Crustaceans are not the same thing as anthropods and their shells generally do not contain the same chitins. Isopods for instance, are land dwelling crustaceans the exoskeleton of which consists mostly of calcium. I have seen video of a Horned Fropg eating a crayfish and I have heard of them being fed as part of a varied diet to Horned frogs, Budgetts Frogs and Pyxie Frogs alike without any mention of a problem.