Have you ever wondered how fish grow? Fish youngsters have been discovered to be included into one three main categories: oviparous, ovoviviparous, and viviparous.
You should know that all fish fit into one of these classifications; however, there can be major differences between species within the same group. We are referring to the length of developmental intervals, parental care, and nesting patterns.
The eggs of these fish grow and develop outside the female’s body. They are also fertilized outside the body – the only exceptions being the oviparous rays and sharks. The eggs hatch quite fast. For example, goldfish usually hatch in 48 to 72 hours.
After they hatch, the offsprings enter the larval stage, when they are unformed, looking like tadpoles. In this period, they feed on a yolk sac that they carry. After the supplies from this sac are finished, they feed on microscopic organisms, called zooplankton.
After a few weeks, when the youngsters are bigger, they start looking more and more like their parents. There is a theory that states that the growth period is so short so that it minimizes cannibalism by other fish. The majority of fish that exist in the world today are oviparous.
The fish that are part of this category are the ones who grow from eggs that develop inside the female’s body. Each embryo grows its own egg and yolk, the source of nutrients.
When the ovoviviparous fish are born, they already have their more or less final shape, and they are able to feed on their own, thus passing the larval stage of their oviparous relatives.
For example, angel sharks and guppies are part of the ovoviviparous family.
The eggs of viviparous fish are fertilized internally, and the youngsters are fed through a uterine form of milk or through a placenta-like organ. In other words, the mother provides the necessary nutrients for their babies in a direct manner.
Two main types of nourishment have been observed in viviparous fish, such as sharks – oophagy and intrauterine cannibalism.
With oophagy, the mother produces eggs strictly to feed the embryos, whereas, with intrauterine cannibalism, the smaller embryos are consumed by their bigger siblings.
Just like the ovoviviparous fish, the viviparous ones are born in a juvenile stage, skipping the larval period.
A common mistake is taking whales or dolphins for fish. They are, in fact, aquatic mammals. This means that they do not lay eggs and they develop in their mothers’ womb. They are born alive in the juvenile stage.
If you are not a fisher or do not have much interest in these creatures, maybe you wondered at one point how fish grow or how they are born. Some of them are born in the larval stage and take just a few weeks to develop into the juvenile stage, while others are released in the environment resembling their parents’ shapes and manage to feed by themselves.