The Horseshoe crab in Ryan's picture may have a shellfish or a barnacle on its back. Fisherman do not like them because they hurt the fishing industry. When I was visiting family in LI, many yrs. ago, some crabs had washed ashore & men were turning them on their back to die.
From Google: Horseshoe crabs' blue blood is so valuable that a quart of it can be sold for $15,000. This is because it contains a molecule that is crucial to the medical research community. Today, however, new innovations have resulted in a synthetic substitute that may end the practice of farming horseshoe crabs for their blood. Mar 18, 2019