Fish are diverse — each species has evolved to live successfully in its specific underwater environment, from streams and lakes to the vast expanse of the ocean. However, all fish share some common evolutionary adaptations that help them thrive in their watery domain. Species of fish also vary from other water-dwelling creatures, such as whales and dolphins (which are mammals) and turtles (which are reptiles). The ocean, alone, is home to around 18,000 species of fish, that humans know of — and there are many more scientists have yet to discover. While there are some exceptions to these rules, below is a list of the traits common across these aquatic animals.
How Fish Breathe Underwater
All fish have gills present from the time they are born until they die. Gills are important organs for a fish, as they are how fish breathe. They help to absorb oxygen from the water and give off carbon dioxide. The gills are located on either side of the fish's head.
Fish Are Coldblooded
Fish are ectotherms, or cold-blooded species. They cannot regulate their body temperature and are dependent on the external environment to get their warmth. A fish's body temperature fluctuates according to the water's temperature and, like lizards, cold water can make them sluggish. Some fish in colder bodies of water, like lakes, will go dormant during winter months.
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Fish Can Detect Nearby Movements
Fish have a specialized sense organ called a lateral line that runs along the length of the body. 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 this specialized organ.
Swim Bladders Help Fish Sleep
All fish have a swim bladder, which is filled with air and helps to ensure that the fish maintains a stable buoyancy in the water, neither sinking nor floating too much. The presence of a swim bladder allows the fish to sleep in water without sinking to the bottom of its habitat. In some species of fish, air is swallowed and sent to the swim bladder. This adaptation also helps the fish to survive in waters that do not have adequate levels of oxygen.
Fins Propell Fish Through Water
Fins are common to all fish. Pelvic and pectoral fins allow the fish to maneuver and maintain its stability while dorsal and ventral fins reduce the rolling motion while the fish is swimming and aids the fish during turns. The tail fin propels the fish forward while swimming.