Fish is not a mammal. Although they belong to the same phylum Chordata and the subphylum Vertebrata, fish mainly includes two classes, namely, Osteichthyes and Cartilaginoidea, while mammals refer to Mammalia, also known as mammals. Unlike fish, which live in the water and breathe with gills, mammals mostly live on land, some in the water, and some can fly.
1. Oviparous: Most fish are oviparous, but the fertilization methods of bony fish and cartilaginous fish are often different. Bony fish are usually fertilized in vitro, while cartilaginous fish are mostly fertilized in vivo. Because there are many kinds of fish, the frequency and quantity of spawning are different. The most spawning is the sunfish, which can produce 300 million eggs each time, but the survival rate is very low.
2. Oviviparous: In addition to oviparous cartilaginous fish, there are also some ovoviviparous or pseudoviviparous cartilaginous fish. This kind of fish looks like it gives birth to small fish directly, but in fact it just puts the eggs in the body to hatch. It does not absorb nutrients from the placenta like a vivipara, but mainly relies on the nutrition of the yolk sac to become small fish.
3. Metamorphosis development: The metamorphosis development fishes are mainly Osteichthyes and Cyclostomata, the former includes lungfish, flatfish, tongue sole, long sunfish and eel, and the latter is lamprey. These fish break out before the embryo is fully developed, and it takes some time for them to become larvae in the water.
