The best of the options laid in front of her, Carrie chose the shell knowing very well what she would become.
As soon as she made her choice, her skin became to crack. Skin became segmented, and then hard. They gained a shiny quality and changed from skin colored to a more translucent blue-white. The scales over her entire body, which was a surprise. Carrie assumed she'd just end up with a tail.
Her breasts quickly grew, becoming quite heavy, and retaining a substantial amount of fat. Along her neck three sharp cuts deepened into gills, and then two more on either side of her ribs. Webbing appeared between Carrie's fingers, and then her toes. Which in turn flattened, and started to flop as she lifted them to get a better look.
Suddenly, her legs were snapped together, and Carrie fell to the floor. Her legs fused into a long, spiny fish tail. Two pelvic fins spring forth, the fins a darker shade of blue than the rest of her body. Her back aches, and she rolls over onto her stomach just in time to watch a spiny dorsal fin erupt from her spine. Long, poisonous spikes pierce through her skin/scales.
Carrie's eyes turn completely black, and her teeth gain rows of sharp teeth like a shark. Finally, the hair on her head falls out, leaving her with a slimy, scaled head. Her entire body has a layer of slime that keeps her moist. Carrie props herself up on her pelvic fins to look down at herself. Her pussy has disappeared into a warm pocket behind two large scales. Her ears are nothing more than two small holes on the sides of her head.
"Huh, not so bad." Carrie thinks. She runs a scaly hand over her body, electricity rushing through her ever nerve as she does. Every touch is so sensitive, and so erotic. Carrie doesn't hesitate to plunge her new hands into the pocket in her tail. The gaping hole opens, and swallows her entire hand.
It's not until Carrie realizes she's still in full few of her friends that she stops. But the "damage" has been done, and cum still leaks from her slit. It's not like normal cum. It's much thicker, and almost clear. It oozes onto the floor, and forms a puddle under the new mermaid.