Poison dart frog evolved

I think that if the water levels were to sink the amazon rainforest. Poison dart frogs would evolve into crabs.Because it would need to adapt to this new underwater sea environment. And what better animal to evolve into then a crab many other species have evolved into crabs. So i think its possible for poison dart frogs to evolve into crab. Its lungs would turn into gills. It would look like a frog but it will have crab legs and claws with crab teeth in its mouth. It would have to get bigger so that instead of eating fruit flies it would eat small fish like tuna. So thats why frogs would evolve into crabs in a underwater environment