Sharks
Sharks are any of the 340 species of fish that, together with the skates, rays, and chimeras, are sharply distinguished from the vast number of bony fish species by their cartilaginous skeletons. Sharks are very keen-sensed fishes, many species of which are able to hunt and eat nearly all the larger water animals in both shallow and deep seas. These two features account for their long evolutionary history. Many of the shark species living today are quite similar to abundant species that swam in seas of the Cretaceous period more than 100 million years ago. Sharks reveal great diversity in behavior and size. The whale shark is the largest shark and also the largest fish in the sea, measuring up to 15-m (49f) in length. The cookie-cutter shark measures less than 50 cm (19in) in length. Sharks are mainly marine fish found in all seas and are especially abundant in tropical and subtropical waters. Many types migrate up rivers. Sharks are best known as aggressive carnivores that even attack members of their own species. Two of the largest sharks-the basking shark and the whale shark-feed on plankton, which they strain from the water with gill rakers. Altogether sharks are very different from other animals because they all ha
Usually five gills slits lie behind the head. Sharks do not have swim bladders and, when motionless, sink to the bottom. The sand tiger shark, for example, bears only two young at a time. The eggs either hatch inside the female or soon after they are laid. Most sharks eat their prey whole or they tear off large hunks of flesh. Sharks coloration often provides a clue to it's preferred habitat. They have strong digestive enzymes and a specialized epithelial fold that spirals the length of the small intestine, enabling the fish to take in different foods. Fertilization takes place internally, the male inserting one of its claspers into the female. Sharks live in oceans throughout the world but are most commonly found in warm and temperate climates and waters. This combination of keen senses accounts for the success of sharks. Males can be identified by claspers, which are extensions of the pelvic fins that serve as reproducing organs. Sharks, to a large extent, feed on dead plants or animals. Sharks are particularly sensitive to sounds of low frequency and have fine directional hearing. The young are frequently born in protected inshore areas away from the males. Sharks have highly interesting daily habits different from that of humans.
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