This is nothing to worry about. Atleast one of my turtles has this. This is caused by the claws of the male scraping the back of the female turtle's shell. This can also be seen in males because males will mount one another in the same manner in order to fight. In wild caught turtles, the tools used to grasp turtles to pull them from holes also cause this condition. To my understanding, it is nothing to worry about. I read an article about the matter on the net once, and with a little research you can probably find it. Good luck.

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