Most fish are poikilotherms, but a small number have the ability to maintain partial or whole-body homeotherms, such as tuna, opah, swordfish, and so on. Although these fish can control their body temperature, their ability is not as good as that of true homeotherms, so they are also counted as poikilotherms.
1. Breathing with gills: Fish live in the water, and their respiratory organs are not lungs, but gills. Gills grow on both sides of the fish head, connected with the fish mouth, the fish will keep the mouth open to swallow water, and then through the two gills discharge. The gill filaments inside the operculum have a large number of gill lamellae, which can complete gas exchange when they come into contact with water flow.
2. Swimming with fins: When fish swim in the water, besides the swing of their trunk, the most important thing they rely on is their fins. Fins are divided into pectoral, pelvic, anal, dorsal and caudal fins, and some fish have adipose fins. Most of the fins can play a role in maintaining balance, once lost, the fish will swing when swimming. The tail fin is responsible for providing power and controlling the direction of swimming.
3. Scales on the body surface: Except for a small number of scaleless fish, most fish have scales, but some scales only cover part of the body, while others cover the whole body. Fish scales are skin derivatives, usually calcium, which can protect the fish body and prevent water loss. In addition, the special structure of fish scales can reduce the resistance of fish when they swim.
