We 'occasionally' have fish that eat them, the odd snapper and lemons
The balance is all out of whack though. There are far more crabs than there are fish to eat them (due to over-harvesting of fish)
We do (or at least I do) use the crabs for bait as well, but crabs will quite happily eat their mates, so floats are still required even when presenting crab baits. And if you're having to use a float, it may as well be a bright blingy one right?