Introduce your students to the anatomy of the horseshoe crab and how it differs from the anatomy of true crabs. Unlike true crabs, horseshoe crabs do not have antennae or a pair of mandibles -- and horseshoe crabs have seven pairs of legs, while true crabs have only five. Pass out an anatomy diagram of the horseshoe crab with the names of its body parts blanked out, and fill in the names with your students.
Have students handle a live horseshoe crab in the classroom. Though horseshoe crabs might look prehistoric and perhaps even frightening, they are completely harmless. Horseshoe crabs can be purchased from dealers for usually about $10 to $12 each, depending on their size. Remove the horseshoe crab from its tank and review with students its anatomical structures. Then, in groups of two to four, have students carefully touch the telson, legs, pincers and shell of the horseshoe crab. Afterwards, make sure that students wash their hands thoroughly with soap and warm water.
Discuss with your students the concept of a “keystone species” -- that is, a species on which many other species in an ecosystem depend, the removal of which would drastically change the ecosystem. Ask your students to think of what sorts of things would cause an organism to be a keystone species -- for example, an organism that other species depend on for food, or a predator whose removal would cause other species to overpopulate. Discuss the horseshoe crab as a keystone species -- its eggs are a primary source of food for migratory shorebirds. Have students draw a food web with the horseshoe crab at the center of the web, then discuss what would happen if the horseshoe crab disappeared.
Organizations such as the New York Horseshoe Crab Monitoring Network and the Delaware Bay Horseshoe Crab Census often look for volunteers to help in conducting censuses of local horseshoe crab populations. If you live near a horseshoe crab habitat, consider contacting one of these organizations for information on having your class volunteer. Volunteering with a horseshoe crab census is an excellent way of giving your students firsthand scientific experience -- and helping out a great cause as well.