Horseshoe crabs often get overturned by ocean waves as they try to reach the sandy beaches where they lay their eggs. In the following photo, you can see one that got flipped over in the lower left corner.
When that happens, the crab cannot always turn itself over and lay there helpless to be stabbed by the sharp beaks of sea gulls. Or they may just die of exposure. Here’s a part of another beach where the shells of dead horseshoe crabs are strewn about randomly.
However, their greatest enemies are not the elements or even the birds. It is us, humans. We don’t usually eat horseshoe crabs, but fishermen like to capture and use them as fishing bait, and in some areas they can catch hundreds of thousands of them each season.
Horseshoe crab blood is blue and is used in the pharmaceutical industry to test that products such as vaccines, drugs, or medical devices are free of bacteria. So the crabs are harvested and bled of part of their blood for such uses. A quart of blue crab blood can fetch $15,000. After losing up to 30% of their blood, the crabs are fed and released back to nature in a weakened state. In regions where such medical harvest occurs, the population of horseshoe crabs has been declining.