From “A Crowning Success for Crayfish” (ScienceDaily, May 18, 2012), we learn,
A team of scientists from the Max Planck Institute of Colloids and Interfaces in Potsdam and the Ben-Gurion University at Beer-Sheva in Israel has now found that the teeth of the Australian freshwater crayfish Cherax quadricarinatus are covered with an enamel amazingly similar to that of vertebrates. Both materials consist of calcium phosphate and are also very alike in terms of their microstructure. This extremely hard substance has apparently developed in freshwater crayfish independently from vertebrates, as it makes the teeth particularly strong.
Note this also:
In human teeth, both the hard enamel on the crown and the underlying softer dentin are composed of calcium phosphate crystals. “Freshwater crayfish, on the other hand, are very economical with calcium phosphate, the production of which is for them metabolically much more costly than the construction of the rest of the cuticle,” says Barbara Aichmayer. Freshwater crayfish renew their cuticle again and again as they grow. They dissolve the amorphous calcium carbonate out of the chitin tissue and partially store it in order to reuse it in the new shell. This is not possible with the crystalline calcium phosphate on their mandibles. They shed their costly enamel with the rest of the cuticle and build it completely anew. So in one respect, freshwater crayfish are way ahead of us. They renew their teeth at a low metabolic cost again and again, whereas our enamel, despite its hardness, gradually wears out and cannot be replaced.