Although the fisheries for, and mariculture of, penaeid prawns are of major commercial importance, there has been relatively little research undertaken on the chromosome number, structure and composition in the Penaeidae (Xiang 1988 , Xiang et al. 1991. One reason for this is due to the relatively small size and large number of chromosomes, which makes production of histological material difficult. In this paper, we report a simple and effective technique for determining chromosome complements

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... uring spermatogenesis in two species of penaeid prawns, Penaeus merguiensis and P. esculentus in Australia. The first estimates of the number of chromosomes in these species are given. Materials and methods Specimens. Specimens of P. esculentus and P. merguiensis were obtained by trawling in Moreton Bay (27°S, 153°E) southeast Queensland. The prawns were sorted from the by-catch and kept alive by placing them in fresh, aerated seawater. Only males were used for chromosome preparations. The total lengths of the individuals were 71-132 mm for P. esculentus and 130-140 mm for P. merguiensis. Tissues. The prawns were injected with 0.1-0.3 ml of 0.2% (w : w) colchicine solution into the muscle of the first or the second abdominal segment, and kept alive in seawater for 4-5 hr. Individual prawns were then sacrificed, the testis dissected out and cut into 2-3 mm pieces. The testicular tubules were placed in 0.7% (w : w) KC1 solution for 30-35 min. The hypotonized testicular tissue were then fixed for 15-20 min in fresh Carnoy's solution (methanol : acetic acid at 3 : 1). The fixative was then changed twice; firstly at the same (3 : 1) methanol : acetic acid concentration for 15 min, and then at a ratio of 1 : 1, for a further 15 min. The fixed tissues were stored in a refrigerator at 4°C until histological preparation. Chromosome preparation and observation. One or two small pieces of tissue were placed on a clean slide with 2-3 drops of fresh fixative (3 : 1), smeared and spread using forceps. The wet slide was left at room temperature for partial drying. The preparations were stained with 5% Giemsa with a phosphate buffer (pH = 7.0). After immersing in the Giemsa stain for 15-20 min the slide was rinsed with running tap water and then with distilled water. Chromosome counts were made using an optical microscope and only on cells that were spread such that individual chromosomes did not physically touch one another or overlap.