蓝蟹搭压舱水来到意大利，吃光了蛤蜊。他们放了八万只小章鱼下去——但章鱼可能不肯去。

Start with a number: seventeen million euros.

That is the loss calculated by the Emilia-Romagna region, from 2022 until now.

The region is in the north-east of Italy, and it is one of Europe’s most important clam-producing areas.

The loss was eaten by a crab. That crab is called the blue crab.

It does not belong in the Mediterranean; it lives on the other side, off North America, in the Atlantic.

So how did it get here?

By ship.

When an ocean-going ship sails empty, it pumps a great deal of seawater into the hull to hold it steady. That is called ballast water.

At the destination, the water is pumped back out.

Blue crab larvae are tiny, and they were mixed into that water, travelling with the ship, all the way across the Atlantic.

It did not decide to come. We brought it.

Once in the Mediterranean, it found the place suited it perfectly.

The key is its pair of blue claws.

They are hard and sharp, and particularly good at prying open shells.

And a clam’s entire defence is that one shell.

Something that can pry shells open arrives, and the clam has no answer.

That is why the loss is so large.

Now, the response.

The Emilia-Romagna region paid for a project launched last year, called Octo-Blu.

The method sounds simple: raise octopuses, then put them in the sea.

In a laboratory in the seaside town of Cesenatico, they hatch octopus larvae in white water tanks.

Once the young reach a certain size, they are released into the sea near the reefs.

So far, eighty thousand have gone in.

They also did something that feels very like building houses for animals:

They laid a great many artificial dens of cement and clay on the seabed, as homes for the octopuses.

Because an octopus needs a den; without one it will not stay.

Why choose the octopus?

The researcher Antonio Casalini put it plainly: in the wild, the octopus is the most formidable predator of crustaceans there is.

A crab has a shell, and an octopus happens to be good at opening shells.

It sounds like a beautiful plan: let nature solve nature’s problem itself.

But the best part of this story is the problem the researchers named themselves.

The places with the most blue crabs are warm, sandy, shallow lagoons.

And the octopus, as it happens, does not tolerate that kind of water well.

Which is to say: octopuses are very good at eating blue crabs, but may not be willing to go where the blue crabs are.

The plan may simply not work.

And the person saying so is not an opponent. It is the people doing it.

I like that a great deal.

A project willing to state its own weakness before it has any results is worth noting for that alone.

Finally, back to the beginning.

The blue crab is not a villain; at home it is a normal part of things.

It was our ships that carried it to a place where it has no natural enemy.

And now we are using ships and tanks again, to send a second animal.

Think about it:

Something that should not be here has already arrived. Is sending something else to eat it a good idea?