Most of the fish reproduce sexually via oviparity. In this mode of reproduction, the female reproductive cells (egg cells) receive the milt (sperm) from the male fish and it gets fertilized inside the female fish body. Then, the female fish releases the zygote into the bubble nest created by the male fish in the water. Some parent fish guard these nest until the eggs hatch.
How do fish reproduce?
We know that all living organisms reproduce. Reproduction is not essential for the survival of fish but it is important to ensure that their species don’t extinct. Fish can procreate both through the sexual and asexual modes of reproduction. The majority of the fish reproduce sexually in which the female partner lays eggs while the male fish releases sperm. This egg and sperm come together for fertilization and produce young fish.
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Before we learn about the mating process of fish, let’s understand the reproductive organs of fish.
Fish reproductive system
Fish reproductive organs are consist of gonad (sex or reproductive gland) and ducts.
Gonad or Sex gland: A gonad is a sexual gland that produces sex cells and sex hormones in a fish. Female fish have ovaries that produce reproductive cells (sex cells) called egg cells while male fish have testes that produce sex cells known as sperm.
Testes: Most male fish have two testes. However, there are some fish such as jawless fish that have one testis. Most of the fish have spherical structures called sperm ampullae which produce a milky white fluid called sperm. This semen of a male fish also known as milt.
Ovaries: Female fish generally have two ovaries that produce egg cells. The fertilization of these eggs can occur both internally as well as externally. The location of fertilization depends on the reproduction strategy which varies from fish to fish.
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Ducts: The ducts are responsible for carrying sex cells both in the male and female reproductive systems. In male fish, the sperm duct connects each testis to the urogenital opening. Similarly in female fish, these ducts connect each ovary to the urogenital opening.
Now let’s understand how fish mate to reproduce?
How do fish copulate?
Most of the fish don’t mate as mammals do. In the majority of fish reproduction, the process of fertilization takes place out of the parent body. In this process, both male and female fish release a large number of eggs and sperms respectively into the water. These scattered eggs and sperms come together with each other to complete the process of fertilization. Note that not all fish lay large amounts of eggs. There are some fish who produce fewer eggs.
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You will be surprised to know that some male fish have evolved external sexual organs called intromittent organ which allows them to deliver and fertilize egg internally during copulation. For example, spinner sharks have modified pelvic fins which also act as intromittent organs during mating. Other fish such as rays and few fishes from the Poeciliidae family (mosquito fish) also have gonopodium, an anal fin which transmits sperm to females.
Now let’s discuss the various reproduction strategies found in the different species of fish.
How do fish procreate?
In the majority of the fish species, the process of fertilization occurs outside the parent body. During the process of reproduction, members of species come together to form a large group. Then, the male and female. Then, the male and female members of the species will release eggs and sperms respectively. This process of laying eggs is known as spawning. The process of spawning in large groups increases the chances of egg fertilization. It also helps them to protect the fertilized eggs from predators. Some female fish lay eggs in a bubble nest created by male fish where oxygen and food are available easily. They guard these bubble nests to increase the chance of survival of the fertilized eggs. Some male fish such as jawfish, sea catfish, cardinalfish, and Arowana belonging to mouth brooder species, keep fertilized eggs in their mouth until they hatch.
How do fish breed?
Fish can be classified into five categories based on their reproduction strategy. This classification depends on the location of egg fertilization, development of the embryo in internal and external conditions, and the nutrients supply to the offspring.
Ovuliparity: In the ovuliparity reproduction strategy, the fertilization takes place outside the parent body. In this process, the female lays the unfertilized eggs into the water and then these eggs get fertilized as they come in contact with sperm. The most common examples of ovuliparous fish are salmon, tuna, goldfish, and cichlids.
Oviparity: The majority of the fish species are oviparous. In oviparity, first, the fertilization process between male and female gametes occurs inside the body of female fish. Then, the fertilized eggs or zygotes are released into the water by the female fish. These zygotes develop outside the parent’s body. Common examples of oviparous are horn sharks and skates. In these cases, the male partner has a modified pelvic fin that delivers the milt into the female body.
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Viviparity: In this strategy also, the egg cells get fertilize as well develop inside the mother’s body. This mode of reproduction can be divided into two based on the nutrient supply.
Ovo Viviparity: In this case, fertilization occurs internally and the zygote also develops inside the body of female fish. However, it is possible that the offspring may or may not receive nutrients from the mother. Therefore, it relies on the food inside the egg. Examples of ovoviviparous are angel sharks, guppies, and coelacanths.
Histotrophic Viviparity: Embryos that develop inside the female fish body by consuming other tissues such as zygotes. Example: shortfin mako, porbeagle, grey nurse shark, and halfbeak Nomorhamphus ebrardtii.
Hemotrophic Viviparity: Embryos that develop inside the female body and receives nutrients directly from the mother. Examples: surfperches, splitfins, lemon shark, seahorses, and pipefish.
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Other modes of reproduction are: Hermaphroditism, Sexual parasitism, and Parthenogenesis
Hermaphroditism: This mode of reproduction occurs in fish species that possess both male and female reproduction organs. These fishes have the ability to switch their sex from female to male. In these groups of fishes when the dominant male fish is removed from the group, the largest female fish will replace the dominant male by changing its sex. Black hamlets and anemonefishes have adapted this kind of reproduction strategy.
Sexual parasitism: Anglerfish is the best example of sexual parasitism. In this case, the small male anglerfish rely on females like parasites for food and protection. The male anglerfish just provide sperm to female anglerfish to reproduce.
Parthenogenesis: It is an asexual mode of reproduction that occurs in fish. In this mode, the growth and development of the egg cell take place without the process of fertilization. Examples: Amazon molly, Zebra shark, and Bonnethead.
Livebearers: The aquarium fish that keep their eggs inside and body and give birth to live young ones. The livebearers can be classified as ovoviviparous and viviparous. Examples: guppies, mollies, platies, and swordtails.
How often do fish breed?
The breading cycle in fish varies with different species. It also depends on the surrounding conditions of the fish. For example, an outdoor goldfish living in a warmer climate may spawn 2-3 times while goldfish living in an aquarium may spawn throughout the year.
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