Diggers and shapers, unite! The Clamshell Foundation’s sand castle contest starts at 9 a.m. on Saturday at Atlantic Avenue Beach in Amagansett. There will be five categories of competition: Sand Fleas, for children up to 8, Sand Hoppers, for 9 to 15-year-olds, Sand Tribes, for families, Sand Shapers, for adults, and Sand Pros, for veterans of the sand castle arts.