Frazer thought for a moment. ‘I wonder what’s stopping them from going back out through the gap in the reef …’

‘The sharks don’t allow it,’ said Oti. ‘See, more of them are swimming in front of it.’

What had been a confused picture was becoming clearer to Amazon. The mother and baby would make sudden darting runs towards the gap, but the sharks would not let them pass. They seemed wary of the mother, and made their lunging attacks at the little one. The mother would butt at any that came too close, but she could not force a way through to the open ocean and safety.

‘Can you get the canoe over there, Oti? Maybe the sharks will clear off if we sit over the gap.’

The Polynesian boy looked doubtful. ‘This is not wise. There is some danger.’

‘Oh, please,’ Amazon implored. ‘I can’t bear it if that little dolphin …’

Oti shrugged. ‘OK. We can try.’

There was still no wind for the canoe’s small sail, so Oti and Mahina used the bamboo poles to punt the canoe along.

When they reached the gap, the sharks, far from being scared off, seemed to become more excited, and gathered round this new intruder, giving Amazon her first good look at them. Some were just a little longer than her outstretched arms; others were a couple of metres from tail to nose.

Amazon shuddered. She knew that it was silly to regard any animal as evil: she understood perfectly well that all creatures were engaged in the same struggle to survive, to get enough to eat and to reproduce. But there was just something unfathomably wicked about the sharp, pointed noses, the staring eyes with their black, slit-like pupils, the mouth, half hidden, but full of pitiless teeth.

‘We’re lucky,’ said Frazer. ‘These are just reef sharks, by the look of them.’

‘So they don’t eat people, then?’

‘Nah. They might bite your foot off if you dangled it in front of them, but that’s about it.’

‘I’d quite like to keep my feet, actually,’ said Amazon, pulling her toes in from the edge of the canoe.

‘If there was a tiger shark here,’ Frazer continued, ‘things would be a lot less pleasant. Hey, I’ve had an idea. Maybe we can use the poles to drive them away from the gap, so the dolphins can escape. What do you think, Oti?’

‘We can try,’ said the Polynesian boy, without much confidence. ‘But we must have care. This small canoe is not good out in the big sea.’

The gap in the coral was about four metres wide. Beyond the shelter of the reef the water was choppier, and now that the canoe was in the gap it rose and fell with the waves.

Mahina gave Frazer her pole, and he and Oti jabbed them down at the sharks. It was almost impossible to hit the creatures. The sharks slipped elegantly aside from the thrusts. Oti was used to spear fishing from the canoe, and easily kept his balance, but Frazer had more trouble. Once he jabbed his pole with such force that he almost fell in among the angry sharks. He instinctively grabbed hold of Amazon, and would have dragged her into the water with him if it had not been for the quickness and agility of little Mahina, who steadied them both.

However, despite their difficulties, they had some success in at least irritating the sharks enough for them to move away.

‘Come on now!’ Amazon cried out to the dolphins, who had been keeping well clear of the action. ‘Time to make a run for it.’

The dolphin mother seemed to be paying attention. When the last of the bigger sharks swam away, she made another of her rushes for the open sea, followed closely by the calf.

And this time it looked like they were going to make it. The sharks realized what was happening and raced back to try to catch the dolphins before they escaped, but they were too late. In a straight race, the mammals were much too quick. They were halfway through the reef opening, and Amazon had already begun to cheer.

But then something huge and ominous loomed from the seaward side of the reef. This was no foot-chewing shark. It was longer than the canoe and dwarfed the mother dolphin.

It was a tiger shark, the most feared killer in these seas. Too big to risk swimming into the lagoon at low tide, it had, with the ancient cunning of its kind, been waiting patiently outside.

And now its time had come.

It was certainly powerful enough to tackle a fully-grown bottlenose dolphin, but very few hunters will take the parent when the helpless child is up for grabs. And so the shark lunged with deadly intent towards the baby, its wide mouth gaping obscenely.

Superb swimmer though she was, the mother was moving too quickly to turn in time to protect her infant. Trapped between the wall of the reef and the jaws of the predator, the calf had nowhere left to swim. Or so Amazon thought, and her joyful shout turned into a cry of horror.

But all was not yet up for the infant dolphin. A couple of strong beats from its tail sent the little creature up and out of the water. The tiger shark’s jaws closed on nothing but the surging wake.

Amazon and Frazer both gasped as the dolphin flew through the air, and landed with a splash on the shallow water – no deeper than the width of a hand – washing over the reef itself.

And there the calf was stuck. It flapped and flopped, but the sharp coral cut into its delicate skin, and soon it lay still.

The shark sensed that its victim was close. It tried to surge up on to the reef, showing Amazon the alternating light and dark stripes on its enormous back. But the calf was too far away, and the shark couldn’t reach it. For a moment it seemed that the predator would be stranded on top of the reef as well. But the shark managed to thrash itself down from the reef, and rolled away, beating its long tail.

And now the calf was making a desperate bleating sound, calling out hopelessly to its mother. The mother dolphin circled round, and made her own clicks and clacks in response. But she could not come too close as the tiger was back on patrol.

And Amazon was sure she could see a new spitefulness in the great predator’s look, as if it had been humiliated by the youngster’s partial escape, and was now intent not so much on a meal as revenge.

The canoe had come to rest right up against the reef. The dolphin calf was tantalizingly close, but out of reach. Amazon could not stand the plaintive calling of the baby. She had to do something. She knew that it was stupid, but she could not leave the little creature there to cut itself to ribbons on the reef or to boil and blister in the hot sun. But her brain was still working. She knew that if she tried to walk on the reef, her feet would be cut to shreds on the sharp coral.

‘I’m saving that dolphin,’ she said, and before the others could stop her, she picked up the useless sail from the bottom of the canoe and threw it on top of the reef. Then – accompanied by a cry of dismay from Frazer – she stepped from the flimsy canoe on to the sail. She felt her feet sink through the woven palm leaves and press on the coral, but the sail gave her just enough protection. In three quick steps she had reached the little calf.

It was so beautiful, so helpless. She knelt by its side and put her hand on its nose. It looked back at her and seemed calmed, as if it knew that she meant it no harm. Amazon would happily have spent an hour there, just gazing at this wondrous creature, but she knew that she had to work fast. Already the dolphin’s skin was cut and torn, and she could almost feel the agony of the hot sun on its back. She worked her hands and then arms under its body, and lifted it up from the reef. It was heavier than she’d imagined, and she almost slipped and fell.

But Amazon was strong and, even more than that, determined. Staggering under the weight, she made it to the edge of the reef. The mother came in close, followed not far behind by the tiger. It was now or never. Amazon hurled the baby into the sea, clear of the reef, hoping it would know to swim away with all its might.

But the calf was still dazed and confused, and it hesitated for a fatal few seconds. The tiger was on it again. But this time the mother was ready. She propelled herself like a torpedo at the shark, ramming it with a crunching force that propelled the giant fish sideways against the reef.

And then mother and baby were away, flying joyfully through the gentle waves, and no shark was ever going to catch them.

‘Take that, you monster,’ yelled Amazon, pumping her fist in the air.

It was a mistake.

The woven leaves that made up the sail were slippery, and coming apart in the thin layer of water that washed over the coral. Amazon stumbled backwards, half-righted herself and then overcompensated, and fell headlong into the sea, outside the reef.

She was only a few metres away from the tiger shark.

Only one thing saved Amazon: the shark was still stunned by the force of the blow from the dolphin. But already it was recovering its poise.

Amazon hit the surface, spluttering, and began to swim frantically back to the gap in the reef. At the same moment Frazer and Oti, seeing what had happened, pushed the canoe urgently through the gap and towards her.

Amazon felt something warm against her foot: she was bleeding, cut by the sharp coral as she fell. And it didn’t take a shark expert to know what happened when you mixed together blood and water and sharks.

Panicking may not have been a good idea, but, unfortunately, panicking is one of those things you don’t have much control over, and so Amazon beat at the water like a five-year-old thrown into the pool for the first time.

She tried to scream, but that just meant that she swallowed a bellyful of seawater. And somewhere close, she knew, the tiger shark was getting ready to attack. She’d read somewhere that, when a shark bites your leg off, you don’t feel it as a pain, but more as a pulling sensation, and victims are sometimes astonished when they look down and see nothing there but a bleeding stump. She could almost feel the tugging now, and dreaded to look, in case …

‘Amazon! Grab my hand!’

The canoe … it was here, so close. Frazer leaned out as far as he could, while Oti held his other arm.

It seemed to Amazon that she wasn’t moving at all. She had no breath; her eyes were burning from the salt. But somehow the canoe and Amazon reached each other, and she clutched Frazer’s hand. He hauled her up, and held her tightly as she coughed and sobbed. And when she looked down she was delighted to see two legs, completely and beautifully attached to the rest of her!

‘I thought dolphins were supposed to save drowning sailors from sharks,’ she gasped, and then she and Frazer laughed as the two Polynesian children looked on, mystified.

But then a bump alerted them to the fact that they were anything but safe. They were in a leaky canoe, afloat on the open sea without a sail and there was an angry tiger shark paying them very close attention.

Frazer’s clear grey eyes clouded with concern. ‘Now how the heck do we get out of this one?’