Jake could only partly see Sally's naked body in the dark. She dropped to her knees, straddling Jake and grinding against him. When she leaned forward to kiss him, Jake stroked Sally's dorsal fin causing her body to shudder. When went for Jake's pants and his belt was soon unfastened, his zipper down, his cock out. Sally stroked it then slid it into her.
After only a few thrusts from riding Jake cowgirl style, Sally could tell something was wrong. She wanted to fuck Jake so badly, but she needed something more. She took a deep breath, inhaling the salt air and listening to the gentle surf breaking nearby...
That was it. She leaned in again, kissing even more passionately and deeply than before.
"We need to do this in the water" Sally said. The slid off Jake, rose to her feet, and slowly walked toward the ocean.
Jake fumbled with his shirt and pants, trying to get out of his clothes as quickly as possible then hurried after Sally. She waded into the water until it was about waist deep then turned to watch Jake following her in. They embraced, kissing hungrily, bodies pressing together. She lay back until she was floating on her back, her legs spread wide and inviting. The salt water felt so good. Jake took Sally by the waist and slid into her.
As he did, Sally started to feel something else barely perceivable through the euphoria, a sensation at the base of her spine. Sally ignored it and wrapped her legs around Jake and they slowly devolved a rhythm. The mysterious sensation continued, but it felt good. Sally was in heaven, muttering "Yes...oh yes..." between soft moans.
Jake felt something tickling his balls, like a little fish swimming against them. It felt strange but kind of arousing.
Two things were building at once within Sally. The first was an orgasm, but the second was as if with each thrust her tailbone was straining to break free. The fish rubbing against Jake's balls was Sally's tail starting to emerge. When her orgasm started, the tail burst forth in response. The sensation against his balls and between his legs caused Jake to erupt deep in Sally, triggering more orgasms. She let out an animalistic scream of pleasure and locked her legs tight to ensure Jake could not pull out before he'd finished mating with her.
Yes, mating, thought Sally. Like two animals.
Jake finally finished and the orgasms did too. Sally released her legs scissors and Jake backed up not entirely understanding what had happened. Then tripped over Sally's tail and fell backwards. He got up sputtering as Sally rolled over, her dorsal fin and vertical tailfin jutting from the water and glistening in the moonlight. Jake cautiously approached and ran his hand down Sally's back then down the shark tail jutting out from between her butt cheeks.
"That feels good..." said Sally dreamily.
"Don't realize what that is?" asked Jake.
It took Sally a moment to realize her shark transformation had gone further than just a dorsal fin. She knew she should feel scared or at least worried about the consequences or the possibility of further transformation, but in the post-orgasmic haze she felt good.