Hilton Head Island’s Amazing Animals-Part II

HILTON HEAD ISLAND’S AMAZING ANIMALS – PART II 

Last post, I talked about our Island alligators. I promised you a post about some living  dinosaurs so I’m thrilled to tell you all about our fabulous Horseshoe Crabs, which have lived  on Earth for almost 450 million years!!

June 20th is International Horseshoe Crab Day, so it is a perfect time to tell you more about  these fascinating animals you can find on our beaches and how best to help them survive.  Horseshoe crabs and their eggs are vital to the survival of many migratory shore birds and  marine animals. And, they may have even saved your life before!

SO, WHAT IS A HORSESHOE CRAB 

There are 4 species of Horseshoe crabs still alive today. Only one species, Limulus  polyphemus, is found in North America along the Atlantic and Gulf coasts, including Hilton  Head Island. The other three species are found in Southeast Asia.

Horseshoe crabs are considered “living fossils” meaning they have existed nearly unchanged  for about 445 million years, even before dinosaurs existed. Since fossils are not alive, I like to  call them living dinosaurs instead.

Horseshoe crabs are not actually crabs at all – they are arthropods but are much more closely  related to spiders than they are to crabs or lobsters! In fact, they are the only living members  of the Xiphosura order.

The body of the Horseshoe crab is divided into three sections. The first section is the prosoma,  or head. The name “Horseshoe Crab” originates from the rounded shape of the head, because  just like the shoe on a horse’s foot, the head is round and U-shaped. It’s the largest part of the  body and contains much of the nervous and biological organs.

The middle section of the body is the abdomen, or opisthosoma. It looks like a triangle with  spines on the sides and a ridge in the center. The spines are movable and help protect the  horseshoe crab. On the underside of the abdomen are muscles, used for movement, and gills  for breathing.

Horseshoe crabs have 6 pairs of legs – they walk on 10 legs and use their last pair, called the  chelicera, to move food into their mouths. They eat worms, algae, clams, and other small prey  that they root out in the sediment on the ocean floor. Horseshoe crabs have no jaws, so they  crush their meals between their legs before eating.

The third section, the Horseshoe crab’s tail, is called the telson. It’s long and pointed, and  although it looks intimidating, it is not dangerous, poisonous, or used to sting. Horseshoe  crabs use the telson to flip themselves over if they happen to be pushed on their backs.

Horseshoe crabs have 10 eyes scattered throughout the body and several more light receptors  on the tail. The two largest eyes are compound, meaning they are made up of many individual  units called ommatidia, each with its own lens. In Horseshoe crabs, each compound eye has  about 1,000 ommatidia. These eyes are used to find a mate. The other eyes and light  receptors are used for determining movement, changes in moonlight and orienting itself while  swimming.

HORSESHOE CRAB FUN FACTS

Horseshoe crab blood is blue – Human blood is red because the protein in our blood cells  contain iron. Horseshoe crabs have a different oxygen-carrying protein called hemocyanin.  Hemocyanin has copper rather than iron, which gives horseshoe crab blood its bright blue  color.

In addition to being blue, Horseshoe crab blood contains a unique enzyme called limulus  amebocyte lysate, or LAL. It causes the blood to coagulate when exposed to bacterial  endotoxins, which can be deadly. Biomedical companies use LAL to test medicines, vaccines,  implants, and more for endotoxins. It’s how they ensure medical equipment is safe for people.  So, you can thank a Horseshoe crabs for keeping you safe!

Female Horseshoe crabs can lay about 80,000 eggs each year – From May to early June,  Horseshoe crabs swarm our beaches to breed and lay eggs. They arrive in huge numbers  during high tides that coincide with the full moon or new moon. Females dig nests in the sand  and bury a cluster of about 4,000 tiny, blue-green eggs. They can lay about 20 egg clusters  each year!

During this same time period, thousands of shorebirds descend on beaches where Horseshoe  crabs lay their eggs to feast on them. Red Knots, Ruddy Turnstones, Sanderlings, and other species rely on the fat- and protein-packed eggs to power their long flights. For Red  Knots, this important stopover is the last chance to fuel up before the final leg of an epic  annual 9,300-mile migration from South America to the Arctic.

Horseshoe crab babies look just like adults – It takes 2 to 4 weeks for horseshoe crab eggs  to hatch. Tiny crabs emerge, smaller than a pencil eraser and with nearly see-through shells.  The baby Horseshoe crabs find shallow, sheltered waters to live in. They molt several times in  their first year, shedding their old shells to reveal new shells underneath. Their shells darken as  they age. The crabs continue to molt, but with less frequency, as they grow older. Once they  reach maturity, they stop molting.

 

Female Horseshoe crabs are about one-third larger than the males. They can grow to be 18 to  19 inches (46 to 48 centimeters) from head to tail, while the males are approximately 14 to 15  inches (36 to 38 centimeters). The reason the females are so much larger is so they can carry all  the eggs they lay.

HORSESHOE CRABS ON HILTON HEAD ISLAND 

Although you can find Horseshoe crabs all along our beaches, they are much more abundant  on the south end of the island, especially in May and June. If you want to do something fun  away from the beach, maybe on a rainy day, you can also see them at the Coastal Discovery  Museum – https://www.coastaldiscovery.org/ – along with other coastal animals, including  Myrtle, their diamondback terrapin.

As you walk along our beaches, you might find what look like dead Horseshoe crabs but could  actually be their molted shells. To grow, a Horseshoe crab has to lose its hard outer shell, a  process called molting. When it is time to molt, they absorb a lot of water, cracking open the  old shell, usually along the front of the shell. The Horseshoe Crab literally walks out of that old  shell, leaving it behind!

You might also find live Horseshoe crabs upside down on the beach. If you see one, please  help it turn over but be very gentle! Please, do NOT pick it up by its tail because it can easily  break off. Instead, place a hand under it back and use the other to gently turn it over and face  it toward the water.

If you would like to learn more about these fascinating animals, in addition to the Coastal  Discovery Museum, here are some other great resources:

https://www.dnr.sc.gov/marine/pub/seascience/pdf/horseshoecrab.pdf https://horseshoecrab.org/

https://libguides.ccga.edu/crabs

And, one of my favorite books, The Narrow Edge: a Tiny Bird, an Ancient Crab and an Epic  Journey by Deborah Cramer – https://deborahcramer.com/

Coming Next…More about that amazing tiny bird, the Red Knot.