Fish are vertebrate and aquatic animals. They have a system with him that can regulate his temperature depending on that of the environment. Their respiration occurs through gills. They have the body covered with scales, and it moves thanks to fins.
We can find both saltwater and freshwater fish, finding a wide range of fish species.
Next, we are going to explain how fish are born, but to understand this phenomenon, we will first explain how they reproduce.
Reproduction in fish
Fish are animals that reproduce sexually, that is, reproductive systems are involved, and dioeciously, there are two different genders, that is, there is a male and a female.
But species of these vertebrates have been found to suffer from sexual hermaphroditism, that is, they change sex throughout their lives. It is a very useful method for fish that live in ponds or small lakes, since with this they ensure the continuation of their species in case any necessary male or female dies.
Apart from this, fish do not have ovulatory organs, therefore they reproduce externally.
Although there is fish that present sexual dimorphism, that is, there are traits that differentiate a male from a female, there are some cases that do not present this dimorphism and it is more difficult to know what sex it is.
In these cases they will only be differentiated if there is a small lump on the female, produced by the eggs, or some other behavior. The fish look for a mate, and will spend the rest of their lives by his side, dying if the other one dies.
Reproductive migrations.
Reproductive migrations, also called genetic, are what show us that the life of fish does not develop in the same place, but that they are born in one place, develop in another, reproduce in another.
There is a lot of variation in these migrations, since there are fish that hardly move from the place where they were born to reproduce, and others that move kilometers.
Once reproduction is finished, the fish, although it is exhausted by the production of its spermatozoa and by the migration process, also returns to its common feeding site, so that when the migration has to be repeated it is ready again.
We differentiate five groups of migrations:
- Anatropous. They are the ones that migrate from the high seas to the upper river, like the salmon that breeds in the upper part of the river. When they hatch, these fish go down to the sea and live there for a few years, then go back up to where they were born where they spawn and die there shortly after. These fish must have a highly developed sense of smell, so that they can distinguish the different types of water they cross.
- Catadromous. They are the ones that migrate from the river to the sea to spawn, like the eel, which once the descendants are born, undergo numerous changes from the ocean depths to torrential waters. They are suspended adrift in the open ocean, until they reach a stream where they become adults and spawn and return to the starting point.
- Potamodromes. They are the ones that migrate within the same hydrographic basin, something that only occurs in fresh waters.
- Oceanodromes. They are the ones that migrate within the sea, like plankton.
- Amphidromous. They migrate several times between the river and the sea and vice versa.
Apart from reproductive migrations, we also find trophic reproductions, that is, those that occur in search of food for individual development and sexual maturation, or migrations to hibernate.
Types of reproduction in fish
These animals are generally oviparous, that is, with their reproduction they give rise to eggs, although one or another case of viviparous and even ovoviviparous fish has also been found. Its reproduction is also determined by some external factors, such as light, temperature, etc.
As we said before, we will look at three types of reproduction, oviparous, viviparous, and ovoviviparous.
- Oviparous development. This process occurs in both saltwater and freshwater. In this case, fertilization is external, that is, the female expels the eggs and the male the sperm, fertilization, as we see, does not occur inside anybody. In saltwater fish we find two ways of spawning. The first is the deferral, in which the eggs are denser than salt water and go down. The other case is that of pelagic eggs, in which the eggs are released into sea currents and spread throughout the sea. There are some species where they protect their eggs in water bubbles, to ensure their continuation, or they also keep their eggs inside their mouths, which are considered mouth brooding fish, where they are released when they hatch, that is, when the fish comes out of the egg.
- Viviparous development. These fish are the ones that give birth to fully form young ready to make their life. Therefore, contrary to the other development, this one occurs within the female. It is easy to differentiate a male from a species that reproduces in a viviparous way, since in the ventral fins they have a modification in the form of anovulatory organ to better introduce the sperm. This reproduction occurs in cartilaginous fish, such as sharks.
- Ovoviviparous development. This development is a mix between the oviparous and the viviparous. In this case there is also an internal fertilization, and the males have a structure to fertilize the female’s eggs, called monopodium. They are the types of fish that present the most sexual dimorphism, since the females are larger than the males, and the males, apart from presenting the monopodium in the anal fin, are more colorful. From the first fertilization, the female can continue fertilizing without the need of a male, since she continues to store her sperm. Cases of fish with ovoviviparous development are the guppies, the siphons and the mollies.
How fish are born:
Depending on the development of each species, they will be born in one way or another. The great evolutionary path has caused great differences between one species and another. Here I explain ways of how fish are born:
- Oviparous species. As we have already said, this is the most common method of reproduction in fish, since most of them live in seas, rivers and lagoons. Thus, the fish develop inside the egg, which, as we have already explained, is fertilized by sperm outside the mother. When they reach the appropriate level of development they hatch. The mothers can take care of the egg until it hatches or not.
- Viviparous species. They give birth to young with a high level of development, which we call parturition. As we have already said, it is a reproduction very similar to that of mammals, in which the males have a modified fin. When the fish is born it can already be independent, although they usually require a season from their parent.
- Ovoviviparous species. These species develop inside eggs, but they are not loose, but inside the mother, but they do not feed on the mother, but on the materials found inside the egg. The egg breaks or hatches inside the mother, and then the offspring come out from the mother’s body.
