetle Regimbartia attenuata can crawl through the digestive tract of frogs and up to 6 hours after being swallowed.
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