Horseshoe crabs are not crabs, but they will always be called by that name.
Horseshoe crabs predate the dinosaurs having been on Earth an estimated 450 million years. That beats out that T-Rex by about 200 million years.
One of their unique features is their blue blood. That blood is used by the pharmaceutical industry for testing drugs and so has caused their numbers to deplete.
They live in the ocean year-round and make one famous annual sojourn ashore to lay eggs in sandy, wet beaches. You may know that the Delaware Bay and New Jersey are very well known for this spring ritual which is quite amazing.
They are not crabs or even crustaceans because they lack antennae. On the arthropod family tree, they are classified as chelicerates, a subphylum that also includes arachnids - spiders.
Although they are popularly called “living fossils,” they have evolved over the almost half a billion years. Fossils show us that they once had limbs that split out into two branches unlike their single one today.
Another misconception is that their spiked tail is used to "sting" prey or people. Not so. It is used as a steering rudder and to help right themselves after getting stuck on its back. Don't pick a live one up by the tail because it can break off and cripple the crab.
They eat aquatic worms, algae, carrion, and lots of clams and mussels. They mash food between the spiky upper regions of their legs before pushing it into the mouth.
The full moons, new moons, and high tides of May and June are when the Delaware Bay has its Atlantic horseshoe crab spawning. It happens at night when a female comes ashore followed by male(s). Like turtles, she digs a hole and deposits her eggs but then males fertilize them.
This event coincides with migrating shorebirds who gorge themselves on the nutrient-rich eggs which power birds, such as red knots, on their annual migration between the Arctic and South America.
The horseshoe crabs lay many, many eggs because so many are consumed by the birds. The estimate is that a female can lay 90,000 eggs per clutch and that probably about a dozen of those will ever make it to adulthood. It's not just birds who are predators. The embryos are also eaten by fish and sea turtles.
Horseshoe crab blood contains copper which turns bluish-green when it oxidizes.
They also have no infection-fighting white blood cells, but have special cells called amebocytes. This is what interested scientists who found that a vaccine or injectable drug is safe if the blue blood doesn't release the gooey amebocytes that means it did not encounter bacteria.
This makes horseshoe crab blood worth about $15,000 per quart. Though the crabs are released after extracting about 30 percent of its blood, about 0–15 percent of captured crabs die somewhere in the process. Harvesting restrictions have been set in places and researchers have been trying to develop synthetic amebocytes.
Admire these amazing creatures. If you see a live crab flipped on its back, listed by the shell and set it back into the water. They won't hurt you at all. And think about them the next time you get a safe vaccine or injection.
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