Shadow boxing

After all the rain yesterday, the Silvermine River was coming down in full spate, and it seemed like these little crabs had been washed down the river out to sea, then back onto the sandy beach. I think it must be the Sandflat Crab (Cleistostoma edwardsii). It's carapace is less than 1 cm wide, but with that huge shadow, it looks quite ferocious!