Horseshoe crabs have a wide range of food resources. As said before, horseshoe crabs are secondary consumers. They mainly consume a wide variety of mollusks. They feed upon many species of bivalves and lots of different clams.Clams include the surf clam, macoma clam, blue mussel, wedge clam, razor clam, fragile razor clam, and soft-shelled clams.They also eat different kinds of worms. Not only do they consume animals, vascular plant material has often been found in sampled horseshoe crabs.lams include the surf clam, macoma clam, blue mussel, wedge clam, razor clam, fragile razor clam, and soft-shelled clams. They also eat different kinds of worms. Not only do they consume animals, vascular plant material has often been found in sampled horseshoe crabs.