The Ancient Crab That Isn't a Crab

Fossil horseshoe crab ancestor Mesolimulus an extinct arthropod

 

You might know that horseshoe crabs go back before dinosaurs. Their fossil ancestors lived 445 million years ago. (Dinosaurs appear about 200 million years later.) And they survived the extinction event that wiped dinosaurs from the planet 66 million years ago. 

There are 4 species today. In New Jersey, we know the species that is found in the Atlantic coastal waters of North America and the Gulf of Mexico. The three others are found in Asia’s coastal waters.

Did you know that are "blue bloods?" They don't have the hemoglobin that makes human blood red. Horseshoe crabs have hemocyanin which is also an oxygen-carrying protein that contains copper (our blood has iron) and that gives horseshoe crab blood a bright blue color.

That unique blood contains an enzyme that causes the blood to coagulate when exposed to deadly bacterial endotoxins, and so biomedical companies use this to test medicines, vaccines, and implants for endotoxins to insure it is safe for people. Unfortunately, many horseshoe crabs die in the process of collecting their blood. Scientists are now looking for synthetic alternatives to the enzyme.

These crabs walk on 10 legs (5 pairs) but they have another pair that they use to bring food to their mouths like arms. Without jaws, they crush worms, algae, clams, and other small prey from the ocean floor with those legs.

Those legs and its pointed tail look pretty scary to a child (and some adults!) but they are harmless. The tail (telson) is only used when a crab gets stuck upside-down so that it can flip over or as a rudder to swim upside down. When washed up on dry land, the tail isn't as effective, so if you see one it's safe to turn it over and get it back to shallow water.

That telson might not work as a sword but their tough shell is a pretty good defense for adults when they are right-side-up against other predators including sea turtles, sharks and gulls. 

Image by Chris Engel from Pixabay

Spotting a horseshoe crab in the water or on a beach is common enough at the Jersey Shore or in the bays that they even appear on t-shirts and as jewelry. Although you may spot this crab at any time of year on a Jersey beach, the most important time for them is from May to early June when they arrive on beaches to lay eggs.

The largest populations live in the Delaware Bay and they hit the beaches there 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, and they can lay about 20 egg clusters each year.

That sounds like more than enough eggs but thousands of shorebirds also arrive there at that time to feast on the eggs. It is a critical food supply for migrating species such as red knots, ruddy turnstones, and sanderlings. The red knots need this final food stop before they complete their 9,300-mile migration from South America to the Arctic.

If eggs survive they will hatch in two to four weeks and the very vulnerable baby horseshoe crabs go to shallow, sheltered waters to live in for their first year as they grow and molt. 

Besides medical uses and marine and bird predators, humans use horseshoe crabs as bait to catch eels and whelk (sea snails), and that has really hurt crab populations in the past few decades.

The Atlantic horseshoe crab is currently considered “Near Threatened” by the IUCN. While not currently listed as a threatened species by the State of New Jersey, there is currently a moratorium on the harvest of horseshoe crabs within the state.

We always refer to this species as a horseshoe crab but they aren't crabs. They are arthropods and are more closely related to scorpions and spiders. They are the only living members of the Xiphosura order.

Sometimes you'll see them with some hitchhiking creatures attached to their shell