The Atlantic, or American, horseshoe crab (Limulus polyphemus) has existed largely unchanged for over 100 million years. Millions of individuals are commonly observed ashore in spring and summer months during spawning events along the entire North American coastline expanding from the East to the Gulf coasts of the United States and Mexico. Other species can be found in the Indian and Pacific Ocean. The massive deposit of eggs in nearshore sand provides a critical source of food for endangered migrating birds, especially the Red Knot (Calidruis canutus rufa) in the Delaware Bay. Horseshoe crabs are also an important component of the sea turtle diet. In addition to the ecological importance, horseshoe crabs are used commercially for bait in eel and conch fisheries and for biomedical purposes in the production of Limulus Amebocyte Lysate (LAL) to detect bacterial toxins in injectable drugs and implantable devices. Commercial demands have led to population declines in some regions. Fisheries are regulated by state and the current International Union for Conservation of Nature (IUCN) listing for L. polyphemus is vulnerable.

A small number of individuals are housed in public aquaria for educational purposes. With growing interest in animal welfare, the health and stability of populations, and potential stressors that can contribute to decline , it is important to have clear and detailed descriptions of horseshoe crab anatomy and necropsy techniques. The purpose of this guide is to illustrate the normal anatomy and the step-by-step technique for dissection of horseshoe crabs. The contents are largely excerpts of the master’s thesis of artist, Katie (Bergdale) Roorda, which was based on photographs from C. Meteyer documenting the sequence and procedure used for necropsy dissection.