Elaphoidella quangnamensis sp. nov. (Copepoda, Harpacticoida, Canthocamptidae), a new hyporheic species from central Vietnam

A new species of the genus Elaphoidella Chappuis, 1929 has been discovered in the hyporheic zone of a stream in Quang Nam Province, central Vietnam. This represents the second endemic species of Elaphoidella recorded in Vietnam, following E. vietnamica Borutzky, 1967. The newly described species, E. quangnamensis sp. nov. , is morphologically most similar to E. brancelji Sanoamuang & Watiroyram, 2021 from Thailand, a member of Group VIII (the sewelli group) sensu Lang (1948). Both species share several key morphological features, including a serrated free margin on urosomites 2–4 dorsally, anal operculum with more than 25 spinules along free distal margin, caudal rami lacking spinules on the inner margin, and similar armature on P1 and P3–P6. The new species differs markedly from E. brancelji in some characters: I) caudal rami parallel in the new species but slightly divergent in E. brancelji ; II) caudal ramus length/width ratio in the new species about 1.2 times but 2.0 times in E. brancelji ; III) base of caudal ramus with two spinules ventrally but four spinules in E. brancelji ; IV) P1Enp-1 about 1/2 length of Exp-2 but 2/3 in E. brancelji ; V) P2 Enp-2 of the new species with three setae but four setae in E. brancelji ; VI) P5 baseoendopod of the new species with prominence at insertion of seta II but absent in E. brancelji . The new species represents the sixth known species of Elaphoidella recorded from Vietnam.