The epic life of European eels is roughly like this: when they are translucent little dolls, they will take advantage of ocean currents and embark on a journey to find their roots in Europe. During this period, no one urged them to get married, so they lived alone for several years or even decades-until one day they suddenly realized (sexual maturity is about equal to death) that they saved all the energy they spent on the road and mating (putting autumn fat), and then they never ate or drank again, and embarked on the road to the sargasso sea, their birthplace, with the aim of "falling in love and dying". The journey takes about six months, starting in autumn and arriving in the following spring. In the second half of the journey, eels gradually fill their bodies with reproductive organs, reach their destinations, then reproduce and die.

Marine carnivorous fish have three breeding methods:

1. spawn. Most marine fish belong to this type, which is characterized in that mature eggs are directly discharged into water for in vitro fertilization to complete the whole development process. However, a few fish (such as some sharks) are fertilized in vivo, and fertilized eggs are still developing in vitro.

2. viviparous. The main feature is that the egg is fertilized in vivo, and the fertilized egg develops in vivo, but the nutrition of the embryo body depends on its own yolk, which has nothing to do with the mother, such as white-spotted star shark, self-spotted shark, Japanese partial shark, squid, crucian carp, black herring and so on.

3. viviparous. It is characterized by the fertilization and development of eggs in the mother, and the embryos formed by fertilized eggs have blood circulation relations with the mother. Its nutrition not only comes from the yolk of the body, but also needs the supply of the mother, such as the gray star shark.

The spawning ability of marine fish is much higher than that of terrestrial vertebrates. The amount of eggs laid varies greatly, from several large eggs (such as many sharks) to 300 million floating eggs (such as molas). Generally speaking, fish that don't protect their eggs after laying eggs have a large amount of eggs; Fish that protect their eggs after spawning have few eggs.