When the going gets tough, eels get going...backwards

By: Leah Turner, University of Guelph

I’m ashamed to admit this as someone who spends most of her time studying fish, but eels really creep me out. I don’t know if it’s their frightening faces, or snake-y swimming habits, or poisonous blood, or tendency to hang out in deep, dark holes, but I really really don’t like them. Or should say really didn’t like them until I read a preprint by Hasegawa et al. Their research has given me a grudging respect for these wily creatures and their ability to back out of a tough spot (literally).

Eels are ecologically and economically important, so there’s a lot of interest in things like how they grow or where they migrate. But other parts of eel life are more mysterious, like how young eels avoid being eaten. If you can avoid being eaten, you get to live another day. If you live another day, you’re one day closer to being able to mate and produce your own offspring. And if you inherited the skills to avoid being eaten from your parents, there’s a good chance you’ll pass those skills onto your young, and so on.

Graduate student Yuha Hasegawa, together with colleagues from Nagasaki University and the Japan Fisheries Research and Education Agency, set out to understand how it is that young Japanese eels (Anguilla japonica) avoid being eaten by predators. Catching eels in the act of avoiding predators in the wild is tricky, so the research team set up an aquarium in the lab with a fish ominously known as the dark sleeper (aka Odontobutis obscura), which they knew from pre-experiment trials would try to snack on eels. During these pre-experiment trials, some eels managed to escape being eaten. But their escape wasn’t at all what the researchers expected - the young eels weren’t just avoiding being eaten by swimming away from the dark sleeper, they were actually escaping through the gills of the dark sleeper after being eaten.

Technically speaking, this isn’t impossible. If you’ve ever had the pleasure of looking at fish gills, you might have noticed that they’re quite open to the outside. Think about vertical blinds pulled across an open window. Air and rain can still get past the blinds and through the window, but the blinds make it harder. It’s the same with fish gills: you can literally stick your finger in a fish’s mouth (10/10 would not recommend if the fish is alive) and hook it back around to the outside again, through the gill. This is basically what the young eels were doing (except they were swimming, but you get it).

Hasegawa and colleagues wanted to make sure that what they were seeing during the preliminary trial wasn’t just a fluke, so they repeated the experiment with 54 more fish. They found that more than half the time, the eels managed to escape through the dark sleeper’s gills, and went on to live another day. The rest of the eels weren’t so lucky (they were never seen again).

The other cool thing that the researchers noticed was that all the eels that managed to escape did so by wriggling out of the dark sleeper’s gills tail-first. Most fish escape from threats by curving their bodies into a ‘C’ shape and darting away, making the eels’ backwards swimming unique among fishes.

The researchers think that the survivor eels might have inherited their ability to swim backwards from their parents (who themselves inherited this ability from their parents), and the eels that didn’t survive might have tried escaping head-first but were unsuccessful, and ended up being swallowed by the dark sleeper.

This is what Hasegawa and colleagues want to pursue in their next study, using an x-ray camera to watch the escape attempts as they happen. They hope to see whether unsuccessful eels fail to make a getaway because they can’t swim backwards, or because they swim backwards and can’t make their way through the dark sleeper’s gills. The researchers hope that this follow-up study will bring them one step closer to unlocking the mystery of these eels! After reading this study, I can’t say that eels no longer scare me, but it brought me one step closer to sort of liking them.

Edited by B.G. Borowiec and A.E. McDonald. Header photo from Wikimedia Commons.