Wild female broodstock that have fully developed ovaries may spawn spontaneously on the night, or the night after, delivery to a hatchery. Generally, however, the females are eyestalk ablated to increase the likelihood of successful spawning in the hatchery. Obviously females spawn successfully in the wild without eyestalk ablation. It is not known what factors inhibit the female from spawning once in captivity, although stress in suspected to be a dominant factor. Eyestalk ablation has the paradoxical effect of both stimulating the reproductive system while at the same time placing the female under additional stress. However, it has the beneficial effect of synchronising spawning and is hence of considerable use in keeping to required post-larvae production cycles. Spawning is generally induced 4-7 days after eyestalk ablation in females with partially developed ovaries (Stage 2-3).

Spawning itself occurs at night. Normally the pending event is heralded by the female swimming continually around in the tank. Swimming activity may occur for several hours before spawning or may only occur a few minutes before. During spawning the last 3 pairs of pereiopods are held tightly together and are flapped vigorously. The unfertilised eggs are extruded from the paired ovipores located at the base of the third pereiopods. Simultaneously stored sperm are released from the thelycum. Both are ejected with considerable force where the eggs appear as a greenish, and the sperm as a whitish, cloud exiting from beneath the females as she swims. The sperm are non-motile and must come into contact by passive collision with the surface of the egg at the moment of ejection. It is believed that the vigorous flapping of the female’s pereiopods creates an intense vortex that increases the likelihood of sperm and egg coming into contact. The entire process lasts approximately 2 minutes, over which time up to 500,000 eggs of 0.3 mm diameter are shed through the 2 ovipores resulting in a minimum flow rate of 2.25 km/hr. Immediately after spawning the fertilised eggs are either slightly positive of neutrally buoyant. Within a few minutes the eggs gradually drift down through the water and collect on the bottom of the tank. Spawning may also occur in females that have not been inseminated. In such cases, although egg output per female may appear adequate, there will be no fertilisation and hence none of the eggs will hatch.

Egg coloration is mainly due to the accumulated carotenoids, which are fat-soluable nitrogen free yellow, orange, blue and red compounds. Carotenoids are only produced by plants and microbes, hence animals can only obtain them from their diets. Within the prawn or egg the carotenoids are complexed with proteins as carotenoproteins. Although the predominant carotenoid in eggs is the red carotenoid astaxanthin, the association with a protein causes a large bathocromic shift in the visual region of the light absorption spectrum producing different colors, ranging from brown, blue, green and yellow. When the protein is denatured, such as by heating, the carotenoid is freed revealing the true color of the underlying carotenoid. Since prawn eggs mainly accumulate astaxanthin the color is orange to red.

Figure 1.7. (a) Individual sperm cells of P. monodon. Note especially the small size (cf egg diameter of 250 – 300 microns) and lack of a flagellum – the sperm are not capable of swimming. The sperm adheres to the egg at the spike end that afterwards undergoes the acrosome reaction. Magnification 400x. (b) Section of spermatophore. The sperm cells appear darkly stained and are surrounded by tissue of the spermatophore itself. Magnification 40x.