But the Portuguese man-of-war was harmless compared to the box jellyfish – Chinmex fhekeri, the deadly Australian sea wasp.

In the course of evolution, nature had developed an impressive array of toxins. Chinmex Jleckeri was the piece de resistance. A single box jellyfish contained enough poison to kill 250 people. It's highly potent venom paralysed the nervous system, causing immediate loss of consciousness. Within minutes, or sometimes seconds, most of its victims suffered heart failure and drowned.

All this ran through Johanson's mind as he stared at the screen.

Fourteen dead and countless others injured in a matter of weeks. Had the death-count ever been so high on a single stretch of coastline from just a single species? And what about the disappearing ships?

Portuguese men-of-war in South America. Box jellyfish in Australia.

Bristleworms in Norway.

It's probably coincidence, he thought. Swarms of jellyfish appeared all over the world. The holiday season wouldn't be the same without them. They had nothing in common with worms.

He tidied away the last few items of clothing, switched off the television and went into the living room to listen to music or read. But he didn't put on a CD or pick up a book. For a while he paced up and down, eventually stopping at the window. The streetlamps lit the street outside.

The lake had been so peaceful…

It was peaceful here too…

When things were so peaceful, there was usually something wrong.

Don't be ridiculous, Johanson told himself.

He poured himself some grappa, took a sip, and tried to forget about the news.

Then he remembered Knut Olsen, a fellow biologist at the NTNU. He knew a lot about jellyfish, coral and sea anemones.

Olsen picked up on the third ring.

'Were you asleep?' asked Johanson.

'Not with the kids still up,' said Olsen. 'It's Marie's fifth birthday today. How was the lake?'

Olsen was a perpetually cheerful family man whose cosy domestic life seemed like a nightmare to Johanson. They never saw each other socially, unless you counted lunch breaks, but Olsen was a nice guy with a decent sense of humour. With four children he needed it, thought Johanson. 'One of these days, you should come with me,' he said, although they both knew it wouldn't happen. 'Have you seen the news?'

There was a short pause. 'The jellyfish, you mean?'

'Right first time. What's going on?'

'It's obvious, isn't it? Biological invasions happen all the time. Frogs, locusts, jellyfish…'

'But Portuguese men-of-war and box jellyfish?'

'It's unusual…'

'In what way?'

"They're two of the world's deadliest sea creatures. And there's something peculiar about what they're saying on the news.'

'Seventy fatalities in a hundred years?' said Johanson.

'Oh, that's rubbish.' Olsen gave a derisive snort.

'Too many?'

'Too few! The real death toll's much higher – ninety at least, if you count the Gulf of Bengal and the Philippines, not to mention all the unreported and unexplained cases. Australia has always had a problem with box jellies. They spawn in the river mouths north of Rockhampton. Almost all the accidents happen in the shallows – they can kill you in less than three minutes.'

'Is it jellyfish season?'

'In Australia, yes – October to May. In Europe they only really bother you when it's too hot to stay out of the water. We were in Menorca last summer and the kids were going crazy. The whole place was inundated with Velella.'

'Sorry?'

'Velella velella. By-the-wind sailors, Quite pretty, really, providing they're not rotting on the beach. Little violet-coloured jellies – the sand was covered with them. Everyone knows I'm a big fan of jellyfish, but there were too many even for me. The Australian story is seriously odd, though.'

'In what sense?'

'You find box jellies on the coastline, where the water's nice and shallow, not out to sea, and definitely not on the Barrier Reef- but they're saying they've been found there too. It's the opposite with Velella. They're an offshore species, and no one understands why they sometimes turn up on the beach.'

'I thought the beaches were protected by nets.'

Olsen roared with laughter. 'They're useless. The mesh stops the jellies, but the tentacles break off and carry on drifting. No one can see them.' He stopped. 'But why are you so interested? You must know a bit about jellyfish yourself.'

'Not nearly as much as you do. Is this a scientific anomaly?'

'You can pretty much bet on it,' said Olsen, balefully. 'Jellyfish distribution is linked to rising water temperatures and high levels of plankton. Plankton thrive in nice warm water, and jellyfish eat plankton, so you can guess what happens next. It's why they turn up in their hordes towards the end of summer and disappear a few weeks later. It's their natural cycle. Hang on a moment.'

Johanson heard shrieks in the background. He wondered what time the Olsen children went to bed – whenever he called, some kind of riot was going on. Olsen yelled at them to quieten down. Then he was back. 'Anyway, I reckon we get these invasions because the sea is being over-fertilised. Sewage encourages plankton levels to rise, and it all takes off from there. You only need a strong westerly or north-westerly, and the jellyfish are on your doorstep.'

'Yes, but those are normal jellyfish plagues. I want to know-'

'You want to know if it's an anomaly – and I think it probably is. But it's precisely the kind that's difficult to spot. Tell me, have you got any pot plants?'

'Yes.'

'A yucca?'

'Two.'

'There you go – an anomaly. The yucca isn't native.'

Johanson rolled his eyes. 'Don't tell me we're being threatened by a yucca invasion. Mine are fairly placid.'

'That's not what I meant. I'm saying that we've forgotten what's natural and what's not. Back in 2000 I was called out to the Gulf of Mexico to investigate a plague of jellies that was threatening the local fish stock. They invaded the spawning grounds in Louisiana, Mississippi and Alabama, devouring fish eggs and larvae, plus the plankton that the fish would normally eat. The damage was caused by an Australian jellyfish from the Pacific that shouldn't have been there at all.'

'An invasive species.'

'Exactly. The jellies were destroying the food chain and slashing the fish yield. It was catastrophic. A few years before that, the Black Sea was on the brink of an ecological disaster because during the eighties a cargo vessel had shipped in some comb jellies with its ballast water. They didn't belong there. The Black Sea countries kicked up a fuss, but before anyone could do anything about it, the region was screwed. Eight thousand jellies per square metre of sea. Do you know what that means?'

Olsen was talking himself into a fury.

'Then this other business. Portuguese men-of-war off the coast of Argentina. That's not their territory. Central America, Peru and Chile, maybe. But further south? Impossible. Fourteen deaths, just like that. A biological invasion. You can bet the locals weren't expecting it. And now box jellies on the Barrier Reef! It's as if someone had magicked them there.'

'What I find peculiar,' said Johanson, 'is that it's the two most venomous species.'

'Absolutely,' Olsen said slowly. 'But I hope you're not about to give me some kind of conspiracy theory. This is Norway, not America. There are plenty of possible explanations for the rise in jellyfish plagues. Some scientists say it's El Niño and others blame global warming. In Malibu the plagues are the worst they've had in years, and Tel Aviv's seeing some gigantic specimens. Global warming, invasive species – it all makes sense.'

Johanson wasn't listening. Olsen had said something that stuck in his mind.