Stranded Horseshoe Crabs

What are Horseshoe Crabs, and Why Do They Get Stranded?

The American horseshoe crab, Limulus polyphemus, is a prehistoric-looking shelled animal that has existed virtually unchanged in appearance for millions of years. Because of this, they are often referred to as “living fossils.” Although called a crab, horseshoe crabs are not crabs at all and are most closely related to arachnids. 

In the spring and summer of each year, during high tides around the full and new moon, horseshoe crabs gather on beaches and in salt marshes to spawn and may become stranded.

Horseshoe Crabs Can Become Stranded and Die

If the weather is rough or if the waves are especially strong, a horseshoe crab may get flipped over (with its legs facing the sky). 

It can be difficult for them to right themselves, particularly if they are somehow impaired (i.e., broken telson). Because the telson, also called the tail, is used to assist the horseshoe crab with flipping itself over, damage to this part of the animal can hinder its future ability to flip over. 

The telson is not dangerous—it does not contain a stinger but does have photoreceptors, which give it the interesting ability to sense light.

If horseshoe crabs are found with their legs facing the ground, they are not stranded, and it is best not to disturb them.

During rough weather, up to 10% of crabs that approach the beach may become stranded. If stranded horseshoe crabs can be flipped back over before the heat of the day and make their way back to the water, they may be able to survive.

How to Flip a Horseshoe Crab

Horseshoe crabs are harmless when handled properly. Grasp both sides of the shell of the head portion (prosoma) of the animal, pick the animal up, and set it down with its legs facing the sand.

Don’t ever pick up a horseshoe crab by its telson! You risk damaging its telson, which could reduce its ability to flip itself over in the future.

Don’t get your fingers caught in the area between the head and the abdomen (the hinge). This is where the crab bends and your fingers could get pinched.

Just the Shell?

The off-cast molts of horseshoe crabs may be encountered on the beach. These are not dead crabs, but rather the shell that has been shed by a growing crab. Molts will have a large opening along the front of the animal, where the horseshoe crab exited their former shell. Horseshoe crabs molt several times during their first years and then roughly once a year afterward.

Report Spawning and Tagged Horseshoe Crabs

Always check horseshoe crabs for a tag, located near the back of the head section. Follow the directions on the tag for reporting the find. Tag studies help with population estimates of this valuable species. 

You can also report observations of spawning horseshoe crabs to the South Carolina Department of Natural Resources. Researchers use these reports to better understand when and where horseshoe crabs spawn in the state.

Horseshoe Crabs Saving Lives

In South Carolina, horseshoe crabs are gathered for their blood, which is able to detect contamination in intravenous drugs and vaccines, making it vital to human health.

Much like when humans donate blood, horseshoe crabs are collected, bled, and then returned to the sea within a day. Horseshoe crabs are critical to the biomedical industry, and the humane bleeding process implemented achieves survival rates in excess of 80-90%.