Revision of the <i>Acanthocyclops vernalis-robustus</i> group (cyclopinae, cyclopoida, copepoda) from continental water bodies of the palaearctic with description of a new species

A revision of the complex vernalis-robustus group of Acanthocyclops, characterized by 17-segmented antennules and elongated caudal branches, from Palaearctic water bodies is carried out. In total, about 10 taxa of this group have been described from this region, including the invasive American species Acanthocyclops americanus (Marsh, 1892). After this revision, six taxa are recognized as valid, including another species, A. borutzkyi Chaban et Alekseev sp. n. from Sakhalin Island, described here as new. Acanthocyclops magistridussarti Stoch et Bruno, 2011, from groundwaters in Italy, is synonymized with A. orientalis Borutzky, 1966, which also inhabits caves in Primorye of Russia and in South Korea. Acanthocyclops trajani Mirabdullayev et Defaye, 2002 and A. einslei Mirabdullayev et Defaye, 2004, both described from France and Germany, are downgraded to junior synonyms of A. americanus and A. robustus (G. O. Sars, 1863), respectively. Descriptions of all six valid taxa are provided and an identification key is given to all species of this group from Palaearctic water bodies.