Mary made a gesture, and Eva began to change. First her clothes disappeared. Then she was compelled to sit down; Mary didn't want her falling over during the next change, in which her legs fused together into one mass of flesh, her feet flattening out and joining into a tail fin. Finally, Eva's new tail became covered in silver scales, matching her dark brown hair perfectly.
Realizing she was naked, Eva let out a soft "eep!" and covered her breasts. "Couldn't you have made me a merman?" she moaned.
"Oh, take it easy!" Mary laughed. Nobody here but us girls, after all. And no, you don't get your manhood back until I do. Now go check out the bottom!"