Atlantic Horseshoe Crabs

These amazing creatures predate the dinosaurs by millions of years. Although we refer to them as “crabs,” they are not crustaceans, and are more closely related to spiders than crabs. Because Horseshoe Crabs live in protected coastal waters, all photos were taken along Port Royal Sound on the north end of the island.

Horseshoe Crabs spend most of their lives moving along the ocean floor like a small tank, eating whatever lay in their way- fish, shellfish, worms, dead and decaying matter, and even algae. These animals have five pairs of walking legs and, a pair in the front, and one pair of pusher legs in the back used for swimming. (Did you know that they swim upside-down?). They have external book gills (so named because they sorta, kinda look like the pages of a book) that are used for breathing, but also assist with swimming.

This video is of the crabs swimming in the pooled water around the jetty at the heel of the island. You’ll notice smaller males clinging to the backs of the larger females. For more about the spawning of the Horseshoe Crabs, visit my post from May 7, 2016.

Spawning at the jetty…

Now for some more about these fascinating creatures…

See here also my related post Horseshoe Crab Tagging Program.

This next video is from the Coastal Discovery Museum.


Although historically Horseshoe Crabs were used for bait and fertilizer, the only current commercial use is for biomedical purposes. Horseshoe Crab blood contains a clotting agent called limulus amebocyte lystae (say that three time fast), or LAL, that is used to detect bacteria in vaccines, intravenous drugs, and internal medical devices. Most people have been exposed to something tested using LAL.  Permitted harvesters collect the live crabs and draw approximately 1/3 of their copper based blue blood and return them live to the coastal waters where they were collected.

See also my related post In Danger of Becoming Endangered.

Even more important than the medical role they play, Horseshoe Crabs have an  important ecological role. The eggs and juveniles are prey for a wide variety of animals (basically anything bigger than it is), while the adults are prey for sharks, Loggerhead sea turtles, foxes, raccoons, and seagulls. Horseshoe Crab eggs are an important food source for at least 20 species of migrating shorebirds, with some timing their migration to the spawning of the Horseshoe Crabs.

Horseshoe Crabs are protected in South Carolina because of their important commercial and ecological roles. Threats include over-harvesting, loss of spawning habitat, and pollution. You can help by doing simple things like picking up litter from our beaches, or by carefully flipping one over if it is stuck on its back on the beach. Just remember not to pick it up by its telson, just grip the sides of the prosoma (horseshoe shape) and flip it over. They are harmless creatures that will not pinch or bite.

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