Fish are diverse, as each species has evolved to live successfully in its specific underwater environment. Fish are categorized based on their skeletal material. For instance, sharks and rays are classified as cartilaginous fish, as their skeletal system is made of cartilage. However, some characteristics are common to all fish, and these characteristics distinguish fish from other water-dwelling animals, such as whales and dolphins, which are mammals, and tortoises and turtles, which are reptiles.
All fish have gills present from the time they are born until they die. Gills are important organs for a fish, as they are responsible for respiration. They help to absorb oxygen from the water and give off carbon dioxide. The gills are located on either side of the head.
Fish are ectotherms, or cold-blooded. They cannot regulate their body temperature and are dependent on the external environment. A fish's body temperature fluctuates according to the water's temperature.
Fish have a specialized sense organ called a lateral line. It runs along the length of the body and is situated just under the scales. It consists of ducts that are filled with a fluid. The lateral line can detect vibrations and movements in the water. Even if there is no light, the fish can detect food and predators, and even navigate with the help of lateral line.
All fish have a swim bladder, which is filled with air and helps to ensure that the fish does not float or sink. The presence of a swim bladder allows the fish to sleep in water without sinking. In some species of fish, air is swallowed and sent to the swim bladder, where the oxygen is extracted. This adaptation helps the fish to survive in waters that do not have adequate levels of oxygen.
Fins are common to all fish. Pelvic and pectoral fins allow the fish to maneuver and maintain its stability. Dorsal and ventral fins reduce the rolling motion while the fish is swimming and aids the fish while turning. The tail fin propels the fish forward while swimming.