Two New Species of <i>Parastenocaris</i> (Copepoda, Harpacticoida) from a Hyporheic Zone and Overview of the Present Knowledge on Stygobiotic Copepoda in Vietnam

The number of freshwater species belonging to the genus <i>Parastenocaris</i> reported from ten countries of Southeast Asia is quite limited. Only two species have been reported so far from freshwater habitats there, compared to over 290 described species of the family Parastenocarididae worldwide. During the first study of the hyporheic zone of two small rivers in central Vietnam, two new species of the family Parastenocarididae were collected, <i>Parastenocaris sontraensis</i> sp. nov. and <i>Parastencaris vugiaensis</i> sp. nov. Both were collected from the gravel bar along the rivers (Suoi Da and Vu Gia river) using the Karaman–Chappuis method. Both the new species belong to the <i>brevipes</i> group of the genus <i>Parastenocaris</i> Kessler, 1913 sensu Lang (1948), and Reid (1995). <i>Parastenocaris sontraensis</i> sp. nov. is similar to <i>P. hinumaensis</i> Kikuchi, 1970 and <i>Parastenocaris jane</i> Karanovic, 2006 in the <i>brevipes</i>-group. <i>Parastenocaris sontraensis</i> sp. nov. differs from both <i>Parastenocaris</i> species by (i) Exp P3 with three segments in the male, (ii) caudal rami with seven setae, and (iii) caudal rami about 2.4 times as long as wide. <i>Parastencaris vugiaensis</i> sp. nov. can be distinguished from its congeners by the unique combination of the following characters: (i) the elliptical shape of caudal rami, (ii) apical seta (V) with bulbous base, and (iii) anal operculum extends beyond the end of anal somite. Until now, 14 stygobiotic species of Copepoda have been recorded in Vietnam (including two new species in this paper), which is relatively few compared with nearby Thailand with 25 species. Short comments on other stygobiotic Copepoda from Vietnam are added.