Redescription of the rare, incompletely described Mongolodiaptomus rarus (Ranga Reddy, Sanoamuang & Dumont, 1998) (Copepoda, Calanoida, Diaptomidae), from freshwater habitats in northeast Thailand, with comments on conservation status

The genus Mongolodiaptomus Kiefer currently contains 17 species, distributed across Asia, predominantly occurring in countries in the lower Mekong River Basin. Herein, Mongolodiaptomus rarus (Ranga Reddy, Sanoamuang & Dumont, 1998), an incompletely described species, is redescribed based on specimens from Thailand. Morphologically, it belongs to the M. gladiolus species group, closely similar to M. calcarus (Shen & Tai, 1965), M. dumonti Sanoamuang, 2001, and M. gladiolus (Shen & Lee, 1963). M . rarus is, however, distinctly different from its congeners, in particular the prolonged oval shape of the male right leg 5-second exopod and the position and shape of the principal lateral spine on this segment. Mongolodiaptomus rarus occurs mostly in temporary-water habitats, although it also inhabits swamps, which are permanent-water habitats. It appears to be rare, restricted to a few localities, and has to date been recorded from 20 sites in five provinces. Currently, M. rarus is endemic to northeast Thailand. It was found during May and September, over a temperature range of 25–38 °C, pH 6.6–8.5, and conductivity of 2–299 µS cm −1 . The recent development of modern highway construction and the introduction of many agricultural crops have resulted in the disappearance of their habitats, so conservation of this rare taxon is proposed.