The only defense that a horseshoe crab has against predators is it’s hard shell. What they do is bury themselves in the mud or sand which makes it difficult for predators to turn them over or to get a good hold of them.

Horseshoe crabs really don’t have many predators when they are mature, but when they are young they are eaten by many other animals and their eggs are particularly vulnerable. Many shore birds
depend on horseshoe crab eggs for nourishment and to put on weight for their migratory journeys.