The horsehoe crab, a close relative of the extant trilobite, is the oldest living fossil in Maryland, living here for approximately 360 million years. Horseshoe crabs are bottom-dwelling organisms that belong to the largest group of all living animals, the phylum known as arthropods. The presence of chelicera (pincer-like appendages), 5 pairs of walking legs and book gills, and lack of jaws and antennae make horseshoe crabs more similar to spiders, ticks and scorpions than to "true" crabs.
At one time, there were many species of horseshoe crabs, however, only four have survived. Three of these can be found along the shores of Southeast Asia and nearby islands. In America, they range intermittently from the Yucatan peninsula to northern Maine. Each major estuary along the coast is believed to have a discrete horseshoe crab population that can be distinguished by adult size, carapace color, and eye pigmentation. Along the Atlantic coast, horseshoe crabs are most abundant between Virginia and New Jersey with Delaware Bay at the center of the species distribution.
Horseshoe crabs molt or shed their shell to grow. Molting occurs several times during the first two to three years and about once a year afterwards. Molting occurs approximately 16 to 17 times over a period of 9 to 11 years before sexual maturity is reached and once mature, it is believed they no longer molt. Females reach maturity one year later than males and consequently, go through an additional molt. Mature horseshoe crabs then repeat what has occurred for years, an annual spring migration to inshore spawning areas. If a horseshoe crab can survive the rigors of spawning, it may live to 18 years of age.
Horseshoe crabs are well known for their highly visible mating activities. Spawning in the Chesapeake usually begins in late May when large numbers of adults move onto beaches to mate and lay eggs. The peak in spawning activity usually coincides with the full moon and evening spring tides. Adults prefer beach areas within bays and coves which are protected from rough water. Eggs are laid in clusters or nests along the beach, usually between high and low tide. Several nests are made during one beach trip and females will return on successive tides to lay more eggs. Females can produce approximately 88,000 per year. Egg development usually takes about a month and once hatched, larvae usually swim around in the shallow intertidal areas near the beaches where they were spawned until they settle to the bottom and molt. Juvenile horseshoe crabs spend their first and second summer on the intertidal flats and then begin moving offshore.
Adult horseshoe crabs feed mainly on marine worms and shellfish including razor clams and soft-shelled clams. Because they lack jaws, horseshoe crabs crush and grind their food items using the spiny bases of their legs and then push the small food particles into their mouths. Horseshoe crabs can tolerate a wide range of temperatures and can survive in low oxygen environments. As long as their book gills are kept moist, horseshoe crabs can survive out of the water for extended periods of time, especially to spawn.
Horseshoe crabs play an important ecological role in the food web for migrating shorebirds and juvenile Atlantic loggerhead turtles. Delaware Bay is the principal breeding location for horseshoe crabs and is also the second largest staging area for shorebirds in North America. If timed correctly, shorebirds will arrive in the Delaware-Chesapeake region at the peak of horseshoe crab mating in mid-May and June. These migratory birds rely on horseshoe crab eggs to replenish their fat supply during their trip to Canadian breeding grounds. Because these areas are regularly used by large numbers of shorebirds at specific times of the year, they are particularly vulnerable to disruption. The collection of horseshoe crabs by hand from spawning beaches disturbs the feeding of shorebirds. A decrease in the number of horseshoe crabs may leave a large portion of migrating shorebirds without the necessary food resources to complete their trip to arctic breeding grounds.
Adult horseshoe crabs also form a significant part of the diet of juvenile Atlantic loggerhead turtles, a threatened species that utilizes Chesapeake Bay as a summer nursery area. Horseshoe crab eggs and larvae are also a seasonally preferred food item of many invertebrates and finfish, including all crab species, whelks, striped bass, white perch, American eels, killifish, silver perch, weakfish, kingfish, silversides, summer flounder and winter flounder.
The horseshoe crab also provides a medical compound critical to maintaining the safety of many drugs used in medical practice. Limulus Amebocyte Lysate (LAL), a product produced from the blood of a horseshoe crab is used by pharmaceutical and medical device manufacturers to test their products for the presence of endotoxins, bacterial substances that can cause fevers and even be fatal to humans. All injectable drug products and all medical devices (such as replacement hips and artificial hearts) implanted into humans must be safely tested for the presence of endotoxins.
Horseshoe crabs have also been to benefit cancer research. Endotoxins are known to inhibit the growth of cancer cells. Therefore, the ability of the LAL test to detect cancer cells could lead to controlled cancer therapy with endotoxins. In addition, another substance found in horseshoe crab blood may have the potential for diagnosing leukemia. This substance reacts with red and white human blood cells, including cancerous white blood cells in leukemia patients. Furthermore, a New Jersey Sea Grant project has recently discovered a rare protein in horseshoe crab blood that traces and binds with vitamin B12. These findings led to the development of a accurate, cost-efficient testing kit for detecting vitamin B12-related deficiencies and diseases, which may include pernicious anemia, gastric and intestinal damage, and even mental disorders.
Although they are neither valued for their beauty nor their delectable taste in seafood dishes, they are very important to the Chesapeake Bay as an integral part of the marine ecosystem and for their use as a medical compound. In fact, without proper management and protection, man may manage to wipe out a resource that has survived not only dinosaurs and woolly mastodons, but dramatic destruction of their living and spawning environment.
Horseshoe Crab Fun Facts:
- The Indians of Roanoke Island, North Carolina tipped their fish spears with horseshoe crab tails, and their remains have been found in the kitchen middens of coastal Indians.
- By recording electrical impulses from the crab's optic nerve in its lateral eye, many principles underlying the functioning of all visual systems was discovered and gave Dr. H. Keffer Hartline a shared part of the 1967 Nobel prize.
- Horseshoe crabs can swim, awkwardly and upside down, but spend most of their time rummaging along the bottom.
- Despite their armament of spines, tail spike and wriggling, weakly clawed feet, horseshoe crabs are harmless!