Reproduction is typically a cyclical process by which species perpetuate themselves and produce one or more individuals similar to itself by means of a new generation. Most teleosts reproduce more than once in their lifetime and in general, the reproductive cycle includes gamete formation, maturation, release and fertilization. Reproductive organs have the slowest ontogenetic development because they are not essential for survival at birth, and only become functional at maturity. Fish have sexual reproduction and generally are dioecious, i.e., sexes are separated. Nevertheless, there are also cases of hermaphroditism, parthenogenesis and other phenomena such as sex inversion. The reproductive system is anatomically and histologically different in each sex. It consists of reproductive organs, gonads or sexual glands, and the ducts that carry the sex cells. The gonads, usually elongated and paired, are contained laterally in the abdominal cavity, ventral to the air bladder, suspended from the dorsal wall and surrounded by peritoneum.
Male gonads or testes are usually white or cream-coloured. Sexual products or spermatozoa leave the testes by means of deferent ducts, which may open into the urinary bladder (Figs. 3), the last part of the common urinary duct, or outside the body through the genital pore.
Each testis is suspended from the dorsal wall of the abdominal cavity by the mesorchium, which is a continuation of the peritoneum (Fig. 4). The testes are surrounded by a thick capsule of connective tissue. According to their internal organization, there are two basic types of testes in teleosts: lobular and tubular, both of which have central cavities that communicate with the deferent duct. In lobular testes, which are typical of teleost, the connective tissue of the testicular capsule extends internally and divides the testes into lobules. Male germinal cells (spermatogonia) and other cells that take part in support and nutrition are arranged inside. By means of successive divisions and maturation (male gametogenesis), the spermatogonia generate the spermatozoa, which pass into the lumen and subsequently through the deferent ducts. The ducts secrete substances which, together with the spermatozoa, form the milt .
In females, the organization of the reproductive system is highly variable, reflecting a wide range of possible reproductive modes. Female gonads or ovaries are usually saccular and paired, although sometimes they may be fused. They are suspended from the dorsal wall of the abdominal cavity by the mesovarium (Fig 5), which is a continuation of the peritoneum. There are four basic reproduction strategies adopted by dioecious (diocious) fish and although most species are ovuliparous, others are oviparous, ovoviviparous or viviparous. The sexual products or oocytes exit through ducts called oviducts, which may take on different forms. In most teleost the oviducts are continuous with the ovarian cavity, but in some, such as salmonids, oocytes are released into the abdominal cavity and collected by two short genital funnels. In both cases, the oviducts fuse medially into a single duct (genital duct) that opens out through the genital pore located between the anal and urinary openings. In species that produce large quantities of oocytes, the females have large ovaries that may take up a considerable part of the abdominal cavity especially at full maturation (Fig. 6). The ovaries are surrounded by a fine, transparent capsule of connective tissue. Internally, ovaries consist of germinal cells or oogonia*, oocytes*, follicular cells and richly vascularized support tissues. After successive divisions and maturation (female gametogenesis), oogonia produce the oocytes, surrounded by follicular cells that supply them with relatively large amounts of fat and protein-rich material in a process known as vitellogenesis.
Oocyte development is not synchronous, and oocytes of various sizes, at different phases of vitellogenesis may coexist. When the oocytes mature, the follicles of the inner surface open and release them into the ovarian lumen, after which they exit the body via the oviducts. Some oocytes remain in the ovary and are eventually reabsorbed. After spawning, the ovary consists of many postovulatory follicles, immature oocytes and mature oocytes that were not released. Fertilization is usually external. Oocytes are released and may be deposited on the substrate, in nests built for the purpose, or they may simply float in the water. The males of some teleost groups have external copulating organs such as a modified anal fin, with which they perform fertilization internally.
Reproduction is a complex process that always involves changes in the gonads, but other additional phenomena occur simultaneously. Many species develop secondary sexual features that may be permanent or only appear during spawning season. For example, in some species male salmon develop a strong red colour and a hump; another common secondary sexual features is a hooked snout (kype) which can be more or less pronounced depending of species (Fig. 7). Additionally, some species display elaborate courting ceremonies before spawning, or build nests to improve fertilization, while yet others practice various forms of parental care (caring for clutches and offspring, incubation). Some species migrate different distances to reach suitable spawning grounds, thus increasing the chance of reproductive success. Some of the most common gonad pathologies are atrophy, parasitic castration, adherences, haemorrhage, inflammation, granulomas and parasitic cysts. Further reading Developmental stages flat fish: http://www.nefsc.noaa.gov/fbp/reproduction/ Manual to determine gonadal maturity of Baltic cod J. Tomkiewicz, L. Tybjerg, N. Holm, A. Hansen, C. Broberg & E. Hansen Danish Institute. The Laboratory Fish (2000) Gary K Ostrander (editor). ISBN: 978-0-12-529650-2