Fish are a group of aquatic animals with skulls, gills and digitless limbs. They are separated into four groups: cartilaginous fish (such as sharks and rays), bony fish, jawless fish, and hagfish. Living in water presents a number of problems such as maintaining salt concentrations and neutral buoyancy and this group of animals have evolved a number of ways to deal with these issues. Fish were the first vertebrate animals to evolve and have since given rise to all the vertebrate animals found in water and on land. Fossils of fish have been dated back to the Cambrian period around 530 million years ago during a time when the diversity on Earth was going through a massive explosion. Currently there are more than 30,000 species found around the world with the largest diversity found in coral reefs in the world’s tropical regions. Sharks are the ultimate predators of the oceans and have developed many adaptations to help them hunt, reproduce and survive in their marine environment. Sharks have a skeleton made from cartilage rather than bone. Eels are fish with elongated, slender bodies. Around 800 species of eels are found all around the world. They have interesting life cycles and complete immense migrations between the freshwater and the oceans. Just like all other animals, fish need oxygen to survive. Instead of breathing air into lungs, they have gills that absorb oxygen directly out of the water as they swim. Most fish species have four gills found on either of their head. No group of fishes have conquered the sea more than the teleosts. They are the most advanced of all fishes and are dominant in both marine and freshwater habitats. An incredible diversity of teleosts are found throughout the world.