Prehistoric creatures of the Lowcountry: 10 facts about horseshoe crabs
Every Spring, tourists and locals alike notice a peculiar sight at South Carolina beaches.
They suddenly start to resemble mass casualty war zones.
Dead jellyfish litter the sand, little blobs lining the water's edge.
But they're not the only casualties.
Horseshoe crab carcasses are also scattered along the surf — or so it appears.
Some of those “carcasses” could have just been empty shells, according to Dawn Brut of the Coastal Discovery Museum.
“Horseshoe crabs molt until they are about 10 years old,” Brut said. “After that final molt they are adults and put energy into reproducing instead of growing.”
Brut explained that the smaller “dead” horseshoe crabs are probably just the shed exoskeletons left over from the molting process.
The best way to tell if it’s just an empty shell or a dead crab is to take a look at the front of the shell, Brut said.
“If there is an opening in the front, it is a molt,” Brut said. “That is where (the shell) opens and the live horseshoe crab comes out of the old exoskeleton.”
But why are the shells suddenly all over the beaches?
It’s spawning season.
Horseshoe crabs make their way to the shore in large groups each spring.
The prehistoric creatures crawl out of the ocean according to the moon cycle.
During full and new moons from March through June, the crabs come ashore during high tide, dig holes in the sand and lay their eggs.
According to the South Carolina Department of Natural Resources, this ensures the greatest chance of survival for the eggs.
A single nest can contain an astonishing number of tiny, green eggs — anywhere from 2,000 to 4,000, according to Brut.
Brut says a female horseshoe crab can produce “as many as 80,000 eggs per season.”
While the horseshoe crab’s clever use of high tides does provide some protection, the eggs still prove to be a vital food source for migrating shore birds such as the Red Knot, according to SCDNR.
The horseshoe crab isn’t just important for the bird population.
Horseshoe crab blood saves human lives, too.
Ever been vaccinated or received a flu shot? Thank a horseshoe crab.
Their blood is used “to test for bacterial contamination in every vaccine injected and medical device implanted,” according to SCDNR.
While a lot of the “carcasses” found on local beaches are likely to be empty shells, SCDNR estimates around 10 percent of spawning horseshoe crabs die on the beach each year.
SCDNR explained that the crabs get flipped over by waves and become stranded. While they can survive for a while if their gills stay wet, “the heat of a sunny day can quickly dry out and kill an upside-down crab.”
Beach-goers should flip over stranded horseshoe crabs and return them to the water, Brut says.
To flip a horseshoe crab, gently grab the outer edges of the shell, pick up the crab and then place it in or near the water, making sure its legs are in the sand, SCDNR explained.
“Horseshoe crabs will not hurt you,” Brut said. “There is nothing dangerous about a horseshoe crab.”