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Genetic protection of improved lines of Giant Tiger shrimp, Penaeus monodon, may be achieved through the reliable production of all triploid families, as triploid shrimp are reproductively sterile. The problem with current triploid induction methods is in producing 100 % triploid progeny reliably and doing so on a commercial scale. All triploid offspring may be achievable by mating tetraploids with diploids, overcoming current limitations. However, tetraploidy in shrimp to date has been unsuccessful, with all tetraploids dying very early in development. As the induction method of heat was found to be lethal, this study investigated an alternative method using cold water to assess induction rates and lethality. Cold-water treatments between −2 and 14 °C were applied at 18, 20, and 22 min post-spawning detection for various durations with the aim of inhibiting first mitosis. Tetraploidy was induced in treatments at or less than 1 °C; however, no nauplii hatched from these treatments. As no diploid or tetraploid nauplii hatched from any of the treatments below 3 °C, it was determined that this cold-water treatment method is not suitable for tetraploid production in P. monodon. [ABSTRACT FROM AUTHOR]

The influence of body size on the consumption of live zooplankton ( Daphnia spp.) by freshwater crayfish was examined using yabbies ( Cherax destructor) ranging from 5 to 45 g. Food preference between live zooplankton and inert pellets was also assessed under experimental conditions. In experimental tanks, yabbies of four size classes (<15, 15–24.9, 25–34.9 and 35–45 g) were presented with live Daphnia. All yabbies were held in separate tanks with five animals per size class. In yabbies less than 15 g, the feeding mode on zooplankton involved rapid searching and probing with the first two pairs of walking legs. Once a prey was located, the chelae on the end of these walking legs would grasp the zooplankton and then rapidly move it towards the mouthparts. Yabbies larger than 25 g tended to use their walking legs to push the Daphnia nearer to their third maxilliped which would then force or scoop the zooplankton towards the mouthparts. A short-term feeding trial showed that there was no significant difference between size classes in regards to zooplankton consumption ( P > 0.05). Capture efficiency of live Daphnia by yabbies less than 15 g was significantly lower (76%, P = 0.008) than the three larger size classes (93.6%). Yabbies less than 15 g consumed a significantly ( P < 0.001) higher percentage (5.2%) of their body weight than the other size classes (1.1%, 0.8%, and 0.6%, respectively). In the presence of both live zooplankton and a pellet diet, yabbies spent significantly ( P = 0.005) more time feeding on zooplankton (85%) than on inert pellets (15%). This was the first study to quantify zooplankton consumption by yabbies and the results provide insights into understanding the trophic role of freshwater crayfish in structuring zooplankton communities and the husbandry management of crayfish farming. [ABSTRACT FROM AUTHOR]