| Those are protein spots...the mantle has the capacity to secrete the protein conchiolin (the dark spots), calcite and nacre (aragonite)...if the oyster's hormones are acting up or if there is an infection or another "problem" (pollution, environmental change, etc.) then you will obtain those spots.
They usually crack because they easily dehydrate. Think of the shell's protein exterior...it flakes and falls from the shell when dehydrated.
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Douglas McLaurin, M.Sc. Aquaculture
Perlas del Mar de Cortez
Guaymas, Sonora, Mexico
perlas.com.mx
The Pearl is a Harsh Mistress...and I am its Humble Servant |