Physiology of Finfish and Shellfish (2+1)

3.1.30. Breeding Biology of Penaeus monodon

The ovary lies dorsal to the gut and extends from the cephalothorax (head and thorax region) along the entire length of the tail. The determination of ovarian development reveals the shadow of the ovary in the tail region and is scored from 1 to 5.

Line drawing outline of female with ovary in situ.

Colour photo of 3 stages of ovarian development

Figure 1.2 The view observed by hatchery operators when female broodstock are graded for ovarian development by torchlight. The wide saddle of ovarian tissue directly behind the carapace (Stage IV) is indicative of an immediate pre-spawning female. A female scored as a Stage IV during the day is most likely to spawn that night.

The intensity of the ovarian shadow is due to the different density of the ovary and the pigmentation of the egg mass. Although the majority of the ovary is found within the cephalothorax area, the intense pigmentation of the shell in this region prevents the visualization of any ovarian outline. In immediate post-spawning females a vague shadow may be seen which is the area the previously enlarged ovary occupied. In some instances a dark shadow is seen with intermittent areas of vague shadow outline. This indicates that a partial spawning has occurred and that only a proportion of the egg mass was ovulated (spawned).

In penaeid prawns the ovaries are paired, but partially fused in the cephalothoracic region, and consist of a number of lateral lobes. In an undeveloped state, the ovary either does not cast any shadow or a thin opaque line is seen along the length of the tail, and is scored as Stage 1. At this point the ovary is comprised of a connective tissue capsule surrounding a soft vascular area containing future eggs, called oogonia, and accessory cells, also called follicle or nurse cells. The internal wall of the ovary capsule is lined with epithelial cells (called the germinal epithelium) containing oogonia. Once the female is sexually mature, the germinal epithelium will produce oogonia by mitosis division throughout the reproductive life of the female. The eggs develop from oogonia in an area known as the zone of proliferation. As the oogonia develop they increase in size and enter the first stage of meiotic division and henceforth are irreversibly destined to become haploid, with only one set of maternal chromosomes. At this point, although the developing eggs are increasing in size they are not as yet producing yolk, and are known as previtellogenic oocytes. At this stage the ovary can be visualized with a light beam as a large centrally located opaque rope-like structure, and classified as Stage 2.

Typically, it is at Stage 2 that inhibitory hormones in the eyestalk, arising from what is called the X-organ-sinus-gland (XO-SG) neurosecretory complex, prevent further ovarian growth, especially if nutritional or environmental conditions – such as those while females are held in captivity in tanks – are deemed unfavourable by the female.

This block on ovarian development can be removed by eyestalk ablation. However, since the prawn is bilaterally symmetric, ie with two eyes, eyestalk ablation only results in the removal of one of the two XO-SG complexes producing an inhibitory substance. Nevertheless, in most cases this is sufficient to induce further ovarian development and spawning, albeit under sub-optimal conditions.

As the oocytes develop further they migrate out towards the margins of the ovarian lobes in preparation for ovulation. During this migration, follicle cells are attached to the periphery of each oocyte. It is believed that the follicle cells produce the yolk that is internalised in the oocytes in a process called vitellogenesis. As vitellogenesis proceeds, oocytes mature synchronously as yolk accumulates and develop a characteristic dark green colour as a result of deposition of carotenoid pigments. It is the carotenoid pigmentation that mainly causes the dark ovarian shadow during illumination of the female by torchlight. The female is now in Stage 3. By the end of vitellogenesis, the eggs develop cortical granules filled with a jelly-like substance destined to form part of the egg shell membrane after ovulation. At this time the shadow cast by the ovary is large, resulting in a very distinct dark thick region extending the length of the abdomen, with an enlarged bulbous region directly behind the carapace, called the saddle. The saddle may not be as apparent in some broodstock, such as those that have made several spawnings after eyestalk ablation or in captive reared broodstock. The female is now in a pre-spawning state and is scored as a Stage 4.

The culmination of ovarian development is marked by release of the fully mature oocytes (eggs) into the oviduct at ovulation. Ovulation is typically followed within a few minutes by oviposition, ie the release of eggs and stored sperm into the water – the actual spawning event itself (see sections on mating and spawning). Within the egg the nuclear membrane disappears in readiness for fusion between the two (haploid) pronuclei, one in the egg originating from the female and the second, which comes from the sperm at the time of fertilization.

The complete ovary extends from the head to the tail. The majority of the ovarian mass is within the cephalothorax region which cannot be observed by torchlight.

A female can spawn several times within a single molt cycle. A new population of primary oogonia is recruited for each spawning event. If the female does spawn repeatedly within a molt cycle the same reserve of sperm within the thelycum, from the initial mating after molt, is utilized to fertilise the eggs. Typically, successive spawnings have progressively decreasing fertility rates as the sperm mass is exhausted. If the female does not spawn within a molt cycle, the developed ovarian mass is reabsorbed a day or two before the next molt. After each molt, the ovary is fully regressed and the ovary must mature a new.