An Ancient Crab PDF by Annie Kiyonaga

# An Ancient Crab by Annie Kiyonaga An American horseshoe crab in the Delaware Bay region The American horseshoe crab, also known as the Atlantic horseshoe crab, has been crawling on the ocean floor for a very long time. In fact, this crab has been around for longer than most other animals on Ear...

# An Ancient Crab by Annie Kiyonaga An American horseshoe crab in the Delaware Bay region The American horseshoe crab, also known as the Atlantic horseshoe crab, has been crawling on the ocean floor for a very long time. In fact, this crab has been around for longer than most other animals on Earth! This sea creature is one of the oldest animal species still alive today. The American horseshoe crab has been around for at least 450 million years. To put that in perspective, dinosaurs were alive between 66 and 245 million years ago. That means that American horseshoe crabs were scuttling along the ocean floor 200 million years before dinosaurs even existed! So why has this ancient species survived for so long? Well, American horseshoe crabs have bodies that are built for survival. Like many other crabs, they have a tank-like shell, or exoskeleton, that protects them. These exoskeletons are skeletons that grow on the outside of their bodies. As the crabs grow larger, they molt, or shed their exoskeletons. They usually molt 16 or 17 times before they reach their adult size. Below these exoskeletons, American horseshoe crabs have five sets of legs, a tail, and one pair of feeding pincers. They use their legs to grind up their food, and then they use their pincers to feed the food into their mouth. For American horseshoe crabs, meals are usually other invertebrates, which are animals without backbones. For example, they often eat worms and mollusks. One especially interesting part of the American horseshoe crab's body is its eyes. American horseshoe crabs have ten eyes. Some of these eyes are on the tops of their shells, and some of them are on the bottoms of their bodies. Even their tails are part of their vision systems! There are photoreceptors on the American horseshoe crab's tail. These photoreceptors aren't "eyes" in the way that we might understand them. The crabs don't use them to see their surroundings. Instead, these photoreceptors help regulate the American horseshoe crab's body clock. American horseshoe crabs usually look for food in the dark, at night. The photoreceptors on their tails are sensitive to changes in light so that the crabs know when to hunt for food. Another unique part of the American horseshoe crab's body is its copper-based blood. Unlike human blood or most animal blood, the blood in American horseshoe crabs' bodies is bright blue when it's exposed to the air. The copper base of the crab's blood gives it its distinctive color. Their blood also contains highly sensitive immune cells. When a toxin from bacteria gets into an American horseshoe crab's blood, its blood cells clot around the invader. This protects the crab's body from the toxin. In addition to protecting the American horseshoe crabs, these immune cells can also help protect humans! In fact, scientists have used these immune cells to test vaccines for contamination. By using the crab's blood, scientists can make sure that vaccines are clean of any toxins from bacteria and are safe for humans.