Reproductive System

The reproductive systems and strategies in fishes are extremely varied. Examples of the salmonid family are emphasized here. Salmonids reproduce sexually. Pacific salmon spawn only once in their life cycle, whereas the trouts and Atlantic salmon spawn repeatedly during their life span. Unlike other fish, lipid levels have little to do with initiating spawning activity or gamete development in the salmon. Lipid levels are, however, correlated to the distance that animals have to migrate up freshwater streams to reach their spawning grounds. Lipids as well as body proteins are consumed in this migration. The primary environmental factor involved in sexual maturation is day length. The changes in day length are probably more important than absolute day length in the processes that affect reproduction. Temperature also plays an important role in gonadal development in some fishes.

Both the testes and ovaries are located along the midline of the fish, ventral to the swim bladder, extending from the anterior to the posterior end, terminating at the vent. Eggs are commonly released into the peritoneal cavity and reach the outside through a funnel and ovarian duct. Sperm almost always stay inside ducts until they are released.

Egg Size and Fecundity

In general, fish that lay smaller eggs lay many of them. Cod, for instance, lay millions of eggs that incur a high mortality rate. Smaller eggs also tend to hatch in a relatively short time and the hatched young require microscopic food almost immediately because the stored nutrients in the way of yolk is limited or nonexistent. Conversely, fish that lay larger eggs lay fewer eggs that take longer to hatch and have higher survival rates. The young also can survive on the yolk for days or weeks and can ingest large food particles when feeding begins.

There is a correlation between the length of parental care and egg size. That is, the larger and fewer the eggs, the greater the parental care of the young. Herring spawn millions of eggs in kelp beds and abandon them. Salmon lay fewer, larger eggs and bury them in the gravel but still die and do not tend to their young. In the other extreme, some fish produce only a few eggs that are incubated in the body and live young are born that are more or less ready to fend for themselves. The yolk material is solid in salmonids. Their eggs sink in water because they are very dense. The yolk material influences the density and the development time. There is only about 30% water in the yolk material of salmon, whereas the herring egg has oil droplets in the yolk, thus allowing the egg to float. Oviparous fish are egg layers such as salmon, viviparous fish are placental live-bearers as mammals, and ovoviviparous fish are live-bearers but are passive parents.

Hormones

Gametogenesis, the formation of gametes, is dependent on pituitary hormones called gonadotropins. The gonadotropins are leutenizing hormone (LH) and follicle stimulating hormone (FSH). Blocking gonadotropins reduces gametogenesis. These names are taken from mammalian sources. Although both, or analogues of both, may be present in fish, it is LH that predominates. This hormone appears to regulate the maturation and release of the egg. The folli cles probably produce the estrogens, estrone, and estradiol under hormonal control from the pituitary gonadotropin. These hormones control the development of secondary sex characteristics in the female fish. They also serve an important function in the production of vitellogenin in the liver. Vitellogenin is then taken up from the blood and incorporated into the yolk proteins. Progesterone, which maintains the uterine development during pregnancy in mammals, may be absent in fish. The corticosteroids testosterone and androstenodione may control the secondary sex characteristics in the male fish.

Gonads and Gamete Formation

The early development of gametes is similar to other vertebrates. The processes of spermatogenesis, (sperm development) and oogenesis (egg development) proceed in the following manner. In spermatogenesis a diploid spermatogonium, through meiosis, gives rise to haploid spermatocytes, which then become spermatids. A process called spermiogenesis is the final step of differentiation of sperm from spermatids. As Figure 9 shows, the Sertoli cells serve as nutritive cells. During this process, the cysts swell and finally rupture, releasing the sperm to the lumen, which is continuous with the sperm duct. The interstitium between the cysts contain interstitial cells, fibroblasts, and blood vessels. In oogenesis, cells destined to become eggs proliferate, enlarge, and each becomes surrounded with follicular, or nutritive cells. The granulosa layer of the follicle cells is thought to give rise to the yolk. The proteins in the yolk are derived from vitellogenin as discussed above. The development of all eggs in the salmonid occurs synchronously, that is all at once.

Ovulation

Ovulation, or the release of all the eggs from the follicles, is triggered by the completion of yolk deposition. Once the ova leaves the follicle, they lose the supply of nutrients provided by the follicular cells. They depend on the ovarian

(2n) Spermatogonium t

(n) Spermatocyte

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