Horseshoe Crab

Natural History and Biology

Illustration

While its hard shell and many appendages with claws may remind us of a crab, the horseshoe crab (Limulus polyphemus) is not a crab at all! Horseshoe crabs belong to the arthropod phylum along with the crabs, insects, and other invertebrates with jointed legs, but their closest relatives are spiders and scorpions. The horseshoe crab is a very unique animal - so unique that it has been put into a category all by itself. Horseshoe crabs are in the class Merostomata (middle mouth) Horseshoe crabs have long been referred to as "living fossils". The species is over 300 million years old. Fossils that have been found vary only slightly from the horseshoe crabs we see today.

There are three divisions to the body of the horseshoe crab: the prosoma (cephalothorax), the opisthosoma (abdomen), and the telson (tail). On the upper surface (dorsal) you might first notice two large and unusual compound lateral eyes. These are used for finding mates. There are 5 additional eyes on the top of its shell (two median eyes that you might think look like nostrils, the endoparietal eye and two rudimentary lateral eyes). These 5 eyes detect the UV light from the moon and sun and help orient the crab to lunar cycles. The tail also has a series of light sensors along the top and sides that keeps its brain synchronized with cycles of light and dark. On the underside of the crab, there are two ventral eyes, located near the mouth, which may help orient the crab when swimming.

On the underside (ventral surface) are 7-paired appendages. The first pair (the chelicera) are used for placing food in its mouth. The next pairs are the pedipalps; these are the first walking legs. They are modified as a grasping appendage in the adult male. The "last" legs are the "pusher" legs, used for locomotion.

Towards the tail are the five book gills. The horseshoe crab uses them both from propulsion when swimming and for "breathing". Similar to the gills in a fish, they are a membrane that allows oxygen to pass through while keeping the water out. Horseshoe crabs are animals of the temperate seas and in the United States, are only found along the eastern coast and along the Gulf of Mexico. During the cold months, they lay half buried in the ocean floor, but as the day lengthens in the spring, they begin to stir and move toward the beaches, just as they have done for eons. Horseshoe crabs begin appearing on the beaches of Cape Cod just after the first full moons in May and June. It is quite a sight going to the beach and seeing hundreds, maybe thousands, of mating horseshoe crabs.

The males arrive first, followed by the females a week or so later. Females average 30% larger than the males. To attract a mate, the females release a pheromone, a natural chemical (perfume) that serves as a sexual stimulant, into the water. Horseshoe crabs also use their relatively good vision to spot potential mates.

Males patrol the near shore waters and use their pedipalps (specialized walking legs shaped much like boxing gloves) to hook onto the female as she heads toward the beach. She drags him to the water's edge and scoops out a series of five to seven nests, depositing several thousand eggs in each nest. The attached male fertilizes the eggs as she lays them. The newly laid eggs are about 1/16 inch in diameter. At first they are opaque, pastel green, but in a few days, they double in size and the outer layer peels off leaving them transparent.

The moisture from the tides and the warmth of the sun allows the eggs to mature and hatch in the two-week period between spring tides (The higher than normal tides that occur with new and full moon). After hatching, the juvenile horseshoe crabs dig their way out of the sand. Unlike many marine invertebrates such as shrimp and sea stars, which pass through several stages before the look like their adult counterparts, horseshoe crabs begin life as miniature adults. At approximately ¼ inch across, they lack only a fully functional digestive system and a moveable tail. For about a week, they swim about, absorbing their yolk sac as their digestive system matures. Juveniles are quite good swimmers. They swim mostly at night and they swim upside down, moving their legs and gills in a progressive wave from front to back. When their digestive systems have matured, the search for food begins. Juvenile horseshoe crabs generally spend their first two summers on the intertidal flats feeding before the daytime low tide and burrowing in the sand for the rest of the day.

To find prey, horseshoe crabs push their way along the bottom, digging little furrows like a farmer plowing the ground. As they do so, they use their first pair of appendages, the chelicerae, as feelers to determine the presence of prey. When a crab feels or smells a worm, clam, or dead fish, one of the claws picks it up and pushed it toward the mouth, which is found in the middle of the body. Since the horseshoe crab has no jaws to chew its food, it must bring all of its legs together and use them to crush the worm or clam. Horseshoe crabs also have gizzards containing sand and small bits of gravel to help grind up their food.

Horseshoe crabs continue to grow for nine to ten years until they finally reach maturity. As they grow they must molt of shed their shells like a lobster or true crab. Young horseshoe crabs molt often, but as they near sexual maturity, molting slows, probably occurring once a year. Males are sexually mature at their sixteenth molt or ninth year. Females need at least seventeen molts, or one more year than the males. Unlike crustaceans, such as the blue crab, which back out of their old shells, the horseshoe crab crawls out in front of its shell through a split that develops along the lip formed at the junction of the dorsal (top) and ventral (bottom) surfaces.

How long can horseshoe crabs live? No one really knows, but some scientists have speculated that 30 years is a possibility. A few horseshoe crabs have been kept in aquarium habitats for 15 years. Obviously, an organism that does not begin breeding until age nine or older should have a life span that enables it to reproduce for a number of years.

Adult horseshoe crabs have very few predators. Some sea turtles and man are the primary predators. Aside from natural mortality due to beach stranding, disease and predation, the largest cause of death currently comes from the bait fishing industry, who harvests the horseshoe crab as bait for eel and conch. Horseshoe crabs are also captured, and bled for medical uses, these crabs are returned to the ocean after bleeding, but an estimated 10-15% will die during the process.

Historical uses Human uses of the horseshoe crab began thousands of years ago. The Native Americans ate the flesh of our local species. They ate the meat of the abdomen, the muscles used to move the tail, and possibly other parts. The Native Americans used the horseshoe crab's shell to bail water and they used the tail as a spear tip. The Native Americans also passed on to the early settlers the knowledge that the horseshoe crab is an excellent fertilizer.

Other uses have included feed for chicken and hogs, and bait for eels. The use for livestock food is no longer viable, as the crab feed did affect the flavor of the meat. However, horseshoe crabs are still being used as bait for eel pots.

Medical uses The horseshoe crab has led to tremendous advancements in eye research, development of surgical suture and wound dressings, and detection of bacterial contamination. Because of their large size, simple construction, and easy accessibility of the optic nerve the compound eyes of the horseshoe crab make the animal an ideal laboratory research subject. Medical researchers have been able to find out much of what we know about how our eyes function.

The shell of the horseshoe crab has also been extensively studied. Other arthropods have chitin as part of the chemical make-up of their shells but the chitin in the horseshoe crab shell is a very rare pure type. Chitin is a substance found in the shells, or exoskeletons, of horseshoe crabs, as well as other arthropods, such as lobsters, crabs, shrimps, spiders, beetles, and mosquitoes. It has received the attention of scientists because is non-toxic, biodegradable e, and when processed to produce another substance called chitosan, can be used to produce a variety of important products. Contact lenses, skin creams, and hair sprays can be made from chitin. It can be used to remove lead and other harmful metals that may be dissolved in drinking water, and clean certain harmful chemicals from wastewater. Chitin joins the fight against fat w hen it is added to foods. It has the ability to bind with fats and then passes them through and out of the body without being digested. Chitin can also be made into string used to sew up wounds and used in wound dressings. People do not have an allergic reaction to the stitches, which dissolve slowly, and the dressings actually promote healing. Many of these products are now available in Japan and are under development in the United States.

The most important discovery was that the blue, copper-based blood of the horseshoe crab contains a clotting agent that attaches to dangerous, fever-inducing toxins produced by many infectious bacteria. This clotting agent is called Limulus Amoebocyte Lysate, or LAL. The discovery of LAL yielded an excellent method of checking any drug for gram-negative bacteria, which are difficult to detect. Gram-negative bacteria are a group that causes a number of human diseases, including spinal meningitis. LAL is the standard test for injectable and intravenous drugs, required by the Food and Drug Administration (FDA).