Finfish Breeding and Hatchery Management (2+1)

Unit 2 - Sexual maturity, breeding season and development of gametes

2.4. Gamete maturation and release in males

• Fish endocrinologist or breeders are normally biased towards events in the female such as vitellogenesis, final maturation and ovulatory (the three important stages of reproduction). This is because gonadal development in the male goes ahead on its own even in captivity.
• In nature, male remains mature over a much longer period than the females and spermatozoa can remain fertile in the testis or outside for much longer periods (up to several days) compared to ovulated eggs which loose fertility very quickly.
• However, milt may be scarce and of poor quality, particularly in fishes which need to be induced bred (carps and milk fish) an understanding of sperm formation will be useful in overcoming these practical difficulties.
• The testis of fish is sac-like and folded.

Like the oocyte, male gametes (sperm cells) develop and mature in the male fish through a series of stages. The stages are given below

1. Spermatogenesis

• Spermatogonia are cells that give rise to spermatocytes through a process called Spermatogenesis.
• The spermatocytes produced by meiosis (I) are enveloped by sertoli cells until their liberation as mature spermatozoa with in the lumen of the testis.
• GtH induces the Leydig cells to produce Testosterone (T), which causes spermatogonia to divide into spermatocytes within cysts of sertoli cells.

2. Spermiogenesis

• Development of spermatocytes into spermatids (spermiogenesis) may continue and occurs through 2nd meiosis in the absence of GtH.
• The growth of spermatids takes place within the cyst of sertoli cells and at this stage the spermatids possess flagellum.

3. Spermiation

• Mature spermatids are liberated into the lumen as spermatozoa through a process called spermiation.
• 11-ketotestosterone (11-KT), produced by the Leydig cells under GtH stimulation, initiates this process.17α-20β-DiOH P, also produced by the Leydig cells (and possibly by mature spermatozoa), maintains continuous low level spermiation.

4. Hydration

• The last step before release of spermatozoa is hydration i. e. dilution of spermatozoa with seminal fluid produced from the walls of the sperm duct under the influence of 17α-20β-DiOH P.
• This diluted sperm suspension is called milt which is released during spawning.
• Males are considered ‘ripe’ once spermiation starts, although low level spermiation can go on for several months in some species.
• Spermiation and sperm hydration increase rapidly when the appropriate environmental cues are present and are often synchronized with final maturation in females.
• Males generally are mature even in captivity, if not, environmental/hormonal manipulation can induce spermiation.
• Hence males need less hormone than females to induce ‘final maturation’.

Mature male with creamy white testis