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The basic technique differs between penaeid species depending on whether the females possess a closed or open thelycum. With the former, for example with P. monodon, females must be inseminated just after they moult and before the new shell hardens. Whole spermatophores, manually extracted from the males, are transferred to the thelycum where they will be retained during the inter-moult period. Females of open-thelycum species, such as P. vannamei and P. stylirostris, require insemination prior to each spawn, and it is usual to apply only the sperm mass to the thelycum after squeezing it out from extracted spermatophores. Although artificial insemination has clearly demonstrated its viability and is routinely used in some P. vannamei hatcheries, most maturation units continue to rely on natural impregnation occurring within the maturation tanks since less labour is involved, mortality rates are lower, and more consistent fertilization rates are obtained (Bray & Lawrence, 1992). |