Atlantic Horseshoe Crab
Limulus polyphemus
Intertidal Zone on Lulu Key
March 17, 2004
Photographed by: Jonathan Francisco
Atlantic Horseshoe Crab
Additional Photos: eyes | underside | tail | mating | digging
Additional Video: mating | female underside
Descriptive Information
Field Observations

On our canoe trip out to Lulu Key we took a break at a sand bar. It is here where we first encountered an Atlantic Horseshoe Crab (an ancient species, fossil record). When we examined this arachnid we noticed several different characteristics. The outer covering (carapace) was brown and very hard. Also it had 2 eyes at the front and they were set far apart. On the underside of the crab we found 5 pairs of walking legs and one pair of pinchers in front. The male has a specialized pair of pinchers for mating. The fuzzy mouth is in the center, therefore, the different appendages bring the food to it. This crab also has a triangular abdomen. The abdomen and carapace are at a hinge. The Horseshoe Crab also has a long hard, pointed tail. This tail is used to flip the organism over when it is upside down. It does so using circular motion of the tail. Atlantic Horseshoe Crabs mate in early spring, at high-tide lines (especially on a full moon tide). The larger female releases a pheromone to attract the male. The male grabs onto the female via his specialized pinchers. The female moves to the highest point of the tide and digs a hole half a foot deep. Into this hole she releases 200 blue-gray tiny eggs. Along with the eggs she releases another pheromone that causes the male to release sperm. The eggs are then covered by the incoming tide and the two organisms "unlatch" and go they're separate ways. This is external fertilization. We caught two Horseshoe Crabs mating on Lulu key at night, but when the lantern light was on them, they moved towards the water. But when the light was no longer on them they moved back to shore. During our observation of the two we accidentally detached the two. The male went back into the water, while female continued to dig a hole.


Alden, P. National Audubon Society Field Guide To Florida. Alfred A Knopf, New York. 1998. Pg 204.

Kaplan, Eugene. Southeastern and Caribbean Seashores. Houghton Mifflin Company. Boston. 1998. Plate 25-26.

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