Physiology of Finfish and Shellfish (2+1)

3.1.18. Ovulation and fertilization

The oocyte mature projects into the lumen of the ovary. The follicle ruptures and the egg escapes from the aperture. This is called ovulation or it refers to the expulsion of the mature oocyte from its follicle into the ovarian cavity. The forces causing rupture of the follicle are not clearly known and the muscle fibres of the ovarian wall may be responsible for it. Fish ovaries may be of three types: gymnovarian, secondary gymnovarian and cystovarian. In the first type, the oocytes are released directly into the coelomic cavity, which then travel through the ostium and enter the oviduct and are eliminated. In the secondary gymnovarian ovaries, ova are shed into the coelom and then they go directly into the oviduct. In the third type, the oocytes are directly deposited into the exterior by traveling through the oviduct. Depending on a female fish’s species, they may have one of these three types of ovaries.

Gymnovaries are the primitive condition found in lungfishes, sturgeons, and bowfins. Cystovaries are the condition that characterizes most of the teleosts, where the ovary lumen has continuity with the oviduct. Secondary gymnovaries are found in salmonids and a few other teleosts.

(ostium An opening, derived from the Latin word for a door or opening).

Fertilization is usually external in water and sperms and ova are discharged close to each other in water. Sperms become very active soon after they are released in water, but survive only for a few minutes during which fertilization takes place.

The released egg is protected by a fairly tough chorion or egg case. Within this the cytoplasm and yolk are contained by a vitelline membrane. Often one or more oil globules are present. The chorion has only one opening, the micropyle for the entrance of sperm. Usually the micropyle closes after entry of one sperm to prevent multiple fertilization. They are collectively referred to as egg activation. The vitelline membrane separates from the chorion creating a perivitelline space. Some elasmobranchs have polyspermy in which a number of spermatozoa may enter the micropyle but only one fuses with the nucleus of the egg and the rest are being resorbed and perhaps used as an additional nutrient. After fertilization the chorion hardens. The egg goes through a process of cleavage and morphogenesis as the cells divide form layers and then organs.