Class Merostomata Horseshoe Crabs

During the full or new moons of late spring and early summer when tides are the highest, vast numbers of horseshoe crabs

FIGURE 2 Representative of the subphylum Chelicerata: ventral view of an adult horseshoe crab, Limulus polyphemus (class Merostomata).

(all Limulus polyphemus; Fig. 2) come ashore in eastern North America to breed in bays and estuaries. The five Asian and North American species of this class are remnants of this strictly marine class. Although formidable looking and up to 60 cm in length, horseshoe crabs are harmless to humans. When not breeding, these chelicerates reside on or in soft bottoms in shallow water. They are scavengers and predators on clams and worms. Although they can swim weakly by flapping their book gills (modified abdominal appendages), their primary locomotion is walking. Extinct merostomates (order Eurypterida) may have been the largest arthropods ever to have evolved (nearly 3 m long) and seem to have given rise to terrestrial arachnids. Despite their name, horseshoe crabs are not closely related to true crabs (subphylum Crustacea).

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