Sea Potato Shell

One of my grandchildren found this perfect shell of a sea potato on the beach yesterday. The sea potato is a type of sea urchin that lives buried in the sand sea floor. When alive it is covered in spines that trap air and prevent asphyxiation. During life it is a fawn colour but the shells found on the shoreline are white and have lost their spines. The patterning on them is beautiful. They are incredibly fragile.