Origin and taxonomic position of Far Eastern island populations of Eurytemora caspica tethysiana subsp. nov.

Abstract

The article describes a new subspecies Eurytemora caspica tethysiana subsp. nova that, according to its genetic and morphological features, is closely related to the recently described species Eurytemora caspica caspica from the Caspian Sea. According to our hypothesis, both of these taxa are relics of the fauna of the Tethys Sea in the last phase of existence (Paratethys).The described form occupies an intermediate position between Eurytemora caspica caspica and Eurytemora carolleeae by its morphological characteristics. The time of divergence of the described subspecies and the original forms according to molecular clocks is in good agreement with the geological events associated with the evolution of the Tethys Sea (8–20 MYA), which indicates a very slow rate of evolution among representatives of Eurytemora. The morphological differences of our subspecies from the nominative Caspian subspecies are: the shapes of the genital double-somite and P5 distal segment in females. In males sexually dimorphic differences are in the shapes of left P5 exopod and distal segment of the right P5, in ratio L abdomen/L caudal rami.At the same time, both forms belong to the affinis group of species, which includes also Eurytemora carolleeae and Eurytemora affinis. The new taxon is described and the problem of the Eurytemora species’ evolution in the affinis group is discussed in the present work.