This is one of the so called sponge crabs, with its rear legs articulating upward to hold something over itself. Instead of a sponge, this species seems to hold a blade of algae. It is possible that these may need to be split; in some, the eye color is red and in others white.
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The crab is on the other side.
Algae leaf flipped over, with the crab still holding on.
Created 14 September 2018