Atlantic Horseshoe Crab
Limulus polyphemus
Atlantic Horseshoe Crab (Limulus polyphemus): Blue Blood, Book Gills, and a 445-Million-Year Body Plan
The Atlantic horseshoe crab (Limulus polyphemus) is a marine chelicerate—not a true crab—with a horseshoe-shaped carapace, book gills, and copper-blue blood. Delaware Bay spawners fuel migrating red knots, and its amebocytes power the LAL endotoxin test.
- Habitat
- Shallow coastal waters, estuaries, sandy and muddy flats
- Size
- Females to ~60 cm total length (incl. telson); males smaller
- Diet
- Worms, bivalves, small crustaceans, detritus (opportunistic)
- Range
- Western Atlantic: Maine to the Yucatán Peninsula
- Conservation
- IUCN Vulnerable (assessed 2016)
- Lifespan
- Often 20+ years; maturity ~9–11 years
Habitat & range
Atlantic horseshoe crabs live along the western Atlantic from the Gulf of Maine to the Yucatán Peninsula, favoring shallow bays, estuaries, and sandy or muddy flats. Adults usually stay in waters shallower than about 30 m, though they have been recorded deeper. The largest spawning aggregation is in Delaware Bay, where adults come ashore in May and June—especially on new- and full-moon high tides—to bury eggs in the intertidal sand ([NOAA Fisheries](https://www.fisheries.noaa.gov/feature-story/horseshoe-crabs-managing-resource-birds-bait-and-blood); [USGS](https://www.usgs.gov/centers/eesc/science/sustaining-horseshoe-crabs-and-supporting-migratory-shorebirds-delaware-bay)). Juveniles spend early years on nearshore flats before moving farther offshore.
Diet & foraging
They are opportunistic benthic feeders, not active pursuit predators. Using walking legs to dig and gnathobases at the leg bases to grind, they take marine worms, small bivalves, crustaceans, and organic detritus ([NOAA](https://oceanservice.noaa.gov/facts/horseshoe-crab.html); [Aquarium of the Pacific](https://www.aquariumofpacific.org/onlinelearningcenter/species/atlantic_horseshoe_crab)). The mouth sits between the leg bases on the underside of the prosoma—typical of chelicerates, not of true crabs.
Behavior & adaptations
The body has three parts: a broad horseshoe-shaped prosoma, a hinged opisthosoma with short lateral spines, and a long rigid telson used to steer and to right the animal when flipped—not a sting ([Maryland DNR](https://dnr.maryland.gov/ccs/pages/horseshoecrab-anatomy.aspx); [UF/IFAS](https://ask.ifas.ufl.edu/publication/SG190)). Five pairs of book gills under the abdomen handle gas exchange and can help juveniles swim. They have about ten eyes and photoreceptors, including large lateral compound eyes. Blood is blue because oxygen is carried by copper-based hemocyanin; amebocytes from that blood are used to make Limulus amebocyte lysate (LAL), the standard U.S. test for bacterial endotoxin in injectable drugs and medical devices ([Smithsonian NZP](https://nationalzoo.si.edu/animals/news/10-incredible-horseshoe-crab-facts); [NOAA](https://oceanservice.noaa.gov/facts/horseshoe-crab.html)). Taxonomy: major databases (ITIS, NCBI, GBIF) place the species in class **Merostomata**, order Xiphosura (or Xiphosurida), family Limulidae; some recent phylogenomic studies nest Xiphosura inside Arachnida. Either way, they are chelicerates closer to spiders and scorpions than to crabs.
Conservation
The IUCN Red List assesses *Limulus polyphemus* as **Vulnerable** (2016). Pressures include bait harvest for eel and whelk fisheries, biomedical bleeding for LAL, and loss or degradation of spawning beaches. Delaware Bay eggs are a critical spring fuel stop for migrating *rufa* red knots (*Calidris canutus rufa*) and other shorebirds ([USGS](https://www.usgs.gov/centers/eesc/science/sustaining-horseshoe-crabs-and-supporting-migratory-shorebirds-delaware-bay); [NJDEP FAQ](https://dep.nj.gov/wp-content/uploads/njfw/delaware_bay_rufa_red_knot_and_horseshoe_crab_faq.pdf)). Handling tip: never lift a horseshoe crab by the telson—that can injure the muscle it needs to right itself.
Fun facts about Atlantic Horseshoe Crab
- Copper-based hemocyanin turns their blood blue; amebocytes from that blood make LAL, used to screen injectable drugs and devices for endotoxin.
- Fossil xiphosurans with a similar body plan reach back to the Ordovician (~445 million years); living Limulidae are often called living fossils, though *Limulus polyphemus* itself is a modern species.
- Despite the name, they are not crabs or crustaceans—they are chelicerates, more closely related to spiders and scorpions.
- Delaware Bay hosts the world’s largest spawning aggregation; those eggs fuel migrating rufa red knots each spring.
- They have about ten eyes and photoreceptors—including large compound eyes—and classic Limulus vision research contributed to the 1967 Nobel Prize work on lateral inhibition.
Atlantic Horseshoe Crab vs Great Barracuda
Speed and cutting teeth beat a slow armored scavenger in open shallow water.
On a sandy Atlantic flat, a great barracuda can rush and rake a horseshoe crab that cannot outswim it. The crab’s carapace and telson help against waves and righting, not against a fast pelagic bite. In a Wilder matchup framed as open coastal water, the barracuda’s Ambush pressure wins; in a buried-in-sand scenario the crab’s armor and camouflage would matter more.
Read more about Great Barracuda.
Common questions about Atlantic Horseshoe Crab
Are horseshoe crabs really crabs?
No. Atlantic horseshoe crabs (Limulus polyphemus) are chelicerates, more closely related to spiders and scorpions than to true crabs or other crustaceans. They lack antennae and crab-like mandibles; major databases place them in class Merostomata, order Xiphosura (NOAA; ITIS).
Why is horseshoe crab blood blue?
Because oxygen is carried by hemocyanin, a copper-based protein, instead of iron-based hemoglobin. When oxygenated, the blood looks blue (Smithsonian NZP; MBL).
What is Limulus amebocyte lysate (LAL) used for?
LAL, made from amebocytes in horseshoe crab blood, detects bacterial endotoxin. It is the standard U.S. screen for contamination in injectable drugs, vaccines, and many medical devices (NOAA; FWC).
How many eyes does a horseshoe crab have?
About ten eyes and photoreceptors: a pair of large lateral compound eyes on the prosoma plus additional simple eyes and light sensors, including on the telson (FWC; Maryland DNR).
Why do red knots need horseshoe crabs?
In Delaware Bay, migrating rufa red knots time their stopover to feast on horseshoe crab eggs—often the bulk of their spring fuel before Arctic nesting. That egg pulse is unmatched elsewhere along the Atlantic Flyway (USGS; NJDEP).
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