Mongolodiaptomus nakhonphanomensis sp. nov. (Copepoda, Calanoida, Diaptomidae) from north-eastern Thailand, with a note on M. pectinidactylus (Shen & Tai, 1964)

A new species of diaptomid copepod, Mongolodiaptomus nakhonphanomensis sp. nov. , is described from surface water bodies in Nakhon Phanom Province, north-eastern Thailand. The new species is assigned to the M. mephistopheles (Brehm, 1933) species group and is morphologically most similar to M. pectinidactylus (Shen & Tai, 1964), a rare species in Thailand. It can be distinguished from M. pectinidactylus by the following characters: I) In females, the new species has asymmetrical lateral wings on pediger 5 and the second exopod of the fifth leg, whereas M. pectinidactylus (Shen & Tai, 1964) shows symmetry in these structures; II) the female of the new species has a shorter and more robust seta on the second exopod of the fifth leg compared to that of M. pectinidactylus ; III) in males, the new species lacks a spinular row on the basis of the right fifth leg, which is present in M. pectinidactylus ; IV) the second exopod of the male fifth leg in the new species is distally expanded, whereas it is oblong in M. pectinidactylus ; V) the lateral spine of the second exopod of the male fifth leg is bent outwards in the new species, whereas it is straight in M. pectinidactylus . A brief re-description of M. pectinidactylus and a comparison with the new species are provided. This discovery raises the number of recognised species in the genus Mongolodiaptomus to fifteen worldwide. A biogeographical map is also presented.