Redescription and taxonomic status of Sinodiaptomus valkanovi, a common limnoplanktonic calanoid copepod in Japan, with comparison to the closely related S-sarsi

The limnoplanktonic calanoid copepod Sinodiaptomus valkanovi (Diaptomidae: Diaptominae) is redescribed in detail from Japan, where it commonly occurs in ponds and reservoirs especially around the Seto Inland Sea. The species has been referred to as a subspecies of S. sarsi outside Japan due to lack of subsequent morphological comparisons. Sinodiaptomus valkanovi is easily distinguishable from S. sarsi by having longer antennules in both sexes, a short projection on the right antennule segment 15 of the male, and a thumb -like posterodistal basal projection of the male left leg 5. These characte rs do not show significant variation, indicating differentiation at the spe cies level. S. valkanovi is restricted to Japan, in contrast to the wide-ra nge of S. sarsi in Asian waters. The Bulgarian population, from which the s pecies was originally described, and the North American population, which w as formerly recorded as S. sarsi, are considered to have been introduced fr om Japan with aquatic plants such as water lilies.