Eels can travel over land, climb walls and take down serious prey. They may be Australia’s most hardcore animal

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Eels can travel over land, climb walls and take down serious prey. They may be Australia’s most hardcore animal

They may be no match for saltwater crocs or great white sharks, but for their size, our freshwater eels are surprisingly hardcore.

These slippery fish can travel over land, take down serious prey, and climb walls — all without any freaking legs!

Their shape-shifting rivals that of insects such as butterflies, moths and cicadas.

And eels undertake one of the most epic migrations known within Australian waters — but to this day, their breeding grounds remain a mystery.

Lurking in the muddy bottom of a river or dam is only part of an eel’s life. It’s like the Clark Kent bit. The rest is the stuff of fishy superheroes.

Life starts with a marathon

If you’ve seen an eel in a river or dam, it’s already the stuff of legend.

From the moment it hatched somewhere in the ocean — probably in the Coral Sea — it was on a mission to swim thousands of kilometres to the Australian mainland. Only around one in every 10,000 hatchlings make it.

The big swim starts with an easy stretch for the leaf-shaped larval eel. Purpose-built for drifting, the leptoscephalus, or “narrow head” larva, rides the South Equatorial Current towards the Aussie coast.

But among the challenges it faces are the stages of metamorphosis — gradually moving from the googly eyes and big mouth that only a mother eel could love to a more classic “eely” shape.

Superpower no.1: invisibility

While still in the ocean, probably near the continental shelf, the larva transforms to a classic eel shape, but with a difference: the eel is transparent.

It’s called a glass eel.

Being transparent probably helps glass eels hide from predators, but along with this seeming superpower, the eel loses the ability to eat — just when it has to get all the way to a river mouth or estuary. It’s not an easy task for a tiny half-starved glass eel!

The eel can only eat again when it reaches a river mouth and undergoes another serious change.

Spending time in brackish (a mixture of salt and fresh) water lets its ocean body adjust to the freshwater life ahead.

At this stage, it starts to darken in colour and becomes known as an “elver”.

Elver can eat, which is lucky because it needs a lot of strength for the next part of its epic adventure: the long trip upstream.

Oh, and puberty.

Changing sex to suit the crowd

Eels are sexless from the time they hatch until they grow about 30 centimetres in length.

Then some version of eel puberty kicks and they transform, becoming either male or female.

And which way they go depends on the population density.

In an area with a lot of eels, the young eels are more likely to become male.

But in areas with fewer eels — like further upstream, which is harder to get to — eels are more likely to become female.

Overcoming tall, slippery obstacles

You know what’s hardcore? Having the body of an eel and still climbing up waterfalls.

These creatures are relentless in their journey upstream to find a home.

And if that means scaling a waterfall — or the odd dam wall — they’re up for it!

Fish gotta swim. But it’s well known to farmers that eels can make their way across seemingly dry land to reach a dam to call home.

Their secret weapon? They can “breathe” through their skin!

Well … not quite, but their tiny scales and slime help them transfer some oxygen into their bloodstream from their skin.

Eels only need the smallest trickle of water to travel, which really gives them an edge as they push their way inland.

They’re ferocious hunters

Eels might start out as vulnerable little wrigglers, but by the time they settle into their freshwater home, they get massive.

Long-finned eels (Anguilla reinhardtii) — one of the two common eels in Australia — can grow up to 1.7 metres long.

Their short-finned cousins (Anguilla australis), more common in south-east Australia, are about half as long.

And adult eels will eat snakes, birds, fish and native water rats (rakali).

This makes them a top predator in freshwater ecosystems.

Swapping guts for gonads

If there’s one thing that should be obvious about eels by now, it’s that they never take the easy option.

Once they find a nice river or dam to call home, they don’t just settle in for the duration.

Eventually, the call to breed compels them to make the long journey back out to sea.

Along the way, these fully grown eels get kitted out for breeding. Their guts shrivel up to make room for their enlarged gonads — where eggs and sperm are made.

At this stage they stop eating, which is probably a good thing as their anus shrinks to prevent water loss.

And their external appearance changes once again as their eyes enlarge, and their heads gets pointier — probably to refine their swimming and deep-water vision.

They leave the way they came, shrouded in mystery

Eventually, the eels make it back to the ocean; until last year, scientists could only guess where they went.

By attaching trackers to short-finned eels, researchers from the Arthur Rylah Institute were able to follow some eels as they swam a whopping 2,620 kilometres from western Victoria all the way to the Coral Sea.

Exactly where they spawn is still a mystery, but it’s possibly near New Caledonia.

We don’t know what happens after they spawn, either. Most likely they die of exhaustion.

Hardcore? One hundred per cent.

Special thanks to eel expert Lachlan McKinnon.

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