The taxonomic position of Asian Holopedium (Crustacea: Cladocera) confirmed by morphological and genetic analyses

The cladoceran Holopedium gibberum Zaddach, 1855 (Ctenopoda: Holopediidae) was once thought to occur broadly in the northern hemisphere, but its cryptic sister species was recently separated from H. gibberum sensu stricto (s.s.) as a new species, Holopedium glacialis. In East Asia, although “H. gibberum” occurrence has been recorded in many water bodies, the identity of the surveyed populations has rarely been confirmed via molecular analyses. Thus, it is unclear whether it is actually H. gibberum s.s. or H. glacialis that is distributed in East Asia. We used DNA-barcoding techniques to check the taxonomic status of Holopedium samples collected in Japan. We sequenced mitochondrial cytochrome c oxidase subunit 1 (mtCOI) and nuclear 18S ribosomal DNA (nr18S) of Japanese Holopedium and compared the results with those of H. gibberum s.s. collected in Norway and H. glacialis collected in Canada. The mtCOI sequence divergences between Norwegian H. gibberum s.s. and Japanese Holopedium were at most 2.4%, which was within the degree of intraspecific differentiation in cladocerans. Norwegian H. gibberum s.s. and Japanese Holopedium shared identical nr18S haplotypes. Individuals of Canadian H. glacialis were genetically different from those of Japanese Holopedium. We therefore concluded that Japanese Holopedium can be identified as H. gibberum s.s.