Feared by most， loved my some and hunted by many， sharks are one of the most mysterious groups of creatures roaming the Earth today. Defined as a fish with a full cartilaginous skeleton and a sleek， streamlined body， a shark can range in size from the two foot pygmy shark， to the colossal 50-foot whale shark. There are more than 250 different species of sharks currently identified， making it one of the most diverse animal genera on the planet. Sharks are found in every major body of saltwater in the world， but are more common in warmer waters. There are a small number of shark species that do thrive in fresh water， and certain sharks have been known to venture from their saltwater homes to major freshwater lakes and rivers. Most members of the Shark family are predatory， though some are more ferocious than others. Ironically， the largest of the shark species， the whale shark and basking shark， are virtually harmless plankton eaters. Sharks have an extraordinary sense of smell when it comes to hunting prey， and have been known to be able to sense visually undetectable amounts of animal blood dispersed into the water from considerable distances.