Crab
Crabs are shy. The hard shell must not be too much a deterrent for predators. I found a couple hiding in the coral.
They were well protected and impossible to get at. Hence, they were also impossible to photograph. And then we chanced upon a lesson.
The empty shell of a crab was all that was left after it had been eaten. So, I do not see crabs out in the open on any dives.