As they swam, following after the unknown scent, Baxter felt a confusing jumble of thoughts in his mind. First was that he was being challenged, somehow, by the scent, and he felt worried, a nagging feeling at the back of his mind that he might not be able to stand up to the challenger. Also drifting through his brain was a tidbit he'd read in abiology textbook some time ago, about a species of fish that could change sex: if a school was left without a male for some reason, one of the females could become a fully functional male. But why was that entering his mind right now?
Baxter and the mermaid made it over the reef the scent was coming from. It was even stronger here, and it was coming from...a little machine on the bottom of the tank. And suddenly, Baxter pegged the scent. It challenged him, but at the same time turned on his female tank-mate (who had begun to tenatively touch her new breasts during the swim and was now in all-out Body Exploration Mode.) It was the scent of an alpha male.
Somehow, Baxter knew that he was not an alpha, but a beta; it had something to do with the testosterone levels, he thought. And with this realization came a terrible sinking feeling that he couldn't quite place...
His hair was growing longer. Growing out, haloing around his head in the water, until it was waist-length (or would have been if it wasn't floating,) the same silver-blue as his scales. His skin was smoother and creamier; his arms and hands had become slender and delicate. His face felt different, too; softer, more refined. It slowly dawned on him what was happening.
"Help!" he yelled to his tank-mate. His voice cracked in the middle of the word. "Help!" he cried in his new soft soprano, "It's turning me into a girl!"
She turned to him, not even stopping the soft gasps and moans she was emitting in the course of her explorations."So what?" she asked. "This...oh! Th-this is great...you'll...ah! love it! N-nothing wrong with being an oh my god...woman!"
Baxter's waist narrowed, and his shoulders rounded and narrowed as his hips (or whatever they were now) expanded. Why is this happening? he agonized. There was a tingling and a pressure, and he clutched his chest with both hands, only to find that flesh was slowly gathering under his nipples, which were becoming larger and more sensitive. It's happening because it has to happen, he realized. Slowly, the mounds of flesh under his nipples were taking on a familiar shape. In other species, power conflicts between males result in the injury or death of one or both parties. The growth stopped, and he was left with a modest pair of female breasts, somewhere around a small C-cup. Realizing that he was now feeling himself up, he quickly go of them. They floated there, buoyant in the water thanks to the fact that they were mostly fat.
Suddenly, there was a tingle down below, and although he could see only a flap of skin covering a slit in the front of his tail, Baxter could feel his penis slowly shrinking inside the flap. This way power conflicts are settled decisively, without injury to either party. The loser lacks the ability to re-challenge for breeding rights. His penis inverted itself, becoming her vagina; the testicles withdrew inside her body, beginning the transformation into ovaries. And not only does the loser lack the ability to challenge for breeding rights, but also the reason to. There is no alpha-beta division among the females; each is equally likely to be impregnated. Beta males actually have a much higher chance of passing on their genes if they become female. The back of her vagina expanded into her new womb, her ovaries taking their new place inside her.
It was finished. Baxter was no longer a man. Baxter was no longer a merman. Baxter was now a complete, fully-functional mermaid.