Okay, so it’s actually just one actress and one mermaid role, but it’s a biggie.  The thesp in question is Astrid Bergès-Frisbey and the role is for Pirates of the Caribbean: On Stranger Tides.  Other than the part about having a tail, no other details were given about the role. 

Bergès-Frisbey has appeared in such French films as La Fille du
, Bruc, Extase, La Premiere Etoile and Un Barrage
Contre le Pacifique
as well as the telefilms La Reine Morte, Elles
et Moi
, Divine Emilie and Sur le Fil.  She has also appeared
onstage at the Theatre Marigny in a production of Peter Shaffer’s Equus
On Stranger Tides begins shooting in Hawaii in June for a May 20, 2011 release.

via THR.com