My bettas ate my nerite!!
Ok, I'm not 100% sure, but I'm about 90% sure.
My female bettas turned out to be excellent snailers- I had a snail overpopulation and then when I added my girls to my tank, I was seeing empty shells and my pond snail numbers were going down. Now I don't know if I have any pond snails, and the only ramshorn snails I have left are the bigger ones.
BUT I hadn't seen my nerite in about a week, and then yesterday I found its shell upside down and empty! I definitely didn't expect them to eat a nerite because they're so big- a lot bigger than the ramshorns! But if the snail had just died, wouldn't its body still be in the shell? Unless it died and the bettas realized it and started eating it, I dunno... This is very weird and surprising.