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Horseshoe Crab

Limulus polyphemus

Horseshoe crabs mate in spring and summer on sandy, protected beaches. (Amy Roe/Flickr)
Horseshoe crabs mate in spring and summer on sandy, protected beaches. (Amy Roe/Flickr)

The horseshoe crab is a primitive-looking arthropod with a hard, brownish-green exoskeleton and a spike-like tail. It visits the Chesapeake Bay’s sandy beaches each spring and summer to spawn.


  • Hard, rounded, brownish-green exoskeleton
  • Spike-like tail
  • Widely spaced eyes that look like bumps on the top of the shell
  • Five pairs of jointed legs
  • Gills have folds of membranes that look like leaves of a book
  • Juveniles are sand-colored
  • Grows to 2 feet long


  • Juveniles are found in shallow, protected waters with a sandy bottom
  • Adults live in deeper waters


  • Found year-round in the middle and lower Chesapeake Bay, as far north as the Chester River
  • Most visible in spring and summer, when it spawns on sandy beaches


  • Eats mostly worms and mollusks such as razor clams and soft shell clams
  • Because they lack jaws, horseshoe crabs use the spiny bases of their legs to crush and grind their food, then push it into their mouths
  • Spends most of its time rooting through bottom sediments looking for food


  • Several types of shorebirds eat horseshoe crab eggs
  • Various fish, invertebrates and sea turtles feed on eggs and larvae
  • Humans catch adult horseshoe crabs to use as bait and for medical research

Reproduction and Life Cycle:

  • Spawning takes place in spring and summer (peaking in May-June), usually during evening high tides when the moon is full or new
  • Large numbers of adults crawl up onto sandy, protected beaches to mate and lay eggs
  • Females lay clusters of about 4,000 greenish eggs in the sand around the high-tide mark. They return to the beach to lay more eggs during high tides throughout the season.
  • Eggs take about one month to develop and hatch
  • Young swim to shallow, sandy, protected nursery areas, where they remain for up to two years

Other Facts:

  • Horseshoe crabs are not true crabs. They are more closely related to terrestrial spiders than blue crabs.
  • Horseshoe crabs have existed for more than 300 million years
  • Although their spikes and spines may look dangerous, horseshoe crabs are harmless
  • Just like other arthropods, horseshoe crabs must molt to grow. They leave their old shells behind and grow a new, larger shell.
  • Horseshoe crabs swim upside down
  • Can survive out of the water for an extended period of time if its gills are kept moist

Sources and Additional Information:

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