Egg Development of Freshwater Fish

All fish begin life inside an egg. Even livebearers start out as an egg, hatching inside their mother before being born; most are ovoviviparous, rather than truly viviparous. Each egg is a complex chemical soup of proteins, fats, and sugars, with a genetic code to fit it all together, and it is all contained within a protective but gas-permeable shell. Life begins for a fish the moment the egg is fertilized, an event that occurs for many species in the water column as the egg drifts slowly towards the bottom. From this point onwards the pace of development is relentless — eggs are an excellent and nutritious food source and it is a race for each developing embryo to hatch before being found by a hungry predator.

What are the first few hours of life like?

When the sperm and egg first fuse and a new life begins, the embryo consists of just one single cell. For many familiar tropical fish, it takes around three days for the egg to hatch. These 72 hours are arguably the most active period of a fish's life. The single cell splits into two soon after fertilization and about 20 minutes later, it divides into four. This rate of division and growth continues for the next few hours until the embryo can be seen within the egg by the naked eye. It appears as a bump on one side of the large round “globe” inside the egg. The globe is the yolk sac, the provision left by the mother fish for her offspring. It is the yolk sac that fuels the early development of the embryo. As the embryo continues to expand, it starts to grow around the outside of the yolk sac, eventually enveloping it within a thin layer of cells. The main body of the embryo is visible as a raised band curling around part of the yolk sac. It may not look like much, but in the first six hours of life, the embryo has gone from a single cell to tens of thousands, all with the original DNA blueprint.

Are all these cells the same?

Until now, all the cells were identical copies of one another, but a functioning animal has many different kinds of cells — skin cells, blood cells, nerve cells, etc. At this point in development, different genes switch on within different cells of the embryo and change the course of development for that cell and its subsequent lineage. This process, known as differentiation, is irreversible; once the genes have instructed a cell to become, tor example, a brain cell, then its fate is mapped out. Gradually the fates of all the embryo's cells are determined and the mosaic of diverse cell types continue to develop, creating specific tissues and organs in the embryo's body.

How does a group of cells develop into a fish?

From being a blob of cells, after one day the embryo begins to look like what it is — a fish. A ribbon of cells along the length of the fish curls inwards to form a tube that will eventually become the fish's backbone.The major organs — the heart, the liver, and the brain — develop apace. After a further 24 hours, the embryo appears impatient to enter the big wide world; tiny muscles are twitching, the heart is beating and eyes can be clearly seen. The embryo is almost ready to hatch. Curled tightly round the yolk sac, the egg offers little room for further growth. The next stage of life occurs beyond the egg, in open water.

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