They’re not really crabs, but modern-day trilobites, more closely related to spiders and scorpions.  These animals show up by the hundreds on the shore during full or new moon nights, corresponding with the highest tides in spring.  They spawn, laying thousands of eggs, and return to the water.  Unless for whatever reason, they don’t – this is an intact, but dead, animal found on the beach at Flag Pond, along the Chesapeake.