The Far East of Russia is a region where boreal and tropical faunas mix; it is also a zone of cladoceran endemism. The present study aimed to compare a set of microcrustacean (Cladocera and Copepoda) associations in three large lakes of the Russian Far East: Khanka, Bolon, and Chukchagir. The associations of the microcrustaceans were identified based on the function of the discrete hypergeometric distribution. Many of the 108 taxa found here were unaffiliated with an association. Interestingly, the portion of taxa involved and “not involved” in species associations differed among geographic faunistic complexes. The rate of endemism was significantly higher among the taxa incorporated into the associations as compared to the “not involved” taxa. In all the lakes, there were large clusters of phytophilous species characteristic of the macrophyte zone (and its margins) and clusters characteristic of pelagic and sublittoral plankton. We found that in the three lakes, the microcrustaceans formed a set of functionally similar associations, but the taxonomic composition of each functional association was specific to each lake. We hypothesize that the composition of functional clusters reflects the history of colonization for each water body. That is, the founder effects and subsequent “monopolization” of habitats have affected species associations.

The aim of this paper is to study the species composition, structure and seasonal dynamics of the cladoceran fauna and species associations in inland water bodies of Jeju Island (Republic of Korea). Only 47 taxa were found in 199 samples. Such faunal paucity could be explained by the existence of only a few types of aquatic environments on Jeju Island as compared to continental China and the Far East of Russia, with their great diversity of water types. We have demonstrated a high significance of the tropical species on Jeju. Our data confirm seasonal faunistic changes in the continental waters of Jeju Island. The rate of tropical taxa is highest in September, after the monsoon season, while the Far Eastern endemic taxa are more common in winter. At the same time, the contribution of Boreal taxa to the fauna of the island is low even in winter. Species associations have been revealed based on binominal distribution; they change significantly from summer to winter. However, a contribution of Boreal taxa to the species associations also is minimal (even in winter), whereas tropical taxa contribute to them greatly (including in winter). We can propose a rough scheme of faunal formation exploring the whole set of obtained information. Initially, at the earlier stages of the island formation, its fauna was consisted of some pre-Pleistocene taxa. Then, during the Pleistocene time, Jeju Island was secondarily interconnected with more southern territories, and tropical species have colonized it. Recent Boreal cladoceran invaders arrived at the island water bodies later when they were already inhabited by formed associations, and for this reason only few of them were able to settle down there. Such a scheme is a hypothesis which needs to be checked by the future phylogeographic studies.

Cladocerans (Crustacea: Branchiopoda) of Northern Eurasia traditionally belong to the most intensively studied groups of freshwater microcrustaceans. However, despite a significant progress in our knowledge of different aspects on their biology, some intriguing questions remain unresolved. The hypothesis of cladoceran faunas differentiation between the western and eastern parts of Northern Eurasia agrees with the continental endemism concept. But to date few cladoceran species groups have been investigated through the whole North Eurasian range. Our current work contributes to the confirmation of this hypothesis through a revision of the poorly studied Pleuroxus trigonellus species group. Based on material from Europe, we redescibed morphology of Pleuroxus trigonellus (O.F. Müller, 1776) s.str. and found that its distribution range is potentially restricted to the western portion of Northern Eurasia. At the same time, morphologically similar populations from the eastern portion of Eurasia belong to a new species, Pleuroxus yakutensis sp. nov. These two species are basically similar in general morphology of parthenogenetic and gamogenetic females, but have clear differences in the male morphology. Additionally we offer an identification key for discrimination of the P. trigonellus group from P. uncinatus (Baird, 1850) according to morphological features of the males.

Based on original data, the most complete and updated species list of the water fleas (Crustacea: Cladocera) of both the southern part of the Russian Far East and Korea is represented. In total, 122 taxa are identified to the species level. The list includes 92 valid species recorded in the Far East of Russia and 71 in South Korea. Each taxon is attributed to one of the six zoogeographical groups: one among four faunistic complexes (widespread Eurasian, East Asian–American, endemic East Asian, and southern tropical), a group of widely distributed unrevised taxa, and a group of invasive species. With a decrease in latitude (i.e., in the north–south direction), (1) the species richness is decreased, (2) the dominance structure (based on species occurrence frequencies) is changed from two–three dominant species to a lack of clear dominants; (3) the rate of representatives of the widespread Eurasian complex is decreased, while that of the southern tropical taxa is increased, and (4) the taxocoenoses of the boreal type are changed to those of subtropical type. In addition to boreal and subtropical type taxocoenoses, three more taxocoenosis types are found in the Khanka Lake basin (the largest water body in the region). The relative abundance and occurrence of taxa from the endemic East Asian complex are similar within the entire region and do not vary with latitude.

Biotic introductions are an ongoing disruption for many ecosystems. For passively dispersed freshwater zooplankton, transcontinental introductions have been common but are poorly studied in the southern hemisphere. Here we assess the hypothesis of recent introduction for populations of the Chydorus sphaericus group (Crustacea: Cladocera) in Australia. We analyzed 254 sequences (63 original sequences) from the cytochrome oxidase I region of mitochondrial DNA of Chydorus sp., which included global representation. Three Australian populations were connected with separate clades in the northern hemisphere, suggesting multiple colonization events for Australia. The timescale of the divergences was consistent with recent (Quaternary) dispersal. As Australian populations are exposed to migrating birds from the northern hemisphere, both avian and anthropogenic sources are candidates for dispersal vectors. We concluded that recent cross-hemisphere dispersal in the Chydorus sphaericus group is more common than previously believed.