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As a young boy, I was fascinated by horseshoe crabs. I spent my summers swimming in a small estuary in Massachusetts. The water was murky, visibility poor and the bottom muddy. Early each summer, the horseshoe crabs would leave the deep water and head for shore. With snorkel and mask I would follow their progress as they plowed along the bottom, sometimes singly, more often with one or more small crabs being pulled along behind a larger one. I never stopped to wonder where they came from, what they were up to or where they went. It didn’t occur to me that they only appeared in early summer, then were gone for the rest of the season. As a young boy, what interested me most was that they looked like little tanks, stealthily emerging from the murk, plowing their way towards the shore, paying scant attention to any obstacle in their way.
As a young college student, I was fascinated by horseshoe crabs. In biology class, I learned that they are much more than tank-like creatures plowing along the bottom. They have been around for a long time, early horseshoe crab fossils date back 350 million years - well before the appearance of dinosaurs. During that time, many creatures have gone extinct, such as the trilobite. Others have changed their appearance radically, certain dinosaurs having evolved into today’s birds. Not so the horseshoe crab. They have changed little in all that time. Unlike most creatures on earth today, they clearly deserve the title “living fossil”.
Although they are called crabs, they are not. Biologically, they are more akin to spiders and scorpions, although in habits they are quite different.
Horseshoe crabs have six pairs of legs that surround the mouth, the legs perfectly placed for grabbing food from the seafloor and passing it directly into the mouth. The first pair, the chelicera, probe the bottom in search of sea worms and clams - the horseshoe crab’s main food. Once found, the food is shredded with the claws of the other legs - the walking legs - before being eaten.
The second pair of legs, the pedipalps, are modified into grasping claws in the male. Males use these claws to hold on to the back of the female during mating season. She pulls him along behind her. Wherever she goes, he goes. It was a mating pair that I had seen so often as a young boy.
Watching them plow into any object in their way, it might seem that vision is not an important sense to the horseshoe crab. Nothing could be further from the truth, although how much they can see in the traditional sense is not known. Still, to any animal with ten eyes (the actual number depends on what is considered an eye), sight must be important.
Two large compound eyes, the lateral eyes, are easily seen on the top of the main shell, the prosoma, and are used mainly to locate a mate. Directly behind each of the lateral eyes is a simple eye whose function is not yet known. Three more eyes, two median and one endoparietal eye, are located centrally at the front of the prosoma. These eyes detect ultraviolet light reflected from the moon. They help the horseshoe crab follow the lunar cycle, allowing it to track the high tides at the new (dark) and full moons. Along the tail, the telson, are a series of light receptors - usually counted as one eye. These light receptors help the horseshoe crab follow the solar cycle, allowing it to track day and night. Finally, there are two more eyes on the underside of the horseshoe crab near the mouth. It is thought that they help the horseshoe crab determine whether it is right-side up or upside down - facing the darker seafloor or lighter surface.
In the late spring and early summer, males will take up station just off the beach, waiting for a female to arrive from deeper water. Since she will lay her eggs in the sand in the intertidal area - the area between high and low tide, it is important for her to time her arrival at the highest tide, allowing a wider expanse of beach for egg laying.
As she passes a male, he will grab the back of her abdomen, the opisthoma, with his grasping claw and get pulled along behind her. Since there are usually many more males than females, it is not unusual to see a train of two or more males being pulled behind the female - sometimes up to 15 males in the train!
When the female has arrived at the beach, she will churn up the sand with her legs and lay her eggs to a depth of about 5 inches. In the course of an evening, each female may lay as many as 80,000 eggs. Once the eggs are laid, she drags the male across them where he will fertilize them with his sperm.
As a young ecologist, I was fascinated by horseshoe crabs. I learned that they are much more than the sum of their biology. The biology of so many other creatures are impacted by the success or failure of the horseshoe crabs.
In an estuary bottom, the fine particles of mud are so tightly packed together that water is not able to circulate to any depth. Without this circulation, the oxygen in the water is soon depleted. With no oxygen, very few creatures are able to survive in the substrate. However, as the horseshoe crab plows its way along the bottom, it stirs up the mud which adds oxygen to the substrate, greatly increasing the diversity of life in the bottom.
Not only do horseshoe crabs increase the diversity of life in the substrate, they also provide homes for many creatures living on the surface as well. Creatures like barnacles, slipper shells, bryozoans, sponges, anemones, oysters, and tube worms need a solid substrate to attach to. In a sandy or muddy bottomed bay, the horseshoe crab shell is often the only solid surface available. Home to all of these creatures, the back of the horseshoe crab can be a self-contained community of amazing diversity.
With thousands of horseshoe crabs often coming ashore to lay their eggs in the sand at the same time and place, many of these eggs will get dislodged from their nests and exposed to predators. Not surprising, many fish, crabs, sea turtles and birds have come to rely on these eggs for food. Studies have shown that as horseshoe crab numbers decline, so do the numbers of many of these other animals - in particular sea turtles and migrating shore birds. Some birds time their migration to arrive along the shores of horseshoe crab nesting areas, where they will feast on the abundance of eggs. One bird in particular, the red knot, travels from the tip of South America north to the arctic each year, a journey of over 9,000 miles. Each spring, they arrive along the coast of Delaware Bay just as the horseshoe crabs are coming ashore to lay their eggs. It is the nourishment of these eggs that will provide the birds with the energy to complete their journey. The horseshoe crab is so important to the survival of the red knot that as horseshoe crab numbers decreased around the turn of the century by 88%, the red knots passing through Delaware Bay decreased by 70%.
The horseshoe crab is a perfect illustration of what it means to really know an animal. First we learn to identify it by sight. Next comes an understanding of its biology. Finally we learn the role it plays in the greater scheme of the natural world. When we gain this level of understanding of any creature, we can’t help be fascinated by it, no mater how common it may seem at first glance.