Check out this huge M. annularis and accompanying fish at Molasses Reef, Key Largo, Florida.
Limulus polyphemus: a “Trash Fish,” or
an Ecological, Commercial,and Biomedical Treasure?
Limulus polyphemus, the American horseshoe crab, has been around for more than two-hundred million years. This “living fossil” not only provides a key to past with its relation to the now extinct trilobite, but a key to the future with its ecological impact, commercial uses, and applications in biomedicine.
A member of Phylum Arthropoda, Subphylum Chelicerata, and Class Meristomata, the horseshoe crab is an animal unique in appearance. (Pechenik 330) Its body consists of two main sections; an anterior section called the prosoma and a posterior section called the opisthosoma. (Pechenik 330) Horseshoe crabs have no antennae and no distinct head. (Pechenik 330) On the dorsal side are appendages modified for feeding (chelicerae), walking (walking legs), and for gas exchange (book gills). (Pechenik 330) Two sets of eyes, simple and compound, are located on the prosoma, and an elongated spike, called a telson, protrudes from the opisthosoma. (Pechenik 330)
Four species of horseshoe crab exist. Tachypleus tridentatus, Tachypleus gigas, and Carcinoscorpius rotundicauda are found in coastal waters of Asia. (Walls, Berkson, Smith 41) The fourth species, Limulus polyphemus, is found along the Atlantic coastline with a habitat range spanning from New York to Florida. The four species differ only slightly in appearance with varying numbers of spines located on the opisthosoma and differing triangular or non-triangular cross-sectional shapes of the telson. (Avise, Nelson, Sugita 1986)
All species of horseshoe crab are marine scavengers that burrow in the sediment on the bottom of the ocean floor. Diet primarily consists of bivalve mollusks, such as the razor clam, soft shelled clam, and wedge clam, as well as worms and vascular plants. An important part of the food web, the horseshoe crab has several predators. Shorebirds along the Atlantic coast feast of the eggs of horseshoe crabs during spawning as well as some larger horseshoe crabs. Mollusks and loggerhead turtles diet on adult horseshoe crabs while shrimp, crabs, and fish will consume the eggs and hatchlings.
Mating occurs during neap tides, when the sun, moon, and Earth are all three in alignment and the combined gravitational pull on the water of Earth by the sun and the moon is the strongest. Increases in water temperature as well as an increase in the amount of daylight are two factors that stimulate the migration of horseshoe crabs to beaches for spawning. (Walls, Berkson, Smith 43) Because males generally outnumber females, competition exists during mating. Males will use a modified set of walking legs, called pedipalps, to grab hold of the female during mating. This often leaves a “mating scar” – a good indicator of a horseshoe crab’s age past sexual maturity. Both male and female will come to shore attached. On the shore, females will dig ditches in the sand 10-20 centimeters deep to deposit her eggs. This ensures that the eggs are protected from ocean scavengers such a shrimp or fish that might eat the eggs as well as protection from the weather or the tide turning up the eggs and washing them away from the shore. The females will produce up to 88,000 eggs per year and release up to 4000 eggs during each mating. (Botton 195) Males will release spermatozoa, which washes over the female’s eggs with the waves coming onto shore. The egg will remain in an incubating stage for 2-4 weeks, after which the hatched larvae will free itself from the ditch by digging or wave action. The larvae will then move to deeper waters and reside there for 9-10 years until it reaches sexual maturity and returns back to the sandy beaches to mate. The exoskeleton of the horseshoe crab will shed during this time approximately 16-17 times before sexual maturity is reached when molting and growing will come to an end. (Walls, Berkson, Smith 44)
The total lifespan of a horseshoe crab is around 20 years. Indicators of the age of a horseshoe crab include number and severity of mating scars, abrasion and scratches on the exoskeleton due to exposure to a harsh environment, and the number of organisms, called epibionts, that attach themselves to the dorsal and ventral portions of the horseshoe crab.
Though the horseshoe crab was viewed for years as being a “trash fish” that was carelessly caught and harvested by fishermen and other commercial users, (Walls, Berkson, Smith 63) several ecological benefits arise from this species. Important to birdwatchers, horseshoe crabs draw shore birds, such as red knots, ruddy turnstones, sanderlings, dunlins, short-billed dowitchers, and semipalmated sandpipers, to the Atlantic coast during their migration northward during the 3rd or 4th week of May each year. (Barash, Bavendame 14) (Walls, Berkson, Smith 47) Timing is crucial between the migration of the shorebirds and the arrival onto the sandy beaches of the horseshoe crabs to deposit eggs. The eggs are very high in protein and provide the migrating shorebirds the energy needed to continue their migration. Without this stopping point and energy source the successful migration of the shorebirds as they journey to their breeding grounds would be jeopardized. Environmentalists see the horseshoe crab as a vital link in the population of the shorebirds, and fear that declining number of horseshoe crabs will result in declining numbers of shorebirds as well.
Commercial uses of horseshoe crabs have resulted in a reduction of horseshoe crab numbers. Between the 1870’s and 1960’s, horseshoe crabs were raised on horseshoe crab farms to be ground up to be used as hog feed or fertilizer. Using horseshoe crabs as fertilizer proved to produce an unpleasant odor and competition with other fertilizers soon took over. Horseshoe crabs have also been used by Atlantic Ocean fisherman in the pursuit of eel, whelk, and catfish. The female horseshoe crab in particular releases a chemical, used primarily to attract males during mating, that attracts eels and whelk. Problems arose when each horseshoe crab used was lost or partially eaten upon each catch. With a noticeable decline in the number of horseshoe crab due to this practice, alternatives have been introduced. Although other types of bait were used, such as green crab or surf clams, the catch did not have the same attraction to this bait. Chemicals similar to those released by the female horseshoe crab can now be synthesized in the laboratory and applied to other forms of bait. In addition, new “bait bags” have been used to reduce the number of horseshoe crabs needed for each catch. (Walls, Berkson, Smith 60). In the “bait bag” the horseshoe crab is held in place with straps to protect the crab from predators on the sea floor. Between bird watching and fishing, the horseshoe crab contributes $93-$123 million to regional economies. (Walls, Berkson, Smith 59) Clearly this species plays a vital role in the success and livelihood of business along the Atlantic coast.
In addition to the ecological impact and commercial uses and benefits, the horseshoe crab has led to advances in biomedical technology. Most aquatic bacteria are gram-negative. Gram negative bacteria contain endotoxins as part of their cell wall structure. Endotoxins can cause symptoms such as fever and ache. Because there can be as many as 1 billion bacteria per gram of sand near the coastal shore, and 1 million bacteria per milliliter of salt water, horseshoe crabs have developed a system of protection. When the horseshoe crab is exposed to endotoxins cells within the blood of the horseshoe crab, called amoebocytes, form a clot that stops and destroys the endotoxin. It is the presence of the blood clot that signals that an endotoxin is present. Scientists have used this cue to their advantage; testing medical devices that are placed within the body for the presence of gram-negative bacteria using the clotting agent Limulus Amoebocyte Lysate, LAL. LAL was approved by the Food and Drug Administration in 1970s and found effective in detecting “one millionth of a billionth of a gram of endotoxin in less than one hour”. (Walls, Berkson, Smith 54)
The process begins when horseshoe crabs are collected and taken back to the laboratory for bleeding. A needle is inserted into the cardiac sinus and 100-300mL of blood is drained from the horseshoe crab with the maximum volume of blood available in the animal between 200-300mL. (Walls, Berkson, Smith 56) The blood “turns blue when exposed to air due to a copper binding agent”. (Barash, Bavendam 14) unlike human blood, which turns red when exposed to air due to an iron binding agent. After the blood has been collected, the animal is returned to its capture site within the FDA requirement of 72 hours after capture. (Walls, Berkson, Smith 57) The blood is placed in a centrifuge to separate the amoebocytes from the rest of the blood. The amoebocytes are spun so that they rupture, forming a “lysate”. (Henkel 30) The lysate is freeze-dried in a powder form that resembles table salt until it is ready for use. To activate the lysate, scientists use sodium and calcium or magnesium salts. (Walls, Berkson, Smith 56) Products are tested for the presence of endotoxins, which will cause the formation of a blood clot if the test is positive.
There are currently five biomedical companies located along the Atlantic coast; Associates of Cape Cod (Massachusetts), BioWhittaker (Maryland), Charles River Endosafe Inc (South Carolina), Haemachem (Missouri), and Limuli Labs (New Jersey). All companies must acquire permits to catch the horseshoe crabs for this purpose and must monitor and evaluate the mortality rates of the released horseshoe crabs.
Several advantages result from the bleeding of horseshoe crabs. Biomedical companies can sell the blood to local fishermen. The blood also contains the chemical known to attract eels and whelk and reduces the number of horseshoe crabs used as bait. Bleeding horseshoe crabs is a more ethical practice than methods used prior to the discovery of LAL. Live animals, such as rabbits, bird embryos, and rodents, were tested for the presence of bacteria. Animals were injected with a test solution and monitored for the presence of fever, skin inflammation, or death. Because no current synthetic compound has been produced to take the place of LAL, it remains the most effective and least harmful methods of detecting bacteria. Its uses include “diagnosis of endotoxemia in conjunction with cirrhosis, cancer, meningitis, eye disease, dental problems, gonorrhea, boutonneuse fever, water quality analysis, urinary tract infections, and bacterially contaminated meat, fish, and dairy products. LAL is also used to test “catheters, pacemakers, and invasive devices” as well as any drugs used in medicine. (Henkel 30)
Activists are concerned about the impact the bleeding has on the overall health and mortality rates of the horseshoe crab after bleeding. It takes the horseshoe crab 3-7 days to regain back its blood volume and approximately 4 months to regenerate its white blood cell count. (Walls, Berkson, Smith 56) Studies following the bleeding of horseshoe crabs have looked at factors such as the effect of the rate of movement that bleeding might cause. Sonic transmitters are attached to the prosoma of bled and non-bled horseshoe crabs to monitor the average rate of movement. Downfalls to this study included the transmitter only working in water – any horseshoe crabs on shore would not send a signal. The transmitter was also only detectable within 1000 meters. Although there was no significant difference in the rates of movement between bled and non-bled horseshoe crabs, it was noted that bled horseshoe crabs tended to have more random patterns of movement, while non-bled horseshoe crabs had more directional patterns of movement. Researchers believe that this difference might be the result of “possible disorientation from the procedure and the loss of blood”. (Kurz, James-Pirri 267) Researchers are concerned that directional movement is necessary for horseshoe crabs to migrate successfully to sandy beaches during mating and that bled horseshoe crabs might result in a reduced number of successful breedings. (Kurz, James-Pirri 267) Additional studies have focused on the possible increased mortality rate in bled horseshoe crabs compared to non-bled horseshoe crabs. Horseshoe crabs are collected, bled, tagged, and released along with horseshoe crabs that were not bled. Out of 10,000 crabs collected, 5,000 bled and 5,000 not bled, researchers found an increased mortality in bled horseshoe crabs by 10%. Further studies have found the mortality rate to reach up to 15% of an increase. Problems arise as each year the number of horseshoe crabs collected and bled continues to rise. In 1983, the number of bled horseshoe crabs was 30,000, by 1989 that number had increased to 130,000, and by 1997 the approximated number of bled horseshoe crabs was 260,000. (Walls, Berkson, Smith 57) Increases in the number of horseshoe crabs caught and bled increases the number of horseshoe crabs that will not survive once released.
Additional uses of horseshoe crabs include the advancement of knowledge about the history and lifestyle of trilobites. Researchers at the University of California are creating 3-D models of the trilobite and the horseshoe crab. The purpose of the study is to “learn about how trilobites interacted ecologically and biologically with other organisms” using “similar creatures in modern environments and comparing them with animals in ancient systems”. (Baumgartner 124) By examining the number of and location of epibionts located on horseshoe crabs, scientists can make prediction about the behavior and lifestyle of trilobites containing similar traces of epibionts preserved in the fossil record.
With the demand for horseshoe crabs putting strain on the supply available, efforts to protect this species are currently in place. In 1998 the Atlantic States Marine Fisheries Commission, ASMFC, established the first coast-wide management plan for horseshoe crabs within nonfederal waters. Under this plan, fishermen must follow quotas that restrict the number of days and areas that they may harvest horseshoe crabs. All sides are represented and included in the overall protection plan. A Horseshoe Crab Technical Committee provides scientific data to the management board. Data such as population status surveys and samplings of egg counts are among a few of the types of data that this committee presents that is of interest of birdwatchers and environmentalists. A Horseshoe Crab Advisory Panel provides guidance to the management board from the viewpoint of fishermen and industries that benefit from the use of horseshoe crabs. Unfortunately, although management plans are currently in effect, collecting data that is sufficient and consistent continues to remain a problem to scientists and concerned activists. Still the awareness of the importance of the horseshoe crab remains an issue and further studies and methods of collecting data are under way.
Horseshoe crabs have existed on this planet for millions of years – outliving its closest relative the trilobite. As resilient and unchanging as this species continues to be, with the changing demands of humans we place stresses on the horseshoe crab that it might not otherwise experience. Because it plays a crucial role in shorebird migration, commercial fisheries, and advances in biomedicine, this “living fossil” continues to leave its trace in ecological and biomedical history. Certainly the horseshoe crab is no “trash fish”.
Avise, John C., William S. Nelson, Hiroaki Sugita. “A Speciational History of “living Fossils”: Molecular Evolutionary Patterns in Horseshoe Crabs”. Evolution. Vol. 48 No. 6. (1994) 1986-2001
Barash, Leah, Fred Bavendam. “Mass Appeal”. National Wildlife. Vol. 31 No. 4. (1993) 14
Baumgartner, Henry. “An Old Fossil Goes High-Tech”. Mechanical Engineering. Vol. 122 No. 9. (2000) 124
Botton, Mark L. “Horseshoe Crabs”. Biologist. Vol. 49 No. 5. (2002) 193-198
Henkel, John. “Drugs of the Deep”. FDA Consumer. Vol. 32 No. 1. (1998) 30
Kurz, W., M.J. James-Pirri. “The Impact of Biomedical Bleeding on Horseshoe Crab Limulus polyphemus, Movement Patterns on Cape Cod, Massachusetts.” Marine and Freshwater Behavior and Physiology Vol. 35 No. 4. (2002) 261-268
Pechenik, Jan A. Biology of the Invertebrates Fourth Edition. New York: McGraw Hill, 2000.
Walls, Elizabeth A., Jim Berkson, Stephen A. Smith. “The Horseshoe Crab, Limulus polyphemus: 200 million Years of Existence, 100 Years of Study”. Reviews in Fisheries Science. Vol. 10 No. 1. (2002) 39-73
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