Figure 1.3 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.

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 (fig 1.3a, b). It is believed that the follicle cells produce the yolk that is internalised in the oocytes in a process called vitellogenesis (fig 1.3c, d). 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 (fig 1.3e, f). 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 (fig 1.3g, h).

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 fertilisation.