October Flash - Day 4

The Flash Fiction Project at Google+ has something new going on! There will be a visual prompt for every day in October, and I'm going to try to write a (very) short piece for it every day as well. Due to time zone differences, I'll try writing and posting mine early in the morning, so it's a day after the original post for me, but still the same for them.

Let the fun begin! Here is the prompt for today:

Today was the day, he knew it. Today, they would catch her, or, well, he would. For the other two, it was all just a game. Huh, games! What's a game worth compared to the hunt? Nothing, he thought. The hunt is the real thing, while the game is just that kind of bad joke you always got these days.
They walked down the beach, always level with the salty water, until they reached the spot. There he hid behind the stones concealing their meeting place from the open, and waited.

She came in a wave of water green with algae. Her smile was wide and excited, and not entirely human. Strands of wet brown hair covered her otherwise naked shoulders and breasts.
She ran aground and propped herself up on her forearms. He couldn't hear what she said, but her laughter rang out loud and clear to his hiding place. Her fin splashed happily in the shallow water. It was time to strike.
He moved quietly behind the stones until his feet got wet. Level with her, he jumped up on the stones, down into the ocean and with two quick strides, he was on top of her. She screamed.
Pongo, yowling with rage, tried to bite his leg, but he held on to his prey. She wriggled under him until they lay facing each other. Her eyes burned their way into his skull, and the remotest part of him felt his grasp on reality slip. Reality? He was holding a mermaid, for god's sake! Reality could go and bother someone else.
Suddenly, he was kissing her.
Pain erupted in the lower part of his spine, spreading out downwards, paralysing him. She pushed him away, into the water. The part of his brain not concerned with the pain wondered briefly why she stayed and watched, instead of fleeing.
Then the pain stopped. Pongo, having retreated to dry land, started howling a lament.
He looked down at the legs he could not feel any more. They had been replaced by a shiny, grey fin. He turned and stared, lost for words, into her smiling face.
"If you want me, you have to be like me," she explained, before turning to his dog. "If you want, you can come, too."
Pongo ducked, his tail beating the ground in confusion.