Superficially, a sea is just a body of salt water. But if you dive deep inside, you will find another world, full of some of the strangest creatures on the planet. Fish of all shapes, sizes and colors swim in the oceans while the crabs scurry into their burrows and giant squids see everything with their large eyes. Aquatic life is much more different than life on land. Aquatic animals are designed and shaped for their type of surrounding. Many of them cannot survive on land, including fishes. Fishes belong to kingdom Animalia and phylum Chordata. They differ from other chordates because they bear gills and lack limbs with digits. The limbs of fishes are modified into fins, which help them swim. Their skins are covered with scales. Slimy mucus covers scales on skin of most of fishes and protect the fish's skin. Many organisms with “fish” in their names are not actually fishes, like starfish, jellyfish and shellfish. Fishes are distributed in a wide variety of habitats, marine water and freshwater. Fishes living in sea differ from those living in freshwater. Even though much of their lifestyle is similar, there is one key difference. Unlike freshwater fishes, a sea fish is at risk of losing its body water through osmosis. To prevent this, it excretes concentrated waste. A fish in sea swims is like the one in freshwater in all other aspects. While swimming, it flexes its body in such a way that some waves of contraction starts forming. These waves help the fish to move forward and push the water backwards. The tail fin helps to steer. Water pushes through the gills, and the dissolved oxygen diffuses into the fish’s blood through the fine capillaries in the gills. This blood goes round once before returning to the gills again. The blood is pumped by a twochambered heart. A fish’s nervous and reproductive system are unique, too. When it hunts, it uses 'Lateral Line System' which consists of some specialized cells that detect prey and avoid predators. It is the fastest reaction known in the animal world. Many fishes lay eggs, but some also give birth to their young ones. A female fish can lay over millions of eggs, which are, then, fertilized by male fishes. Those female fishes who give birth to young ones, retain their eggs and nourish the embryos. A fish’s life in sea is fascinating, but dangerous as well. A fish has to acquire its food while keeping a lookout for potential predators. It has to create a new generation of fishes. Add to that the strain human meddling with their ecosystem. It is not easy being a fish.