M.Sc. (Zoology) / The alien fish parasite Argulus japonicus Thiele, 1900 has been introduced to the African subcontinent via exotic fish species. This parasite is now found widely distributed throughout the Transvaal and has assumed near epidemic proportions among fish in the Vaal River system. . Aspects of the reproduction of this parasite were studied. Infected fish from the Vaal River system were maintained in the laboratory in order to establish a stock of A. japonicus. These infected fish, together with Cyprinus carpio Linn. ,1785 from fishponds in Lydenburg, were maintained in a 2125 cubic meter culture tank. This culture of A. japonicus was maintained for more than a year from which specimens were used for experimental work and scanning electron microscopy (SEM). The morphology and anatomy of the sexual organs of both the adult male and female parasites were described. A. japonicus males have thoracic appendages which are functional during copulation. A peg is present on the pre-axial margin of the basis of the fourth thoracic limb and a socket occurs on the dorsal post-axial side of the coxa of the third thoracic limb. The conventional concept concerning copulation is that during copulation the above structures are used only as a clasping mechanism. The peg fits firmly into the socket, keeping the limbs of the male in position, while clasping dorsally round the last thoracic limbs of the female...

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