The earliest horseshoe crab fossils, from Devonian times, show very little difference from the living horseshoe crabs of today, thus making it a "living fossil." Horseshoe crabs were among the first animal life to experiment with leaving the sea. The only time we are likely to see one is at spawning time, when they crawl up onto the beach to deposit eggs in the sand. At this time, we may see that the females are larger: they come up in pairs, the big females crawling up the sand berm, the smaller male clinging to her back. From above, it looks like just two rigid sections of shell, and a spike tail; but flip one over, and you will see the numerous jointed legs flailing about. By arching its back, it can turn right-side up again, using the spike tail as a pivot.
When you look at a horseshoe crab up close, you wonder: where, exactly, is the living part of the animal? It looks like the legs are attached directly to the inside surface of the domed shell. Can there really be living flesh inside? And the two largest eyes sit directly on top of the shell, as if the creature was all shell and nothing else. A third, tiny eye is hidden at the back of the "head" section. If the legs are attached inside the dome of the front section, the back section is filled with flapping layers called book gills. There is a highly specialized flatworm called the horseshoe-crab leech which lives on these book gills. Where is the mouth? It is at the center of the cluster of legs, the bases of the legs acting as jaws and "teeth" for "chewing" the food up.
Once the eggs are laid, the horseshoe crabs return to the sea. The eggs rest in the sand until it is time to hatch; then, they hatch on a high tide, which washes the planktonic larvae out to sea. There they remain near shore, gradually moving further and further out as they grow. They live like their parents, plucking tiny bits of food from among the sand grains. The horseshoe crab can, if necessary, swim by flipping upside-down and waving its book gills, but it prefers to remain on the seafloor, walking.
Horseshoe crabs seem to be affected by the rotation of the earth, because they only occur on east coasts: one species on the east coast of North America, the other on the east coast of Asia. If you slide a pan of water, you see that the water has more inertia than the pan itself; you see the water get shallower at the leading edge of the pan, deeper at the trailing edge. So it is with the oceans: because of the direction of the earth's spin, the Atlantic sea level is higher on the shores of the Americas, lower on the European and African coasts; likewise, Pacific sea level is higher on the Asian side than the American side. The horseshoe crab's distribution may have something to do with this.
Unlike vertebrate animals, which have red, iron-based blood, the horseshoe crab's blood is copper-based, and therefore bluish-green. This unusual blood is used in medical labs for detecting certain toxins.
The biggest danger to the horseshoe crab may stem from its need to come ashore for spawning. As more and people concentrate in coastal zones, beaches become more and more developed. And there are always the senseless ones who kill for fun: I remember walking down a beach during the horseshoe crab spawning season, and seeing a boy there, smashing the defenseless horseshoe crabs with rocks. Along the entire beach, I could not find a single horseshoe crab that he had missed; every one lay broken, many with the rocks still embedded in them. This kind of pointless mass slaughter, if repeated over and over along the whole coast, could spell disaster for horseshoe crab populations. This creature, which survived the eons - from before any plant or animal had colonized the land, through the great mass extinctions - may meet its doom now, at this late date called the Age of Man.