Two New Species of the Taxonomically Ambiguous Mongolodiaptomus birulai Group from Southwestern China and Southeast Asia (Crustacea, Copepoda, Calanoida, Diaptomidae).

Currently, the synonymy of the two morphologically analogous copepod species, Mongolodiaptomus birulai (Rylov, 1922) and M. formosanus Kiefer, 1937, remains doubtful. While examining the detailed morphology of diaptomid copepods from China, Cambodia, and Vietnam, we came across two undescribed species whose morphology is closely similar to that of M. birulai from Northeast China and M. formosanus from Taiwan. In this paper, descriptions of M. parabirulai sp. nov. from Yunnan, Southwestern China, and M. longiserratus sp. nov. from Cambodia and Vietnam are presented. The first new species, M. parabirulai sp. nov., can be distinguished from its congeners in the male P5 by it having the distal outer portion of the second right exopod produce a wing-shaped expansion and its strongly robust left and right basis. The second new species, M. longiserratus sp. nov., can be differentiated by the following characteristics in the male: the spiniform process on segment 20 of the right antennule is longer than that of segment 21 and has a serrate outer margin, and the right P5 basis has two (longitudinal and semicircular) hyaline lamellae. Furthermore, the status of M. birulai and M. formosanus is reviewed and recommended.