Xiphosura – Horseshoe crabs

These creatures are often called “living fossils” because they have remained relatively unchanged since the time of the dinosaurs. One of their most distinct features is their hard, dome-shaped shell and long, rigid tail spine called a telson. While the telson might look like a weapon, it’s not used for defense—it actually helps the animal flip itself over if it gets stuck on its back. Unlike crabs, which they are often mistaken for, horseshoe crabs are more closely related to spiders and scorpions, sharing more features with arachnids than with crustaceans.

Their unique body structure sets horseshoe crabs apart from other sea creatures. They have compound lateral eyes, like insects, but also five additional simple eyes on top of their shell. This gives them a wide range of vision, allowing them to sense both light and movement very effectively. Their mouth is located in the center of their legs, and they actually use their legs to push food toward it—almost like walking and eating simultaneously. Another striking difference is their blue-colored blood, which contains copper instead of iron. This blood is not just unusual; it’s also extremely valuable in medicine. Scientists use it to test for harmful bacteria in vaccines and medical equipment, making these animals vitally important to human health.

Despite their armored look, horseshoe crabs are gentle creatures that spend most of their lives on sandy or muddy sea floors, feeding on worms and small clams. They play an important role in coastal ecosystems, particularly during their spawning season. When thousands of horseshoe crabs come ashore to lay eggs, shorebirds, like the red knot, rely on those nutrient-rich eggs as a critical food source during long migrations. Their ancient survival strategy—emerging in massive groups—ensures that even though many eggs are eaten, enough survive to continue the species.