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ATLANTIC HIGHLANDS HERALD
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THE HORSESHOE CRAB: 350 MILLION YEARS OLD AND COUNTING If you have spent time on Sandy Hook Bay beaches recently than you have most likely seen our population of horseshoe crabs. Around May, June, and July the species appear to be all over the bayside area, as they are in the process of mating and depositing eggs in the sand. Yet, horseshoe crabs are more charismatic and interesting than just their propagation activities. For starters, horseshoe crabs are not really crabs at all. So extraordinary is the natural history of horseshoe crabs that they are grouped between crustaceans (which are mainly arthropod invertebrates, including the crab, lobster, and shrimp) and arachnids (which are eight-legged land invertebrates that include scorpions, spiders, mites, and ticks). Horseshoe crabs are classified in a group by themselves, called Merostomata. Our local species is classified as Limulus polyphemus. Since the horseshoe crab is one of the oldest species on earth that has not evolved in structure or anatomy, it is a living fossil. Take a time machine to the age of dinosaurs and you will find horseshoe crabs existing in the same shape and lifestyle they are presently living. Horseshoe crabs are over 350 million years old, which makes them 100 million years older than dinosaurs. In fact, the species may be over 350 million years old, as fossil horseshoe crabs have been found in rocks that date back to 500 million years ago. Certainly, horseshoe crabs are an historic species. With horseshoe crabs existing so long, you would think they must have the perfect physique. Yet, the species is notorious for having bad eyesight, for being an awkward swimmer, and for not being able to defend themselves against predators. All this seems to be okay, however, for the remarkable horseshoe crab has as equally remarkable shell. On the outside, the dark brown shell camouflages the species. The head and thorax is covered above with the hard, thick, horseshoe-curved shell and the small abdomen is covered with a narrow shell hinged from the main shell at the front and terminating in a long, sharp spine. The animal has a large compound eye on each side of the shell, with two pairs of smaller, simple eyes between the compound eyes, and five light-receptive eyes beneath the shell. The mouth is in the middle of the underside, which is placed perfectly for seizing and grinding food (such as clams, mussels, and worms) as it walks along the sea floor. During the winter months, the species will burrow under ocean or bay sand, where it feeds on small invertebrates. It takes approximately ten years for a male horseshoe crab to become an adult and approximately a year longer for females to mature. Females are generally two-thirds larger than males, since they sustain and bear eggs. In late spring, both males and females will travel to beaches in the intertidal zone of bays and estuaries that their ancestors have traveled for thousands of years to mate and lay eggs. Females will deposit about 10,000 eggs during several trips to sandy shores. During each trip, a female will dig into the sand and drop about 4,000 eggs. Sand will cover the shells to protect the eggs from predators, and keep them moist and allow oxygen to reach the young. In about 14 to 30 days, the young will break free of their shell and travel to the sea. While 10,000 eggs may seem like a lot, only one out of 130,000 eggs will reach adulthood. The eggs of the horseshoe crabs are an important source of food for turtles and migrating shorebirds, some of which are endangered, such as the red knot (Calidris canutus). Birds will travel thousands of miles from their South American wintering grounds to Sandy Hook Bay to rest and specifically feed on a plentiful supply of horseshoe crab eggs before they depart to breeding grounds in Alaska and northern Canada. Without Horseshoe crab eggs, most migrating birds will not have enough energy to continue their migration path. Consequently, species of birds can quickly become threatened with extinction. Unfortunately, the population of horseshoe crabs in the Northeast is declining. While they have survived over 350 million years of enormous changes to the earth and have seen the dinosaurs come and go, the species cannot survive the next 100 years without our help. Their population is heading downward due to excessive human development causing water pollution, loss of habitat, and the overharvesting of the species for fishing bait and fertilizer. What can be done to help save the horseshoe crab? For starters, believe it or not over 10 percent of the population will die because they are unable to flip themselves over onto to their feet due to rough surf turning them over or humans handling them in a wrong manner. By gently flipping over a stranded horseshoe crab by their shell (not their tail) when you see one on the beach you can easily promote the survival of the species. Don't be afraid of getting hurt, they are peaceful creatures that do not bite or sting. In addition, since the extinction of most species is caused by changes in the environment, such as habitat loss or disruption, the best way to save horseshoe crabs is to preserve their natural environment. It is clear that simply preserving a few unique habitats is not working. A more comprehensive approach is needed. Therefore, the use of buffer zones and natural corridors to create more wilderness areas, the purchase of environmentally sensitive sites for open space, and better land-use management practices will help to increase horseshoe populations and the general health of the Sandy Hook Bay ecosystem. For more information on horseshoe crabs, you might want to check out the following three web sites: http://www.state.nj.us/drbc/crab/crabhome.htm sosap@gateway.net - contact Joe Reynolds |
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