noun
1. : any of several closely related large marine arthropods of eastern No. America and eastern Asia constituting the only surviving members of the order Xiphosura and class Merostomata and having a broad crescentic cephalothorax with a pair of large compound eyes and two simple eyes on the upper surface and six pairs of legs arising from the lower surface about the centrally placed mouth, a small abdomen articulated to the cephalothorax with its segments fused into a single piece, swimming appendages to which the flat leaflike gills are attached, and a long stiff movably articulated caudal spine ; especially : an arthropod ( Limulus polyphemus syn. Xiphosurus polyphemus ) found on sandy and muddy bottoms on the coasts of No. America from Maine to Mexico and the West Indies that attains a length of nearly two feet — called also horseshoe crab
2.
a. : a large European spider crab ( Maia squinado )
b. : a large scarlet anomuran crab ( Lopholithodes mandtii ) of shallow water from California to Alaska
c. : the largest of the edible crabs ( Paralithodes camtschaticus ) often measuring five feet from claw tip to claw tip, weighing up to 15 pounds, and widely distributed in the No. Pacific