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…
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- Notice the 3 main body parts: the prosoma (the front horseshoe shape), the opisthosoma (middle), and the telson (tail). People mistake the telson for a weapon, but it is actually used as a rudder for steering, and to help right itself if it gets flipped over. NEVER, never, never pick up a live Horseshoe Crab by its telson.
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- Notice the walking legs, the large pushing legs, and the book gills. There are 6 light sensing eyes, including several on the telson, 2 compound eyes (like an insect) on top of the prosoma used for finding a mate, and 2 eyes on the underside of the body that are possibly used to orient itself while swimming, for a total of TEN eyes.
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- Bottom Closeup with Tiny Crab. You can think of the Horseshoe Crab as a “crawling hotel,” with other creatures such as barnacles and snails living on its hard exterior, and creatures such as this little crab lurking underneath.
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- Tagging Program. If you see a Horseshoe Crab with a tag such as this, please call the phone number on the tag and report the tag number. Taking a picture as I did or writing down the numbers will aid you. Reporting helps the SC Department of Natural Resources learn more about the activities of these incredible creatures.
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- Clustering on the Beach at the Spring Tide. Horseshoe Crabs are numerous on our beaches during the spring spawning season that runs from March through June.
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- Mating Behavior. Every 2 weeks during the new and full moon high tides (called spring tides), the females come out of the water, often dragging a male attached to her with his hook-like front legs. On the beach she will dig a hole in the sand beneath her to lay her eggs. As she lays her eggs, the attached male (or males) release sperm to fertilize the eggs. The eggs develop under the sand for 2 weeks until they hatch just in time to catch a ride out on the next high tide. A female may lay as many as 80,000 eggs per season.
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- Juvenile Tracks in a Tidal Pool. Horseshoe Crabs molt their exoskeleton as they grow bigger. They molt more often when young, and molt 17 times during their first 10 years of life. Females molt one additional time, leaving them larger than the males.
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- Juvenile in a Tidal Pool.The crabs are mature after their final molt around age 10, with a lifespan lasting as long as 20 years. Once mature, the energy formerly used for growing is now focused on reproduction.
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- Before molting, a new shell begins to form. When ready, the crab absorbs water through its gills, making itself bigger. The old. hard shell cannot expand and splits at the front where the top and bottom join. The crab crawls out the front , leaving the old shell behind. It takes about 24 hours for the new shell to harden. With each molt the Horseshoe Crab increases in size about 25%.
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- It is not possible to visibly determine the gender of a Horseshoe Crab until after the final molt when the front pair of legs will look like pincers in the females and hooks or boxing gloves in the males.
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- Tracks in the Sand. Prior to their final molt it is possible for Horseshoe Crabs to regrow lost appendages.
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- Burrowed in the Sand. If still on land once the tide was receded, a Horseshoe Crab may burrow in the sand to wait out the next tide.
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- Body Pit with Tracks
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.
