Sharks

SharksSharks are surely one of the most fearsome creatures on Earth. But they are also one of the most sophisticated and enduring. The oldest shark fossils date from more than 300 million years ago, before the age of the dinosaurs. And a few existing shark species, such as the hornshark, have kept the same basic physical characteristics for more than 150 million years. Scientists attribute this remarkable longevity to the shark's superior physiological developments. Sharks are outfitted with several special characteristics that make them highly effective, both as hunters and survivors. In this article, we'll examine these unique qualities to see why the shark is top dog in the ocean. Shark AnatomyWhen people think of sharks, they generally imagine an animal like the one in this picture. The tall dorsal fin, torpedo-shaped body and giant teeth of this great white shark are familiar to most everybody. But there are actually more than 400 different shark species alive today, and they vary considerably in size and appearance. In fact, roughly 50 percent of shark species are less than a meter long. So what makes a shark a shark? One of the most well-known sharks, the great white用心 爱心 专心Sharks, along with rays and chimeras, are distinguished from other fish primarily by their body composition. Most other fish have skeletons made of bone, just like mammals, reptiles, amphibians and birds. Sharks and rays, on the other hand, have skeletons made entirely of cartilage, the same flexible material in your nose and ears. Cartilage is sturdy like bone, but it has a much lower density. This material keeps sharks relatively lightweight, so they don't sink in the ocean and they don't need an air bl...