Jake grimaced at the taste of the witch's concoction. He twisted his head away but it was too late. He had already swallowed most of it./ And that was enough. The witch just grinned. It had the taste of seawater. Too salty and full of things not fit for a decent human drink. But spitting it was useless. It was in Jake's belly and he barely had the time to demand what it would do top him that already he could feel the chances affecting his body. The witch answered him not, at any rate.
First, he felt sharp pains at his neck, which he clutched as he screamed in agony. It was as if a knife had sliced him repeatedly on the sides of his neck. Slits appeared, painful at the touch. Gills, although Jake did not know it yet. He was so focused that he did not notice his cloth rip and become rags until it was too late, indeed leaving him naked. His chest felt heavy and he coughed with much difficulty and pain, which heralded the renewed pressure on Jake's legs. The poor boy was on the ground as his legs could not support him any longer. Powerless to halt it, he watched in horror as his legs forcefully fused together and molded into one piece of ugly flesh. He even felt his leg bones fuse and become one. From his waist down, he had the tail of a worm, a clumsy, useless tube of flesh with puny bones. He could sense his veins and arteries vanish and rearrange and branch out. His anus remained placed where it should but his penis and testicles retracted into his own horrid tube of flesh. He could feel them but they were gone. There not for pleasure but pure reproduction, to emerge just to fertilize eggs. The opening was simply to let his penis show itself when aroused. Urine would just drip-drop wherever he was. Ugh! How vile! He could only drag himself with his hands. Hands which he saw were now webbed. Even his chest and arm hair were utterly gone! As for what were his legs, the skin was smooth. But not for long. From his hips, scales, first little bumps but then free to show their rich blue and transparent hues , spread downward, ending in two fins and letting more erupt from Jake's former legs two. Odd, but not to painful. And then the young man understood. A mermaid. He'd become a mermaid. Well, merman. His tail still flopped uselessly on the floor of his cage. Water. He'd be much more at ease in water, even if he traded a cage for a fishing bowl. The witch, for one, cackled.
"Well, it appears that I won't have to take a trip to the sea for fish scales now!"