Mermaid Line is a collection of stories that ran in Yuri Hime magazine. They are collected together as short mini-series, done in an episodic, almost soap-opera-y way.
In "Megumi and Aoi " Megumi confesses to her friend Aoi that, despite her ungainly swimming, she feels as if she is a mermaid. Aoi finds herself fantasizing about being the prince to Megumi's mermaid, but Megumi runs to the arms of a boy, trying to put a beard on her feelings.
Ayumi, in "Ayumi and Aika," wants to get married to her boyfriend so naturally, she's shocked when he tells her that he wants to become a woman.
"Yukari and Mayuko" are both OLs. Mayuko's between boyfriends and bored, so she asks Yukari to pretend to be her girlfriend. It works out well, until Yukari realizes that her feelings for Mayuko are more than just pretend.
And finally, in "Miura-san and Me" Okabe loves Miura's beautiful hair. When Miura-san cuts it all off, Okabe learns to love her for herself.