Fish make up the largest percentage of vertebrates on earth. There are roughly 22,000 species of fish, and they come in every conceivable shape, size and color. Fish are an integral part of all aquatic ecosystems, and they have served as a food source for people for thousands of years.
Beyond an abundant food source and a suitable habitat, fish need specialized breeding systems to support their massive populations. The mating, hatching and growth patterns of fish are fascinating processes that boast some surprising facts.
Mating and Breeding
The vast majority of fish reproduce by spawning, which is a mating process that occurs at specific times of the year. During spawning, males release sperm and females release eggs at the same time into the water. The sperm and eggs find each other, and the eggs become fertilized. The fertilized eggs either float along with plant matter or sink to the ground below. Since most spawning fish provide no protection for their eggs from the vast number of predators that feed on them, they must produce millions of eggs to ensure some breeding success.The production of eggs that develop and hatch outside of the mother is called ovopartity.
Because fish eggs are laid underwater and are in no danger of drying out, most do not have a shell. Instead, they are surrounded by a protective permeable membrane that lets gasses in and out. A fertilized egg contains cells that divide continuously and develop into an embryo. It also contains a yolk, which provides the embryo with its nourishment.
When a fish is ready to hatch, special enzymes are triggered into action by secretions from the embryo's hatching glands. These enzymes work to dissolve the egg's protective membrane, significantly weakening it so that the baby fish (called a larva), can easily break through and swim away. Once hatched, the larva still carries the yolk sac with it as a food source. Once the yolk is gone, the larva must quickly learn to find food for itself, or risk death.
Not all fish are hatched from eggs. Some fish species, including many sharks, are born live.
There are two ways through which fish produce live babies. The first--which is common in sharks and rays--is for fertilized eggs to remain inside the mother until they hatch, at which point she gives birth to the already-developed babies. In this process, called ovoviviparity, the young are not nourished internally by the mother, but by the egg yolk.
A rare number of fish reproduce through vivipartity, in which the mother carries her embryos inside her, nourishing them directly from her own body by way of a placenta.
Some fish mate in pairs and lay their eggs in small nests, which they vigorously defend from predators.
In some fish species, the females carry their fertilized eggs in their mouths until they hatch.
Only 10 to 20 percent of fertilized fish eggs survive to maturity in the wild.