Fishes are cold-blooded vertebrate animals that comprise around 34,400 species found in freshwater and saltwater across the globe. Fish fall under the superclass Agnatha (jawless fish) and phylum Chordata which includes hagfishes, sharks, bony fishes and many more. Except for the fish Opah, all other fishes are cold blooded vertebrates. The characteristics of fish are gill slits on the side, skeletal supporting cord and a tail. The study of fishes is known as ichthyology, which has brought light to the importance of fish in the world’s food supply.
Now let’s look at the life cycle of a fish to better understand them.
Life Cycle of a Fish:
1. The Egg – Stage 1 –
In this stage, the tiny embryo develops inside the hard shelled egg. During this process, the embryo develops different organs of its body. Many eggs do not survive this stage. Fishes which live in warmer regions hatch their eggs faster, compared to fishes living in colder regions.
2. The Larvae – Stage 2 –
After the egg hatches, the larvae emerges from the eggs that have a yolk sac attached to their bodies. This attached yolk sac provides nutrition to the growing larvae. The larvae survive for about 4 days on the nutrition provided by this yolk sac. The eyes and mouth parts of the larvae develop in this stage. When the yolk sac is completely absorbed, the young larvae is called fry.
3. The Fry – Stage 3 –
Unlike the fish larvae that depended on the yolk sac for food source, the fry has the ability to feed on its own. In the first few months a young fish is called fry. In this stage, the fry undergo many developmental stages.
4. The Juvenile – Stage 4 –
In this stage, the fish develops more adult-like characteristics like fins, color and other body parts. This is the stage where a fish transforms from a fry to an adult fish. Time period of this stage varies from species to species. Most species do not survive this stage due to inevitable predators.
5. The Adult – Stage 5 –
Here, the fish is fully grown with all body parts and is sexually mature. The adult fish is capable of mating and reproducing. Sexual maturity depends on the lifespan of the fish. Species with shorter lifespan mature faster compared to species with longer lifespan.
6. The Spawning – Stage 6 –
Spawns are the eggs and sperm released or deposited by fishes and other aquatic animals. The female fish releases eggs into the water and the male fish releases milt (semen from male fish) to fertilize the eggs. Not all eggs are fertilized. Spawning in fishes varies, depending on the species. While some fish spawn in intervals, some spawn annually and there are some fishes which spawn once in a while.
Fishes are over 450 million years old vertebrates and have constantly evolved over the years to adapt to any living conditions. Almost all water bodies consist of fish, except for the Dead Sea in Asia and Great Salt Lake in North America.