Sharks essays

Sharks essays 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 at tack 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 have unique physical characteristics, mating habits, and feeding habits. Most sharks are gray in color and have leathery skin covered with small, sharp, pointed scales, which, unlike those of bony fish, do not enlarge during the animals growth. Usually five gills slits lie behind the head. The tail is asymmetrical, with the backbone extended into the upper lobe. Many species have rows of sharp teeth embedded in fibrous membranes instead of in the jawbones. When the sharks teeth are lost in their prey, other teeth quickly shift into position to replace the teeth that they lost. The fins and tails of sharks are rigid and not erectile, like those of the bony fish. Contrary to popular image, the dorsal fins rarely stick ...