In this section, you will learn and understand ways, how fishes reproduce sexually. Fishes are animals that have gills, fins, and vertebral columns in their bodies. They belong to the class Pisces of the kingdom Animalia. Their heart has only two chambers. Now let’s start the topic, i.e. how do fishes reproduce sexually?
Among fishes, approximately 90 percent of bony fishes are oviparous reproducers. In this method of reproduction, females lay eggs, which will be then fertilized by males. Most fishes may lay hundreds of eggs at one time. It is because it takes much less energy than growing an embryo in an egg inside her body.
In fishes, reproductive organs are the testes and ovaries. There may be a range of secondary organs that increase reproductive fitness as well. In some fishes, there is a genital papilla, which is a small, fleshy tube present behind the anus, from which the eggs or sperms are released. The shape of the genital papilla helps to find the sex of a fish.
Some fishes are hermaphrodites, having both testes and ovaries either at different phases in their life cycle or, as in hamlets, having them simultaneously.
Testes:
Testes are present in male fishes, which are similar in size. Testes on the right side are usually large in the case of sharks. The primitive jawless fish has only a single testis, located in the midline of the body. The testis of some teleost fish, under a tough membranous shell, contains very fine coiled tubes called seminiferous tubules.
The developing sperm travel through these coiled tubes to the rete testis located in the mediastinum testis, to the efferent ducts, and then to the epididymis where newly produced sperm cells become mature. The sperm move into the vas deferens and are finally expelled through the urethra and out of the urethra by means of muscular contractions.
Ovaries
These sex organs are present in females and are common to all vertebrates There may be thousands or even millions of fertile eggs present in the ovary of a fish. Most normal female fish have two ovaries. In the primitive jawless fish and some teleosts, there is only one ovary, In some elasmobranchs, only the right ovary develops completely.
There are basically three types of ovaries, gymnovarian, secondary gymnovarian or cystovarian.. The oocytes are released directly into the coelomic cavity and then enter the ostium, then through the oviduct, and are eliminated in the first type of ovary. Secondary gymnovarian ovaries shed ova into the coelom from which they go directly into the oviduct.
In the third type, the oocytes are transferred through the oviduct.
Eggs
In fishes and mostly amphibians, the eggs are jelly-like. Cartilaginous fishes (skates, rays, sharks, and chimeras) eggs are fertilized internally and exhibit a wide range of both internal and external embryonic development. Air-breathing amphibians lay their eggs in water, or in protective foam as with the Coast foam-nest treefrog,
Reproductive strategies
Nearly all fishes produce by sexual reproduction, in which the fusion of sperm is produced from the testes and eggs produced from the ovaries. In most fish fertilization is external. Both males and females release large quantities of eggs and sperm into the water.
Fertilization of eggs can be either external or internal in fishes. In most species of fish, fins have been modified to allow internal fertilization. The development of the embryo can be external or internal, however, some fishes show a change between the two at various stages of embryo development.
Ovuliparity
Ovuliparity means the female lays unfertilized eggs, which then be fertilized externally. Examples of oviparous fish include goldfish, eel, salmon, tuna, and cichlids. Amongst the species of these fish fertilization takes place outside the mother’s body, with the male and female fish shedding their gametes into the surrounding water.
Oviparity
In Oviparity, fertilization takes place internally and so the female sheds zygotes or newly developing embryos into the water. It is estimated that over 97% of all known fishes are oviparous. In oviparous fish, internal fertilization needs the male to use some sort of intromittent organs to transfer sperm into the genital opening of the female.
Examples include the oviparous rays, such as skates, and the oviparous sharks, such as the horn shark. In these cases, the male is equipped with a pair of modified pelvic fins generally termed claspers. Marine fishes can produce a very large number of eggs which are released to the open water column.
The average diameter of the eggs is about 1millimetre. The eggs are generally surrounded by the extra-embryonic membrane, which lacks a shell, hard or soft, around these membranes. Some fishes have leathery, thick coats, especially if they must withstand physical force. These types of eggs can also be very fragile and small.
Types of viviparity in fishes
There are two types of fishes regarding viviparity (1) Viviparous fishes (2): Ovoviviparous fishes. Viviparous fishes give birth to live young, the eggs develop whilst receiving nutrition from the parent, while ovoviviparous fishes give birth to live young which do not receive nourishment from the parent whilst in the womb.
Most Selachii ( stingrays, eagle rays, the majority of sharks, and giant rays) are viviparous fishes. On the other hand, certain species of sharks (such as the basking shark), as well as guppies and other fishes, are examples of ovoviviparous fishes.
Parthenogenesis (An asexual mode of reproduction)
Parthenogenesis is a type of asexual reproduction, in which the development and growth of an embryo occur without fertilization. In Parthenogenesis, the development of an embryo takes place from an unfertilized egg cell.
About 50 species of unisexual vertebrates have been described, including at least 20 fish, 25 lizards, a single snake, salamanders, and frogs.
Parthenogenesis in sharks has been confirmed in the zebra shark and bonnethead.
Gynogenesis
Gynogenesis is a special type of Parthenogenesis. In this type of asexual reproduction, offspring are produced by the same mode as in Parthenogenesis, although, the egg is stimulated to develop simply by the presence of sperm. The sperm does not transfer any genetic material to the offspring.