Vampires and Mermaids and Angels OH MY!

Random thought of the day: What's the deal with mermaids and angels all over the place now in YA fiction? Has anyone else noticed this silent take-over of the supernatural/paranormal/fantasy genre? As a juvenile and YA cataloger in my current position, I have noticed a TON of mermaid/siren and angel themed lit coming through. How do you think it will fare against the ever popular vampire/werewolf/magic themed YA books out there? Is Twilight a thing of the past? And how will this take-over effect YA books-turned-movies? Will we be seeing Robert Pattinson take on the role of merman or fallen angel on-screen?