Biology of Finfish and Shellfish

4.1.4 Female reproductive organ (Ovary)

In case of females, a pair of elongated sac like structures found in the abdominal cavity just ventral to the kidney which are called ovaries. They are attached to the body wall by mesovarium. The ovaries are free anteriorly but each ovary continues posteriorly as oviduct. Two oviducts fuse and open exteriorly by a genital aperture. The wall of the ovary consists of three layers.

1. Peritoneum (outermost thin covering layer)
2. Tunica albuginea (made up of connective tissue, muscle fibres and blood capillaries)
3. Germinal epithelium

Germinal epithelium is the vital part in ovary where development of the eggs takes place. The germ cells in the germinal epithelium are called oogonia which undergo a number of maturation stages to become a ripe ovum. This process is called oogenesis.

Breeding : The term breeding refers to successive stages of courtship, mating and spawning.

Courtship: It is the heterosexual reproductive communication system which ultimately leads to mating. It is a process during which mature males try to attract mature female or vice versa by displaying brilliant colour or by swimming around the other. Sometimes mature males fight among themselves to attract the females. In case of Betta, the brilliant coloured males swims around the female with his beautifully coloured fins fully extended and mouth widely opened and the branchiostegal rays protruding out to expose the bright red gills.

Mating : It refers to the sexual act itself in which at least one male and one female come close together and release their gametes more or less simultaneously into the surrounding media (external fertilization) or by the transfer of sperm from male into female (internal fertilization).

Spawning

It is the process of release of gametes i.e. sperms and eggs from the sperm duct or oviduct respectively. There are two types of spawning-

1. Oviparous: It is the process of emission or release of male and female gametes (sperm and eggs) from the body of the fish to the exterior or outside.
2. Viviparous: Releasing of developing young ones to the external environments (In case of internal fertilization).

Fertilization

There are two types.

1. External fertilization: Eggs from female and milt (liquid containing sperm) from male are released simultaneously to the exterior environment where they meet each other for fertilization.
2. Internal fertilization: Sperms are deposited into the reproductive organ of the female where fertilization takes place . In case of internal fertilization following three things are usually found

(a) Fertilized eggs may be released (embryos) immediately after fertilization. or

(b) In some fishes, developed embryos (before hatching) are released. Here fertilization and incubation takes place inside the fish.

(c) In some fishes, hatchlings come out (in live bearers).

Spawning Stages

Broadly, it can be categorized into three types. They are:-

1. Pre-spawning
2. Spawning
3. Post spawning

In pre-spawning, different stages of development of gonad (Testis and Ovary) takes place. During spawning stage, release of sperms and eggs occurs. Post spawning stage is the recovery stage where maturation of gonads begins from the initial stage.

Prior to the spawning, sperms are released into the sperm duct which is called spermiation. Similarly, the eggs are released into the oviduct prior to spawning which is called ovulation.

The spawning behavior differs from species to species. Some fishes spawn only once and die. But some other fishes spawn several times in their life time. During pre-spawning stage gonads occupy maximum space in the viscera cavity.

Study of reproductive biology of fishes is very important for better understanding of the annual regeneration of the stock.