Answer:
They are brown horseshoe-shaped arthropods that can grow to about 12-18 inches in diameter. They crawl along the bottom of shallow water and come ashore to mate. They are mostly harmless, but can stick their tail/spine almost straight up if they feel threatened.
Their blood is frequently harvested for medical reasons. This does not harm the crab and they are released after taking about 20% of their blood.
They're not much fun to play with.