This is a very old, ancient creature. Fossil records suggest they may have existed, in the present form they are now, 450 million years ago. This predates the fish and dinosaur fossils. Once found in several oceans and seas, today they are found along the eastern seaboard of the United States and along the Pacific coast of Asia from Korea to the Philippines. The species that inhabits the American coastline is Limulus polyphemus.

Range

• U.S. Mid-Atlantic seaboard, south and including the Gulf of Mexico

Habitat

• Shallow areas of protected estuaries
• Sand or mud bottoms
• Seagrass beds

Anatomy

• 60cm (25in) in length
• Hard shell, two body sections – cephalothorax (head) and abdomen (tail)
• Long spine called a telson, non-venomous, non-defensive, used to right itself and push in sediment
• Two small chelipeds underneath near front of head
• 10 legs, small chelipeds (claws), males have a hook to grasp female during mating
• Last pair of legs with leaf-like paddles used for pushing and burying into the sediment
• 5 elongated “book-gills” beneath the abdomen
• Each ridge possess a compound eye; the anterior ridge possess a median eye (less vision)
• “horseshoe” shape designed to plow through the sediment looking for food

Diet

• Scavengers – feeding primarily on small mollusk, worms, inverts, fish pieces, and algae
• Mouth located beneath and center of cephalothorax; they possess a crop (storage) and a gizzard (with tooth-like denticles) where food is ground down; the flesh is swallowed and shell is regurgitated.

Reproduction

• Dioecious (can tell males from females): males smaller and possess a hook on front arm to grasp female during breeding
• Approach intertidal beaches during full and new moons in spring and early summer; usually late afternoon and evening – may still be present at dawn
• Females dig a depression and deposit 200-300 eggs; eggs are 2-3mm (.08-.12in)
• Males (riding backs of females) will fertilize in the depression and then buried. Sometimes many males will follow a female to breed (called satellite males).
• Incubate until next spring high tide. Offspring about 1cm (0.4in) in length
• Reach maturity in 3 years

Predators

• Eggs and young – shorebirds, crabs, fish
• Adults – loggerhead and kemp’s ridley sea turtles

Economic Uses

• The blood is copper based (thus blue) and possess many properties useful in the medical field; including Limulus Amebocyte Lysate (LAL) used for clotting during surgeries and injections.
• Used has bait in the commercial eel fishery

The state of Florida is interested in sightings of these animals and particularly interested in where they are nesting. If you discover a nesting beach, please contact the Sea Grant Agent at the local County Extension Office.

Resources

Barnes, R. (1980). Invertebrate Zoology. Saunders College/Holt, Rinehart and Winston, Philadelphia PA. pp. 1089.

McLaughlin, J. F. Lanting, D. Griffis, M. Main. (2015). The Horseshoe Crab. University of Florida EDIS publication. WEC182/UW200.

FWC; http://www.myfwc.com/research/saltwater/crustaceans/horseshoe-crabs/

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