Science on the SPOT: Rendezvous With Horseshoe Crabs

Horseshoe crab
Horseshoe crabs are more closely related to spiders and scorpions than to crabs.

DELAWARE BAY, DELAWARE - The empty shells of an unfortunate horseshoe crab or two are a common sight on the popular Jersey shore (that’s what locals call the beach in these parts) during the early summer beach season. Occasionally a child may tentatively investigate one, but most steer clear of what’s left of the dangerous-looking arthropod.

What many people never see, and probably aren’t even aware of, is the frenzy going on in the nearby (and less frequented) Delaware Bay. Hundreds of thousands of the crabs come ashore there every year in May and June to mate and lay eggs, and then mostly disappear as the summer gets into full swing.

The horseshoe crab is a bit intimidating at first, looking like a cross between an amphibious assault vehicle and a creature out of the movie Alien. In reality, you’d need to go out of your way to get harmed by one – though Jim Hewes, the science teacher and volunteer crab surveyor whom we interviewed for this video, confided in us that he did once get stabbed through the boot by a crab’s spike-shaped tail, or telson, when he stepped on it by accident.

The crabs use their tails to try to turn themselves over if they get flipped upside down, though they aren’t always successful, as documented by the beautiful clock-face pattern in the sand that surrounds the crab that Jim rescues in the story. The telson is attached to the crab by tendons, which tend to get injured if the crab is picked up by it. So if you ever attempt to rescue a flipped horseshoe crab, the best way is to hold it by its shell.

Horseshoe crabs, as it turns out, aren’t really crabs at all but a separate subphylum, which makes them more closely related to things like scorpions and spiders. The males and females can be told apart by their size (females grow much larger) and their appendages. The male possesses specialized ones designed to latch onto the back of the female while mating. The Atlantic horseshoe crab (Limulus polyphemus) has its main migration in the Delaware Bay.


Conch and eel fishermen now face restrictions on the amount of horseshoe crabs they can harvest to use as bait, but the animals have also become attractive to the biomedical industry. Researchers have found that the crab’s blue blood contains a clotting agent with anti-bacterial properties. One quart of horseshoe crab blood can sell for about $15,000. While the bleeding process itself doesn’t usually kill the crabs, there is some debate over how harmful the process is to their health.

It could take a decade or more to begin to see the results of the regulations that went into place in the early 2000s to protect the crab population. So scientists and volunteers like Jim Hewes will continue to monitor the health of the crab population - and of shorebirds like the red knot, which depend on their eggs - for signs of recovery.

Thanks to their efforts, curious children at the Jersey shore should continue to have strange, alien-looking life forms to investigate for many years to come.