1. Living in water

All fish live in water, even if some of them can leave water for a short time, but long-term dehydration is not feasible. Mudskippers, for example, can come ashore to feed at low tide, but they will return to the water before they are killed by the sun. The African lungfish, which spend the longest time out of the water, burrow into the soil during the dry season of the river and rest on a small amount of water until there is water again.

2. Breathing

with gills one of the reasons why fish live in water is that they mainly breathe with gills. Unlike the lungs of terrestrial animals, the gills need to be wet to work properly, and when water flows through the gill filaments, the blood vessels inside can complete the exchange of oxygen. However, some fish can breathe through other organs, such as loach, which can breathe through the intestine, and gourami, which can get oxygen from the air.

Since it lives in the water, it always needs some means to help it swim. Fins are the helpers of fish swimming. It is usually divided into five categories, namely, pectoral fins, pelvic fins, dorsal fins, anal fins and caudal fins. A few fish also have fat fins. Different fins have different functions, but they can basically maintain balance, among which the tail fin can also provide power and turn, which is particularly important for fish.

4. The reproductive mode of

most fish is oviparous, and a few are ovoviviparous. Ovoviviparous looks like viviparous, but in fact it is oviparous, but the eggs are not hatched in the outside world, but in the fish body, so they are all small fish.