Ask the Aquarium: Horseshoe Crabs Called 'Living Fossils'
Q. I found what I thought was a dead horseshoe crab on the beach. A passerby said it was a molt. Do horseshoe crabs molt?
A. Yes. Horseshoe crabs (Limulus polyphemus) outgrow their shells and must molt to accommodate their increasing size.
Usually, you can tell if you’ve found a molt by looking just beneath the upper front edge of the shell. If you find a long, curved slit, it’s where the crab squeezed its body out of the old shell. Soft at first, the new shell hardens in about 24 hours and the crab is about a fourth larger than before.
Oddly enough, the horseshoe crab isn’t really a crab at all. It’s from the large group of animals called Arthropoda, which includes lobsters, crabs and insects; however, this armored tank of a crab has 10 walking legs and is more closely related to spiders and scorpions than to crabs.
It’s often referred to as a “living fossil, and” some scientists theorize it has remained virtually unchanged for more than 300 million years. It feeds by crawling along the ocean floor in search of worms, mollusks, dead fish and other morsels.
Also surprising is that this ocean-dwelling crab comes ashore to mate.
From spring to early summer, females lay 2,000 to 3,000 eggs in a series of sandy nests along the water’s edge. There the eggs are fertilized by males. Some two weeks later baby crabs emerge looking like miniature, tail-less forms of their parents.
The little crabs make their way to the ocean, where they remain until sexual maturity — some 9 to 11 years — when they migrate back to the beach to lay eggs of their own.
Despite its menacing look, the horseshoe crab is harmless. Its stiff, formidable-looking tail is used as a lever to right itself, when strong waves or currents flip it on its back.
Information is provided by the North Carolina Aquarium at Pine Knoll Shores.
The state operates three public aquariums: one in Pine Knoll Shores, another at Fort Fisher and a third on Roanoke Island. The aquariums are administered by the North Carolina Department of Environment and Natural Resources, and are designed to inspire appreciation and conservation of North Carolina’s aquatic environment. For more information about the aquariums, visit www.ncaquariums.com, or call (800) 832-FISH.
Sherry White works for the public affairs office of the N.C. Aquariums.
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