Limulus polyphemus

(Linnaeus, 1758)

Atlantic horseshoe crab, American horseshoe crab

Limulus polyphemus is a marine and brackish chelicerate commonly known as the Atlantic or American horseshoe crab. Despite its , it is more closely related to spiders, ticks, and scorpions than to true crabs. The has remained morphologically unchanged for approximately 200 million years, earning it the designation of a 'living fossil.' It is ecologically and medically significant, with distributed along the Atlantic coast of North America and in the Gulf of Mexico.

Pronunciation

How to pronounce Limulus polyphemus: //ˈlɪmjʊləs ˌpɒlɪˈfiːməs//

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Identification

Distinctive horseshoe-shaped prosoma (- region) with a hard, smooth . Large compound lateral on each side of the prosoma, plus smaller simple eyes. Long, pointed (tail spine) extending posteriorly. surface bears six pairs of appendages including book gills. can reach up to 75 cm in total body length. Distinguished from true crabs by chelicerate mouthparts (no ), lack of , and overall body plan.

Habitat

Marine and brackish coastal environments including sandy beaches, estuaries, and shallow coastal waters. Breeding occurs on sandy beaches, particularly during high tides. Found in intertidal zones and subtidal areas to depths of approximately 30 meters.

Distribution

Atlantic coast of North America from Maine to the Yucatán Peninsula, including the Gulf of Mexico. A major concentration occurs in Delaware Bay, which serves as a primary and spawning area.

Seasonality

Explosive breeding synchronized to spring new and full moon high tides, restricted to a few hours per day during these periods. Breeding activity concentrated in spring months.

Behavior

Males search for females as they approach breeding beaches, grasp them with modified clasping claws, and remain attached for extended periods (sometimes weeks). Females deposit multiple clutches of in sand, which males fertilize externally—the only extant with this . Unattached 'satellite' males cluster around nesting pairs and compete for access; experimental evidence indicates these males can fertilize eggs, suggesting sperm competition. Males may grasp inappropriate objects when searching.

Ecological Role

serve as a critical food source for migratory shorebirds, particularly in Delaware Bay. The has been described as important to , though specific functions beyond egg provision require additional documentation.

Human Relevance

Blood contains amebocytes used to produce Limulus amebocyte lysate (LAL), a reagent for detecting bacterial in medical applications. from has commercial uses. Subject to decline in some regions, including Long Island Sound.

Misconceptions

Despite the 'horseshoe crab,' this is not a true crab (order Decapoda). It belongs to class Merostomata within subphylum Chelicerata, making it more closely related to arachnids (spiders, ticks, scorpions) than to crustaceans.

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Sources and further reading