For reasons you may not want to know but can possibly guess, I was just looking into mermaid biology on Google, figuring that if I had a question, other people had probably already done the heavy thinking. There's some cool stuff! Like this
anatomical illustration by Brazilian artist Walmor Corrêa (
nightdog_barks, you especially might enjoy the
companion illustrations), this
tongue-in-cheek anatomy lesson (responding to
this one), and a
blog post that includes a since-deleted Wikipedia article on "The Mermaid Problem."
My favorite part of that post was this bit of etymology:
A French idiom, finir en queue de poisson (to end with the tail of a fish), makes reference to this difficulty [of mating with a creature that doesn't appear to have genitalia]; it refers to a promising start that ends in disappointment. It originates from a line in Horace's Ars Poetica: Desinit in piscem mulier formosa superne (the beautiful woman ends in a fish's tail)
Another tangential find was this
musculoskeletal wetsuit.
Also, a pretty sexy Little Mermaid
femslash fic with inspiration from seahorse mating habits. Reminds me a little of the cloaca that frequently featured in Garak fic.