Why is the horseshoe crab called a living fossil?
Horseshoe crabs are marine arthropods with dome-shaped shells that resemble crabs but are closer to spiders and scorpions.
Body plan, feeding and the marine environment
Fossils with broadly similar body plans extend hundreds of millions of years, leading to the term living fossil. It does not mean evolution stopped; modern species have undergone genetic and ecological change. Walking legs and book gills lie beneath the body, while several light-sensitive eye structures occur above. Relationships among marine invertebrates cannot be inferred from appearance alone because similar forms can evolve in different lineages. Pressure, light, temperature, salinity and currents shape feeding and movement. Laboratory observations may not represent the full behavior seen in nature.
Ecosystem function, defense and conservation
Mass spawning on beaches supplies abundant eggs to birds and fish. The clotting system in their blue blood has been used to test medical products for bacterial toxins, but harvest and bleeding can cause mortality. Synthetic alternatives, harvest limits and protected spawning shores can reduce pressure. These animals may shape ecosystems through filtration, decomposition, sediment mixing or reef building. Warming, deoxygenation, acidification, plastic, seabed damage and overharvest are considered together. Conservation targets habitat processes as well as individual species.