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In this paper, two new species of Onychocamptus Daday, 1903 are described from Thailand: Onychocamptussatunensissp. n. and Onychocamptustratensissp. n. The following features mainly distinguish O.satunensissp. n. from known species: internal sausage-like and internal rounded structures on cephalothorax and one outer seta on the male P5 exopod that is as long as the supporting segment. In contrast, the cephalothorax of O.tratensissp. n. is smooth but has rounded integumental window-like structures, and the outer seta on the male P5 exopod is two times as long as the supporting segment. Onychocamptusanomalus shows the highest similarity with the two new species, but in contrast to both Thai species, it has only one seta on the exopod of the antenna. In addition, in the present study, two additional species, O.bengalensis and O.vitiospinulosa, are newly recorded in Thailand. Thus, the number of Onychocamptus species recorded in Thailand increases to five species. A key to all known species of this genus in the world is also proposed.

Parategastes pholpunthini sp. n. is described and illustrated based on material collected in the Thale Noi Lake, Phatthalung province, southern Thailand. This species can be distinguished from its congeners by the number segments of female antennule, the lengths of rami and basis of P1, the shape of middle inner seta of P4 exp-3, shape of P5, and relative lengths of spine at apically of baseoendopod of P5. The differences among Parategastes species are pointed out and they are compared with the new species. An identification key to species of the genus Parategastes are proposed.

Tropodiaptomus is one of the most specious genera in the family Diaptomidae, but it is often rare in terms of distribution and abundance. Moreover, Tropodiaptomus species show a noteworthy variability in some of the morphological characters considered of prime importance in diaptomid taxonomy, and the presence of cryptic or pseudocryptic species is likely. Thus, through a geographically-wide sampling in Thailand, we aimed to investigate the local diversity of the genus and to compare the morphological and molecular diversity pattern based on mitochondrial and nuclear genes sequences. DNA taxonomy was also implemented in order to check whether the Tropodiaptomus lineages were independent species according to the “evolutionary genetic species concept”. Six Tropodiaptomus morphospecies were found, three of which are putative species new to Science pending a formal description. The finding of such a high incidence of undescribed species stresses the existence of a significant “Linnean shortfall” affecting Thai diaptomids. The molecular results showed that most of the studied species could be identified consistently with their morphology-based taxonomy. However, Tropodiaptomus vicinus and T. cf. lanaonus showed a high level of genetic diversity, suggesting that traditional morphological techniques might be inadequate for correctly assessing their taxonomical status.

Tropodiaptomus is a genus of diaptomid copepods with 10 species currently recorded in Thailand. A recent study on DNA taxonomy revealed putative new species among specimens collected from freshwater habitats throughout Thailand. This study examined the morphological characteristics and confirmed the taxonomic status of the two putative new species of Tropodiaptomus. Results showed that the two new taxa were different from other species in the genus Tropodiaptomus. These two new species, T. pedecrassumsp. nov. and T. longiprocessussp. nov., were described and illustrated based on material collected from a swamp in northern Thailand and a pond in western Thailand, respectively. They were distinguished from their congeners by the length of the spinous process on the antepenultimate segment of the adult male right antennule, the number of lobes and serration pattern on the inner margin of the adult male left P5, and the shape and supplementary process on the surface structures of basis and distal exopod segments of the adult male right P5. These discoveries increased the number of records of this genus in Thailand to 12 species. A pictorial key to all species is provided, and their ecological and biogeographical distributions are updated and discussed.