Domesticated fish can produce some conditioned reflexes, for example, when people walk in, they will become active, or there are interactive situations, but from a scientific point of view, the brain structure of fish is relatively simple, and they can not have complex emotions, so they have no feelings for the owner.
1. Breathing with gills: One of the major characteristics of fish is to breathe with gills. They live in the water. When the water flows in through the mouth of the fish and is discharged from the gills, the densely covered gill filaments inside the gill cover will come into contact with the water flow. Because the direction of blood flow in the capillaries inside is just opposite to the direction of water flow. Therefore, gas exchange can be carried out efficiently, oxygen can be obtained from water, and carbon dioxide can be discharged.
2. Scales on the body surface: Not all fish have scales on the body surface, but scaleless fish are only a very small number. Most fish have scales on the body, some are all over the body, and some are only covered in some places. Fish scales are derivatives of the skin, mainly calcium. It can protect the fish body from injury, but also can avoid the loss of water, for the invasion of microorganisms and pathogens, can play a defensive role.
3, can not leave the water: fish can not leave the water, even if some fish can survive in the air for a period of time, but not too long, and must be moist. Out of the water, the gill filaments do not work, causing the fish to suffocate. Some deep-sea fish, adapted to the water pressure in the deep sea area, once the water pressure is too low or in the air, will lead to excessive internal pressure and death.
