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Why are horseshoe crabs more closely related to spiders and scorpions than to true crabs?

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Horseshoe crab (Limulus polyphemus) with a hard, rounded shell and long pointed tail lying on sandy ground. — horseshoe crab Horseshoe crab (*Limulus polyphemus*).

Despite their common name, horseshoe crabs are not true crabs, which belong to the subphylum Crustacea. Instead, horseshoe crabs are classified within the subphylum Chelicerata, making them more closely related to spiders and scorpions than to crustaceans.12

The primary reason for this classification is their shared evolutionary and anatomical heritage as chelicerates. Unlike crustaceans, which possess antennae and mandibles, horseshoe crabs and arachnids lack antennae and are characterized by a body divided into a prosoma (cephalothorax) and an opisthosoma (abdomen). Their defining feature is the presence of chelicerae—the first pair of appendages flanking the mouth—which are used for feeding.21

While crustaceans are generally aquatic and possess biramous (branched) appendages, horseshoe crabs share a more fundamental body plan with arachnids, reflecting their common ancestry within the Chelicerata subphylum.2