The Live Cycle of Horseshoe Crabs

     In late spring and early summer (May-June), during the new and full moon high tides, thousands of female horseshoe crabs come to shore to lay their eggs.

    One or more males use claspers to attach to the rear of the shell of a female. (The males are smaller than the females-about 2/3 the size.) The female uses her shell to scoop a shallow hole in the sand. As many as 4,000 eggs may be laid in the nest. Then the male, or males since more than one male often hangs on, fertilizes the eggs as he is dragged behind the female. The water covers the eggs with sand as the female drags on to the next site. The six or seven inches of sand protect the eggs from predators and keep them moist. One female may lay up to 20,000 eggs in one night – 90,000 in a year.

     Horseshoe crab eggs are pearly blue-green, soft and gooey, and about 1mm in diameter which is about the size of a BB. Incubation takes anytime between two weeks to months. The eggs swell to about 2-3 mm and usually break out of their outer layer in a few days.

    
                                                                                              photo permission: Michael Oates
   While they are  larvae, they molt four times in the egg. Next the young “trilobyte larva” swims for about six days. It floats on its back and paddles along with its little gill plates. It will continue to grow by shedding its shell.
   With each molt it becomes more like the shape of the adult. A one year old crab is about one inch.  The young hatchlings stay close to the beach for two years before they join the adults in the deeper water on the Continental Shelf.

This horseshoe crab is 4 years old. You can compare its size to the brick in the wall behind the aquarium.

   Horseshoe crabs are slow to mature. It takes nine or ten years for a male to become an adult and another year longer for females.  They molt 16-17 times during that time. They increase in size 25-30% with each molt. Once they become adults, they will not molt anymore. Females might grow to be about 24 inches long and weigh up to 10 pounds. The females are bigger than the males. Horseshoe crabs live for 15-20 years.


How A Horseshoe Crab Molts
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