In Hawaii Mud Crabs are often referred as Samoan Crabs as they're the largest shoreline species. Legal size is 6in across & you can only collect males. This helps to sustain a healthy population. Every once in awhile you'll see one just sitting in the open like this. I've noticed they often do this in preparation of shedding it's shell. This one looks like 4in across the back but hasn't thicken its shell yet.
https://youtu.be/RdG1FEI0I3Q