Fish reproductive organs include testes and ovaries. In most species, gonads are paired organs of similar size, which can be partially or totally fused. The genital papilla is a small, fleshy tube behind the anus in some fishes, from which the sperm or eggs are released; Unisexuality in fish sex of a fish often can be determined by the shape of its papilla.
Most male fish have two testes of similar size. In the case of sharksthe testes on the right side is usually larger [ citation needed ]. The primitive jawless fish have only a single testis, located in the midline of the body, although even this forms from the fusion of paired structures in the embryo.
Under a tough membranous shell, the tunica albugineathe testis of some teleost fish, contains very fine coiled tubes called seminiferous tubules. The tubules are lined with a layer of cells germ cells that from puberty into old age, develop into sperm cells also known as spermatozoa or male gametes.
The developing sperm travel through the seminiferous tubules to the rete testis located in the mediastinum testisto the efferent ductsand then to the epididymis where newly created sperm cells mature see spermatogenesis. The sperm move into the vas deferensand are eventually expelled through the urethra and out of the urethral orifice through muscular contractions. However, most fish do not possess seminiferous tubules. Instead, Unisexuality in fish sperm are produced in spherical structures called sperm ampullae.
These are seasonal structures, releasing their contents during the breeding season, and then being reabsorbed by the body. Before the next breeding season, new sperm ampullae begin to form and ripen. The ampullae are otherwise essentially identical to the seminiferous tubules in higher vertebratesincluding the same range of cell types.
In terms of
Unisexuality in fish distribution, the structure of teleosts testes has two types: Fish can present cystic or semi-cystic spermatogenesis in relation to the release phase of germ cells in cysts to the seminiferous tubules lumen. Many of the features found in ovaries are common to all vertebrates, including the presence of follicular cells and tunica albuginea There may be hundreds or even millions of fertile eggs present in the ovary of a fish at any given time.
Fresh eggs may be developing from the germinal epithelium throughout life. Corpora lutea are found only in mammals, and in some elasmobranch fish; in other species, the remnants of the follicle are quickly resorbed by the ovary. In some elasmobranchsonly the right ovary develops fully. In the primitive jawless fishand some teleosts, there is only one ovary, formed by the fusion of the paired organs in the embryo.
Fish ovaries may be of three types: In the first type, the oocytes are released directly into coelomic cavity and then enter the ostiumthen through the oviduct and are eliminated. Secondary gymnovarian ovaries shed ova into the coelom Unisexuality in fish which they go directly into the oviduct. In the third type, the oocytes are conveyed to the exterior through the oviduct. Cystovaries characterize most teleosts, where the ovary lumen has continuity with the oviduct.
The eggs of fish and amphibians are jellylike. Cartilagenous fish sharks, skates, rays, chimaeras eggs are fertilized internally and exhibit a wide variety of both internal and external embryonic development. Most fish species spawn eggs that are fertilized externally, typically with the male inseminating the eggs after the female lays them.
These eggs do not have a shell and would dry out in the air. Even air-breathing amphibians lay their eggs in water, or in protective foam as with the Coast foam-nest "Unisexuality in fish," Chiromantis xerampelina.
Male cartilaginous fishes sharks and raysas well as the males of some live-bearing ray finned fisheshave fins that have been modified to function as intromittent organsreproductive appendages which allow internal fertilization. In ray finned fish they are called gonopodiums or andropodiumsand in cartilaginous fish they are called claspers. Gonopodia are found on the males of some species in the Anablepidae and Poeciliidae families.
They are anal fins that have been modified to function as movable intromittent organs and are used to impregnate females with milt during mating. The third, fourth and fifth rays of the male's anal fin are formed into a tube-like structure in which the sperm of the fish is ejected. The male shortly inserts the organ into the sex opening of the female, with hook-like adaptations that allow the fish to grip onto the female to ensure impregnation.
If a female remains stationary and her partner contacts her vent with his gonopodium, she is fertilized. The sperm is preserved in the female's oviduct. This allows females to
Unisexuality in fish themselves at any time without further assistance from males. In some species, the gonopodium may be half the total body length.
Occasionally the fin is too long to be used, as in the "lyretail" breeds of Xiphophorus helleri. Hormone treated females may develop gonopodia. These are useless for breeding. Similar organs with similar characteristics are found in other fishes, for example the andropodium in the Hemirhamphodon or in the Goodeidae.
Unisexuality in fish on the males of cartilaginous fishes. They are the posterior part of the pelvic fins that have also been modified to function as intromittent organs, and are used to semen into the female's cloaca during copulation. The act of mating in sharks usually includes raising one of the claspers to allow water into a siphon through a specific orifice.
The clasper is then inserted into the cloaca, where it opens like an umbrella to anchor its position.
The siphon then begins to contract expelling water and sperm. Oogonia development in teleosts fish varies according to the group, and the determination of oogenesis dynamics allows the understanding of maturation and fertilisation processes.