Fish have many characteristics, but the most important ones can be summarized in four main aspects. First, all fish live in water, and even though some can leave the water for short periods, they must return to the water after a while. Second, fish breathe using gills, which allow them to extract nutrients from the water. They do not have limbs, but instead have fins, which help them swim and maintain balance in the water. Additionally, most fish reproduce through egg-laying.
- Living in Water
All fish live in water, and while some can leave the water for short periods, they cannot survive without water for long. For example, mudskippers can go ashore during low tide to search for food, but before they dry out from the sun, they must return to the water. The fish that can stay out of water the longest are African lungfish, which burrow into the mud during dry seasons and survive by entering a state of dormancy with only a small amount of moisture until the waters return. - Breathing through Gills
One of the reasons fish live in water is because they primarily breathe through gills. Unlike the lungs of land animals, gills need to remain moist in order to function properly. As water flows over the gill filaments, the blood vessels inside facilitate the exchange of oxygen. However, some fish can breathe using other organs; for example, loaches can breathe through their intestines, and members of the bass family have a special structure above their gills that allows them to extract oxygen from the air. - Swimming with Fins
Since fish live in water, they need mechanisms to help them move. Fish fins serve as their swimming aids, and there are generally five types: pectoral fins, pelvic fins, dorsal fins, anal fins, and caudal fins. Some fish also have adipose fins. Different fins have different functions, but they mainly help fish maintain balance. The tail fin, in particular, provides propulsion and helps with turning, making it especially important for fish. - Reproduction through Egg-laying
Most fish reproduce by laying eggs, though a few are ovoviviparous. Ovoviviparity may seem like live birth, but it is actually still a form of egg-laying—only the eggs hatch inside the mother’s body, rather than outside. As a result, the offspring are born as small fish.
