Fish are cold-blooded aquatic animals, which inhabit various water bodies on the planet. There exist thousands of fish species in the world, which have adapted to the specific environment that they inhabit. Different varieties of fish are found in different habitats, ranging from deep oceans to shallow streams. It is very important to get well-versed with their anatomy to understand how they are able to sustain in diverse habitats. Shape The shape of their body corresponds to the habitat that they dwell in. Surface-dwelling fish have a flat back, while bottom-dwellers have flattened bellies. Species living in slow-moving water have broad, laterally compressed bodies, while those living in fast-moving water have slender bodies. These variations make it easier for them to swim in their respective habitat. An air filled bladder, known as the swim bladder, helps them remain in a neutral state of buoyancy, wherein they neither rise to the surface, nor sink to the bottom. Mouth Their mouth indicates the feeding habit of the species. Surface-dwelling fish, which feed on insects present on the surface of the water, have an upturned mouth, which makes it easier for them to swoop on their prey. In contrast, bottom-dwelling fish have an underslung mouth, often accompanied with whiskers, which act as radars, helping them locate their prey in deep dark waters. Their suction cup-like mouth helps them feed on algae and plants growing on the ocean floor. Similarly, the mouth of a predatory fish is wider than that of an omnivorous fish. Fins The fins are utilized for movement, stability, and, in some species, as tactile organs. The tail fin, also known as caudal fin, is used as a propeller. Forked tail fins help the fish swim faster and thus, is usually noticed in fast-swimming species. Predatory fish have rounded caudal fins, which help them get into action swiftly. At times, large, elongated fins are noticed in some female species. They use these to attract males. The single anal fin, located on the underside of the body, helps them remain stable while swimming. In some species, anal fins are relatively long and are used as propellers along with caudal fin. The paired pelvic fins, also known as the ventrals, are long and thread-like in nature. These multipurpose fins act as tactile organs, providing stability to its movement, and also help species like catfish to carry their eggs while breeding. The paired pectoral fins, which are used by the fish for maneuvering, are located near the gill cover. Pectoral fins, armed with spines, are used for defense against lurking predators. The dorsal fin, located on the back, helps them balance their body while swimming. Gills These are the respiratory organs, which help fish extract oxygen dissolved in water and excrete carbon dioxide. Though many aquatic animals do not require gills for respiration, as their modified skin allows them to breathe through the entire body, it is observed that those species which use gills are more active than those who don't. Some species of fish swallow water and store it in the swim bladder, where the oxygen is extracted. Scales Scales are rigid plates which form a protective covering on their body. In species like the catfish, scales are replaced by bony plates as their protective covering. The fluid-filled ducts located under these scales are designed to feel and pick up vibrations in water. This helps them detect predators and find food. Because of this mechanism, many species of fish can move through the dark parts of the ocean that are too deep for the sunlight to penetrate. Color Color plays an important role for several species of fish. The color of the fish is determined by pigmentation for species that are dark in color, and by light reflection for those having a silvery appearance. Color helps in camouflaging, which is essential for predators as well as prey. And lastly, in some species, bright colors are used to attract members of the opposite sex. Over the period, fish have adapted themselves to a wide range of conditions, which, in turn, has helped them survive against the odds. It is this very adaptability that has helped them survive for more than 450 million years now, while other species, which came after them, including the dinosaurs, have become extinct.