Each spring horseshoe crabs leave the ocean depths and enter shallow waters to spawn. Their arrival is awaited by shorebirds, like the red knot, who feed on nutrient-rich horseshoe crab eggs to fuel their migration north, and by fishermen with permits to use horseshoe crabs as eel and conch bait. More recently, the crabs have also been greeted by volunteers for Project Limulus, a research project based at Sacred Heart University in Fairfield, CT. The goals of Project Limulus include determining population trends of horseshoe crabs (Limulus polyphemus) in Long Island Sound, and discovering whether they return to the same beach each year to lay their eggs.
Eager citizen scientists search the shoreline each May and June during the full and new moon high tides, when the crabs are most likely to come ashore to breed. After counting the crabs, they tag them, by making a small hole in the carapace with an awl, and inserting a round, white plastic tag. Each tag has a unique number, as well as a phone number (1-888-LIMULUS) to call if the tagged crab is found in the future. As more and more tagged crabs are recaptured each year, the researchers get a better idea of where the crabs travel from one summer to the next.
For the past two years, Madison fourth-grade students at Jeffrey, Ryerson and Island Avenue Schools have assisted with the research. Each class visits Circle Beach during a spring high tide to search for and tag the crabs, as well as study the coastal ecosystem. The field study complements the fourth-grade science unit on wetlands and creates a memorable hands-on experience for the students. The children measure each crab, determine if it is male or female, and estimate the percent cover of epibionts (barnacles and other living organisms attached to the carapace), which may indicate the animal’s relative age. In addition, students use nets and shovels to sample horseshoe crab prey, and measure the estuary’s temperature, salinity and pH. Just like Project Limulus researchers, the students come to understand the horseshoe crab’s population ecology, or the ways it’s linked to other organisms in its environment. And hopefully, they also discover that science is fun!
Fortunately, all classes have discovered horseshoe crabs, with 81 animals tagged so far this year, exceeding the number tagged at Circle Beach last year. As exciting as it is to find new animals to tag, the focus of their quest is to discover an animal that had been tagged in years past. The lucky student who finds a tagged crab must report its number to Project Limulus. Soon afterwards, he or she will receive a thank you certificate with the date and place the crab was originally tagged, as well as a pewter horseshoe crab pin. Four tagged animals have been recaptured already this year.
Over 300 million years old, horseshoe crabs truly are living fossils. Despite their name, they are more closely related to spiders and scorpions than to crabs. The animal’s tail, or telson, is not a stinger, but is simply used to turn itself over. Not only are horseshoe crabs harmless to humans, but we actually benefit from cells found in their blood. Horseshoe crab blood is blue, due to the presence of copper-based hemocyanin. The blood is used by the pharmaceutical industry to test vaccines and medical equipment for the presence of bacteria. In fact, federal law requires that before any drug or device is inserted into the human body, it must be tested for bacterial contamination with Limulus Amoebocyte Lysate (LAL), derived from horseshoe crab blood! Madison students have been able to observe the blue blood before inserting the tag into the horseshoe crab’s shell.
So if you find a horseshoe crab at the beach this summer, don’t be afraid – they don’t bite, pinch or sting. If the animal you find is tagged, call the phone number on the tag, and report where and when you found the crab as well as the number on the tag, and the condition of the animal. You can be part of the education and research involving this prehistoric animal that is an integral part of our environment. To learn more about Project Limulus, visit www.projectlimulus.org.