Stunning book slightly marred by a series-teasing ending

There was a moment in the middle of this book (which I read in a night) when I was HOOKED. I was all in, and I loved the world, the building horror, and the characters. I loved how the title seemed to play around with the biological threat the girls in this school/island were facing, how they were growing wilder internally and externally, and how that wildness could be interpreted as a bucking of the heteropatriarchal system that was trying to control them (even as the wildness itself was destroying them). I LOVED it. Then, that ending happened...it knocked a 5 star book to a 3.5, for me. I didn't realize (slight spoilers) that this would probably be a series--I thought it was a stand alone. And this is by no means an ending for a stand alone novel. The stakes I thought had been established fell through, postponed for a book 2 or 3. However, that doesn't fully take away from the rest of the reading experience, or the fact that this book has some bizarrely memorable horror scenes (some are a bit OTT for my taste, but at least those were somewhat funny, though I'm not sure they were meant to be...). That cover is freaking gorgeous, too.