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I think the little sand balls are their fortress perhaps or else they are just OCD lol? It's weird because once they have finished digging their tunnel home, it's like they can't quit making balls. You can see that they scrap the sand just to make more balls, but they always leave paths so they can run and hide. High tide comes in and destroys most of their balls, so they simply get at it and make more balls and tunnel homes :)

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