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    Limulidae – Horseshoe crabs

    Their tail, though it looks like a weapon, is mainly a balancing tool to flip them upright when waves knock them over

    One of the most remarkable groups in the animal kingdom. They are often described as “living fossils” because their ancestors first appeared over 200 million years ago, long before the first dinosaurs roamed the Earth. Unlike many other arthropods that evolved into countless shapes and forms, Limulidae stuck with a design that simply worked. Their protective dome-shaped shell, hinged body, and long, spiky tail have barely changed since prehistoric times. This ability to survive multiple mass extinctions makes them stand out from other marine creatures, even among arthropods. What looks like a crab is, in fact, more closely related to spiders and scorpions, reminding us how surprising evolutionary relationships can be.

    One of the defining characteristics of Limulidae is their unusual body structure. They have ten legs, which they use for both walking and pushing food toward a mouth located in the center of their body. Their long, sword-like tail, called a telson, may look dangerous, but it is mostly a balancing tool to flip themselves over if a wave knocks them upside down.

    Unlike true crabs, Limulids have compound eyes on the sides of their shells, giving them excellent vision in murky waters, along with five simple eyes on top that can sense changes in light. Perhaps the most famous feature of Limulidae is their blue blood, which contains copper rather than iron. This blood clots instantly in the presence of harmful bacteria, making it one of the most valuable substances in modern medicine. Scientists use it to test vaccines, surgical tools, and implants for contamination, meaning these ancient creatures play a vital role in keeping humans safe.