I've been reading up on this cause I want to convert one of my big tanks to a shark and ray tank. If you're looking for something to keep the sand clean and turned over and not get eaten. Sand Sifting Star is one thing i came across. Another things is people have had success keeping inverts in the tanks as long as they stuff the ray or shark with plenty of mixed fleshy foods(clams, scallops, squid fish, etc.) If you give her something better to eat and than scaveging for your crabs or snails she should leave them alone. Should being the key world. Sand Sifting stars are a safe bet though with rays. They are under the sand and also not a normal diet for them.