This is a spanner crab, Ranina ranina to be precise, the only known living member of its genus.
One of the things that makes this species of crab unique is that it doesn’t scurry sideways like you’d expect. These crabs walk, sometimes run, in a forward direction.
…often kicking up quite the sandstorm when they’re on a mission:
Probably due to the way these crabs appear when they’re ambulatory, they are also called red frog crabs. Cute, huh?
These crabs, which are considered delicacies in many areas, live in coastal waters of the Pacific and Indian Oceans down to about 100m depth.
For camouflage and protection, they use their reinforced rear ends to shovel out holes in the sandy/ rubbly substrate, where they bide their time while waiting to scavenge dead stuff or grab the occasional unsuspecting prey.