Adrian awoke during the night. Faint light shone in his bedroom through the blinds from the moon and the dock lights. He marveled at the beautiful mermaid slumbering in bed next to him. He ran a hand along the faint blue scales of her tail. The medallion shimmered ever so slightly, resting on her chest. Marina had declined to remove it earlier, but Adrian still wanted to know whether that would change her back to human form. His curiosity got the best of him. He took the medallion in his hand and slowly, carefully lifted it over Marina's head.
The scales began to fade and Marina's mermaid tail seemed to slowly melt back into her legs. Adrian had been impressed by Marina's firm swimmer's legs the first time he'd laid eyes on her. He ran his hand along her inner thigh, feeling her warm, smooth skin. The young woman remained fast asleep. Adrian began to feel aroused by her naked human body.
Something more occupied Adrian's thoughts. What if he put the medallion on? He wasn't a woman, wasn't in the water, and wasn't wearing a wetsuit - all things that could have factored into Marina's transformation. Adrian's curiosity got the better of him and he put the medallion around his neck.
Adrian lay in bed waiting for something to happen. Nothing seemed to happen at first, but after a few minutes his legs began to feel strange. He realized he couldn't separate them. He felt his feel turn outward as they began to form a tailfin. Soon his legs - no, his tail - began to spasm. Adrian tried to lay still, but couldn't. His tail begin to thrash uncontrollably, knocking the bedsheet away, smacking into Marina.
Marina jolted awake. "What's going on?" In the dim light she could see Adrian next to her, his legs kicking in unison into her legs... Her legs? The medallion! She stumbled off the bed and fumbled for the light switch.
Adrian blinked at the sudden brightness that revealed what had happened. He had turned into mer with a smooth, gray, scaleless tail. A merman as large gray erect penis attested.
"You took the medallion!" Marina exclaimed.
"I couldn't help myself" pleaded Adrian. "I wanted to see if your change was permanent. And then I wanted to see what would happen if I put it on."
Marina walked back to the bed. "It looks like it turned you into a dolphin merman. I wonder why?"
"Maybe because I'm horny? Dolphins are frisky animals."
Marina wrapped her hand around Adrian's engorged member. "I heard dolphins were bigger relative to their body mass but...wow..." She stroked Adrian, causing him to shudder with pleasure.
"It's really sensitive."
Marina climbed onto the bed, straddling Adrian's tail. She took his penis and rubbed it against her lower lips until she felt herself grow wet. Then she rolled over on her back next to Adrian and spread her legs. Adrian rolled over and crawled between them. They kissed as Marina guided Adrian into her. He pushed slowly and carefully, but she still gasped.
"It's so big!" she exclaimed then scissored her legs behind his back. Marina thought sex as a mermaid was remarkable because of her tail's extra sensitivity, but Adrian's dolphin dick filled her so well. They thrust on to a climax.