#anguillidae

European Eel (Anguilla anguilla), family Anguillidae, order Angulliformes, River Severn, England, UK

CRITICALLY ENDANGERED.

In Aotearoa, it's whitebait season, and that means eels, eels, and more eels! Perhaps the most famous is the longfin eel, which is one of the longest eels in the world and is only found in New Zealand. During its migration, juvenile eels, aka elvers, make a perilous journey from the ocean through inland rivers, sometimes climbing up 20 m (65 ft) waterfalls to reach the lakes where they will spend the majority of their lives.

The news of the day (or a few days ago, rather) is that some scientists looked at how some eels narrowly escape death after being eaten. Yes, after.

They gave Japanese eel juveniles contrast medium and then fed them to dark sleeper fish (side note: these are sometimes called toadles, which is delightful). Then they x-rayed the dark sleepers to see what the eels did!

Dark sleeper [source]

Lo and behold, nearly a third of the eels escaped. It was known they could do this beforehand, but the exact mechanism wasn't.

Many predatory fish eat other fish head first. Since eels are so long, their tails might get left within the oesophagus, which might aid them in their escape. They maneuver their tails out of the gills of the predator (40% managed this!) and coil themselves up to pull the rest of their body out. The majority that got their tails out managed to fully escape.

Japanese eel [source]

Even if their tails weren't left in the oesophagus, the eels could be seen circling the stomach looking for an exit, with quite a few being successful and a couple unfortunately finding the wrong exit.

All of this has to be done very quickly as the eels can only withstand a few minutes in the predators digestive tract.

AKA ōrea.

Aotearoa's only endemic freshwater eel (the other two are variously also found in Australia, or introduced from Australia) and the largest. Females can be over a meter and a half long.

A very important food for the Māori, but now numbers have seriously declined and the species is considered endangered. One reason is that they only mate once, and they take 20 to 60 years to reach adulthood. When they do, they leave freshwater and head far out to sea to the ocean near Tonga, breed, and die. Only a handful of the 1 to 10 million eggs each female lays will survive to repeat the process.

If they can't find a way back to the sea (rare, since they can cross open ground if they have to) the females reabsorb their eggs and can live past a century. The ones in the tank at the Hotitika National Kiwi Centre used to live in a pond behind a canning plant where food was abundant and life carefree.

The returning young are pretty agile too, as their swim upstream - elvers have been seen climbing near vertical surfaces over 40m high.

The eels are omnivorous and opportunistic feeders - you may have seen the video where they descend on a pizza like elongated pirahna.

Two large aquatic animals, Long-finned eel and Sooty Grunter. Found around on the river shore feeding on food people had thrown to them.

Anguilla reinhardtii (top) Hephaestus fuliginosus (bottom)

[ID: A digital illustration of a brown, curled American Eel, surrounded by tufts of green eelgrass, on a background that looks like a scrap of burned parchment. End ID.]

I can finally upload some art now that I have a working phone again!

The longfin eel doesn't get its name for nothing! Known originally as the ōrea, this species is one of the largest freshwater eels in the world, up to 2 m (6.5 ft) in length. Their extraordinary size makes them excellent climbers, capable of scaling steep waterfalls and rushing rapids in order to get to their breeding grounds each year.

(Image: A feeding ōrea, aka the New Zealand longfin eel (Anguilla dieffenbachii) by Tony Forster)