I have a pair motoro and they just ate the live mussels and apple snails i added to the tank to control excess food. seems that they became food for the rays. it's not really a pest for my tank and it cost a bit to buy these shell creatures. The shells seems to be intact and they just suck out the internals. I have seen them catch one attached to the side of the tank and forcing the internals out. Hope this solve your mystery.
