There was a chink at the edge of the outcrop. Anawak hovered in front of it. Then he reached down to his ankle, where his knife was in its sheath. He pulled it out and plunged it into the shells.

The outer crust split open and something shot out towards him, twitching frantically. It collided with his face, almost pulling off his breathing apparatus. Anawak jerked backwards. His head hit the bottom of the boat. A harsh light exploded in his eyes. He wanted to get out of the water straight away, but the keel was above him. Kicking desperately, he swivelled round and was confronted by another mound of shells. Their edges seemed to be stuck to the hull with a jelly-like substance. Forcing himself to calm down, he set about hunting among the floating particles for traces of the thing that had attacked him.

It was gone. All he could see were strange formations of mussels.

Suddenly he realised he was clutching something. It was his knife. A scrap of something dangled from the blade – a blob of milky-coloured, semi-transparent material. Anawak stowed it in the collection jar with the tissue. He couldn't wait to get out of there. His heart was pounding, so he ascended slowly, with small, controlled movements, following the hull upwards until the lights of the two divers appeared in the distance. He headed towards them. They'd found the mussels too. One was using his knife to prise individual shells away from the crust. Anawak tensed, steeling himself for something to hurtle towards them, but nothing happened.

The second diver motioned upwards with his thumb and they ascended slowly to the surface. Gradually the light grew stronger, but the water remained murky.

At last Anawak found himself blinking in the sunshine. He pulled off his mask, and breathed in gratefully.

Roberts and the others were waiting on the jetty.

'So, what does it look like down there?' Roberts asked. 'Did you find anything?'

Anawak coughed and spat out a mouthful of harbour water. 'You could say that.'

THEY WERE STANDING AROUND the tailgate of the truck. Anawak had been nominated as spokesman.

'The rudder was blocked with mussels?' asked Roberts, incredulously.

'That's right – zebra mussels by the look of them.'

'How the hell did that happen?'

'Good question.' Anawak got out his collection jar, opened it and carefully emptied the blobs of jelly into a larger container filled with seawater. He was anxious about the tissue: it looked as though decomposition had already set in. 'There's no way of knowing, of course, but I'd picture it like this. First, the helmsman tries to apply five degrees rudder. The rudder doesn't move. As it turns out, it's blocked by countless mussels that have settled all over the shaft. Now, you guys know more about boats than I do, but a rudder is pretty easy to disable – although in practice it rarely happens. Consequently it never occurs to the helmsman that the rudder might be blocked. He still thinks he hasn't shifted it far enough, so he tries to shift it further. Again, nothing seems to happen. Then the helmsman goes all out, and the rudder breaks free. As it swings across the shaft, it crushes the mussels in its path, but they don't fall off. A paste of ground molluscs clogs the rudder. The blade is wedged tight and can't move back across the shaft.' Anawak pushed strands of wet hair out of his eyes. 'But that's not what really bothers me.'

'What then?'

'The sea-chests are clear of mussels, but the propeller is covered with them. It's completely infested. I don't know how they managed to latch on to the boat, but one thing is certain: a rotating propeller would be too big a challenge for even the most determined mussels. Either the molluscs climbed aboard in Japan – which seems unlikely, since the rudder was in fine working order right up to the two-hundred-mile zone, or they clung to the propeller when the engine cut out.'

'The ship was invaded by mussels in the middle of the ocean?'

'Right, although "appropriated" might he a better term. I'm trying to picture how it happened. A gigantic swarm of mussels settles on the rudder. When the rudder jams, the ship heels. Within minutes the engines are turned off. The propeller stops rotating. More and more mussels descend on the rudder, reinforcing the blockade. In no time they extend across the propeller and along the hull.'

'But the ship was out to sea,' said Roberts, confused. 'Where would tonnes of adult mussels come from?'

'Why would whales scare off tugs and jump on a tow line? You're the one who started telling stories, not me.'

'I know, but. . .' Roberts bit his lip. 'It all happened simultaneously. It almost makes you think there's a link. But it doesn't make sense. I mean, whales and mussels?'

Anawak hesitated. 'When was the last time you inspected the Barrier Queen's keel?'

'There are constant inspections. Besides, she's coated with a special paint. Before you ask it's environmentally friendly. But there aren't many things that can latch on to it. At most, a few barnacles.'

'You've got more than a few barnacles down there.' Anawak stared at Roberts. 'But that's just it. . . By all rights, they shouldn't be there. The Barrier Queen looks as though she's been exposed for weeks to hordes of mussel larvae. And in any case… there was something else down there…' Anawak described how something had shot out towards him from inside the crust of mussels. While he was talking, it all came back to him. First the shock, then hitting his head on the boat – he had seen stars.

No, not stars – flashes of light.

A single flash of light.

Then it struck him: the creature had flashed.

For a moment he was speechless. The flow of words dried up as it dawned on him that the creature had luminesced. But if it had luminesced, it must have come from the depths. It could scarcely have found its way on to the Barrier Queen while the ship was in dock. It must have latched on at the same time as the mussels. Maybe it had been drawn there by them. Perhaps they were a food source. Or a shield. The creature could have been a squid…

'Dr Anawak?'

He stopped staring into space and turned to Roberts. Yes, he thought.

It must have been a squid. It had been too quick for a jellyfish and too strong. Like a single elastic muscle, it had burst through the shells. Then he remembered something else: the creature had appeared as soon as he reached into the chink. He must have cut it with his knife. Had he hurt it? Either way the thrust of the knife had triggered a reflex.

No need to get carried away, he told himself. It was too murky to see down there.

'I recommend you have the dock checked over,' he said to Roberts, 'but first you need to send these samples' – he pointed to the sealed containers – 'to the laboratory in Nanaimo. Have them taken by helicopter. I'll come too – I know exactly who should look at them.'

Roberts drew Anawak aside.

'Leon, what do you really make of all this?' he asked quietly. 'There's no way that tonnes of mussels could have accumulated in such a short time. It's not as though the ship had been neglected for weeks.'

'Those mussels are a pest, Mr. Roberts.'

'Call me Clive.'

'Well… Clive… zebra mussels don't show up in small groups. When they find somewhere new to settle, they march in like an army. That much is known.'

'But not as fast as that, surely?'

'Every single one of those damned things can produce a thousand young every year. The larvae drift with the currents or stow away on the fins of fish or feathers of birds. In some lakes in America there are nine hundred thousand of them in a single square metre. And they appeared there overnight. They colonise waterworks and irrigation plants, and get into the cooling systems of factories built near rivers. Entire pipes are blocked and ruined by them. And from what we see here, salt water suits them just as well as fresh.'