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 visualisation 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 visualised with a light beam as a large centrally located opaque rope-like structure, and classified as Stage 2.