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The presence of four taxa at the species and subspecies level was revealed based on morphological investigation of a small collection of the representatives of the genus Bythotrephes Leydig, 1860 from Austrian subalpine lakes. The presence of a new species, B. inexpectatus sp. nov. was revealed in two lakes and the presence of a new subspecies, B. longimanus austriacus ssp. nov., in six lakes. Specimens of the population from the Lake Halstättersee, distinguished by the presence of peculiar features, are provisionally assigned to the species B. brevimanus Lilljeborg, 1901 of which the main range is confined to Circumbaltic region. Few morphologically specific specimens from Wolfgangsee could not be accurately identified and therefore assigned to the taxon Bythotrephes sp. requiring further investigation. Previously described taxa from the region, B. styriacus Ischreyt, 1939 and B. longimanus var. carnica Ischreyt, 1939, were considered either required further taxonomic evaluation or having an unclear taxonomic status, respectively. In one lake, the co-occurrence of two species was recorded, which is a rather rare case for the representatives of the genus. The potential reasons for the high species richness of spiny waterfleas in Austrian subalpine lakes and their possible origin are briefly discussed.

Climate change and other anthropogenic stressors have led to long-term changes in the thermal structure, including surface temperatures, deepwater temperatures, and vertical thermal gradients, in many lakes around the world. Though many studies highlight warming of surface water temperatures in lakes worldwide, less is known about long-term trends in full vertical thermal structure and deepwater temperatures, which have been changing less consistently in both direction and magnitude. Here, we present a globally-expansive data set of summertime in-situ vertical temperature profiles from 153 lakes, with one time series beginning as early as 1894. We also compiled lake geographic, morphometric, and water quality variables that can influence vertical thermal structure through a variety of potential mechanisms in these lakes. These long-term time series of vertical temperature profiles and corresponding lake characteristics serve as valuable data to help understand changes and drivers of lake thermal structure in a time of rapid global and ecological change., (© 2021. The Author(s).)

A new species of the genus Bythotrephes is described based on material collected in the mountain lakes of Tyva Republic (Southern Siberia, Russia). The representatives of the new species are especially close to those of B. longimanus having long tl I, straight caudal process, and only two pairs of claws on postabdomen and caudal process, respectively. At the same time, the new species is, on average, smaller with shorter caudal process and fairly long apical setae of second endopodital segment of the thoracic limbs of first pair (tl I). Supplemental data on morphology, taxonomy, and geographic distribution of other species of the genus, e.g., B. arcticus, B. cederströmii, B. brevimanus, and B. lilljeborgi, are presented. For B. cederströmii, in particular, females of first generation hatched from resting eggs are described for the first time. Classification of the genus is discussed and an updated key for species and a hybrid form is presented. The highest species richness of the genus, observed within the Scandinavian Peninsula and in the north of European Russia, may indicate the central region of primary speciation which generally coincides with the region of the last Quaternary maximum glaciation. The isolated occurrence of three species, B. longimanus, B. transcaucasicus, and B. centralasiaticus sp. nov. on the southern border of the genus' range, in pre-Alpine and mountain lakes of Europe, Transcaucasia, and Southern Siberia (Tyva) and isolated localities of B. arcticus in Northern Kazakhstan, could also be due to the effect of glaciation(s). Regarding the origin of the genus Bythotrephes, it is hypothesized that it occurred in pre-Pleistocene time in the ancient Ponto-Caspian basin, experiencing large-scale transgressions, where the intensive radiation of ancestral Onychopoda probably took place.

The invasive species of the genus Bythotrephes introduced in the North American lakes in 1970s-early 1980s was named variously either B. cederstroemi or B. longimanus. The investigation of individuals of the genus from 15 Canadian lakes has allowed us to identify all of them as B. cederströmii Schödler, based on the detailed taxonomic redescription of the Eurasian representatives of the species. The available documented data (figures and photographs) on Bythotrephes from other North American lakes, made it possible to recognize their identity with this species as well. The possible introduction of interspecific hybrids of the genus Bythotrephes in the North American lakes has not been confirmed.

The North Eurasian interspecific hybrid forms of the genus Bythotrephes Leydig, 1860 are described morphologically in detail with consideration of the previous data on their allozyme analysis and brief morphological examination. In total, three hybrid forms have been encountered, among which B. brevimanus x B. cederströmii appear to be most common and widespread whereas two others, one of which is new to science, are known from single localities. The high morphological variability of the former hybrid form is analyzed and its diagnostic features, distinguishing their representatives from those of parental species are determined. Within their vast range, the hybrids B. brevimanus x B. cederströmii seem to be one of the most common and numerous members of the genus. For instance, they dominate in all reservoirs of the Volga River where their biology was investigated in some detail. The hybrids are distributed widely being known from Sweden, Finland, and north-west of European Russia to Central Yakutia in Eastern Siberia. The southern borders of their range extend to the south of Eastern Siberia and Eastern Kazakhstan. Further more detailed investigations of hybrids, both genetic and morphological, are highly desirable.

Two species of the genus Bythotrephes Leydig, B. brevimanus Lilljeborg and B. cederstrӧmii Schӧdler, are analysed. The former of them is redescribed on the basis of type material and other materials from Northern Europe while the latter one is reevaluated, in comparison with the previous recent investigation which was based on limited material being not enough for the adequate species assessment. The species under consideration were investigated with the application of detailed morphological analysis, including intra- and interpopulation variability and the original scheme of morphometric measurements. The lectotype and paralectotypes of B. brevimanus were selected. Both species are characterized by considerable morphological variability which was especially prominent in taxonomically important features, such as structure and armament of thoracic limbs of first pair (tl I), caudal process, and claws of postabdomen and caudal process. It was concluded that the taxonomic identification of such variable units like B. brevimanus, B. cederstrӧmii and hybrid forms close to the latter species should be based only on comparison of sets of specimens of particular populations; the usage of single specimens can lead to incorrect results. Pedogenesis, in B. brevimanus and in family Cercopagididae in general, was recorded and described for the first time.A new species, B. lilljeborgi, is described based on material from Southern Sweden and Norway. It differs from the closely related species B. longimanus and B. brevimanus in number and size of claws of postabdomen and caudal process.The ranges of the above considered species overlap, especially prominently in North-Western Europe where all three species under consideration co-occur. B. brevimanus dominates in the Circumbaltic region and eastward through Central European Russia and the Ural Mountain region, possibly to the Yenisei River which requires further confirmation. B. cederstrӧmii is distributed further to the east, being recorded in Central Yakutia (Eastern Siberia). Data on co-occurrence of Bythotrephes species are limited and not always adequate because identification of taxa was often confused. Based on the information contained in the present paper, in particular the reevaluation of B. cederstrӧmii taxonomy, it may be suggested that both interspecific hybrid forms and B. cederstrӧmii s. str. could have invaded North American inland waters.

We present an annotated checklist of the Cladocera (orders Ctenopoda and Anomopoda) from the continental waters of Algeria, based on published records and original data from analysis of samples from 112 water bodies collected in 2012-2016 in humid and semi-arid regions of the country. Thirty six species have been identified in this study. Three taxa (Daphnia obtusa, D. mediterranea and Ceriodaphnia cf. quadrangula, being an undetermined taxon belonging to the Ceriodaphnia genus) are new to Algeria and eight (Daphnia curvirostris, D. galeata, Macrothrix dadayi, Scapholeberis rammneri, Acroperus angustatus, Ovalona nuragica, O. orellanai and Coronatella anemae) are new to the Maghreb in general. The number of Cladoceran species recorded in Algeria has been raised to 81. More sampling efforts are needed to make this list more complete.

Diaphanosoma turkanae sp. nov. is described from Lake Turkana (Kenya, East Africa). This species is the second Afrotropical endemic in the order Ctenopoda. It is characterized by a distinctive feature, the presence of an unusually small number of setae on the upper two-segmented antennal branch (exopodite); the proximal segment of the branch always bears three setae while the distal segment bears either six or seven setae. Morphologically the new species is most similar to D. orghidani Negrea, also occurring in Africa and presumably sharing an evolutionary affinity. D. turkanae sp. nov. co-occurs with two other species of the genus, D. lacustris Kořinek and D. excisum Sars, which are briefly described here. Temperate-subtropical species of the genus Diaphanosoma penetrate far south along the Nile River system up to the equatorial and Ethiopian lakes and further southwards, often forming multi-species assemblages.

Two species of the genus Bythotrephes Leydig, B. arcticus Lilljeborg and B. transcaucasicus Behning, forming a group of close species, are redescribed on the basis of type material and other materials from Northern Eurasia. They were investigated with the application of detailed morphological analysis, including intra- and interpopulation variability and the original scheme of morphometric measurements. The lectotypes and paralectotypes of the species were selected. B. arcticus compared with other species of the genus, seem more evolutionary primitive and less specialized, judging from its large body size, comparatively short tl I, short and straight caudal process, well developed distal setae on two proximal endopodital segments of tl I, and large number of claws. B. arcticus tends to occur in small and shallow water bodies, while B. transcaucasicus was predominantly found in pelagic zone of large and deep Transcaucasian lakes. B. arcticus is probably mainly distributed along the tundra and northern forest area of the Eurasian continent and the nearest islands from Scandinavia to Eastern Siberia. Its documented easternmost locality is situated near the northern part of the Yenisei River, whereas B. transcaucasicus was recorded in the second half of 19 th -first half of 20 th century in three closely situated mountain lakes in Turkey, Georgia, and Armenia. The presence of B. arcticus in Northern Kazakhstan and B. transcaucasicus in the distant Transcaucasia is linked with their (or their ancestor) hypothetical displacement to the south in the period of ice ages. More recent surveys of Transcaucasian lakes have not revealed the presence of B. transcaucasicus. The rare findings of this species in the past can imply that it could also be missed in the process of collecting but, on the other hand, it cannot be excluded that B. transcaucasicus become extinct as a result of an anthropogenic or other impact on Transcaucasian lakes.

Two core species of the genus Bythotrephes Leydig, B. longimanus Leydig and B. cederströmii Schӧdler, are redescribed on the basis of the type material, topotypic material, and other materials from Western and Northern Europe. They were investigated with an application of detailed morphological analysis, including intra- and interpopulation variability and the original scheme of morphometric measurements. The neotype of B. longimanus was selected from the topotypic population of the species from Bodensee (Switzerland-Germany). The taxonomy of B. cederströmii and of the whole genus is complicated by the presence of a variety of similar forms with intermediate morphology supposed to be interspecific hybrids, which much confused previous researches. The evolutionary transformations of some morphological structures, such as thoracic limbs, abdomen, and caudal process, are analyzed and discussed. In particular, it is suggested that the internal side of the endopodites of the thoracic limbs of the second-fourth pairs grew in proximal direction, occupying the part of protopodite with the "gnathobasic" process, which remains to be homologous to those of the thoracic limbs of the first pair. The strongly reduced thoracic limbs of the fourth pair lost two terminal segments and have proved to be represented by three segments, two proximal of which are those of protopodite, while the third terminal segment is composed of the fused distal part of the protopodite and the first proximal endopodital segment, bearing transformed and specifically arranged setae. The specificity of the "gnathobasic" processes of Bythotrephes is that they are the derivatives of the second, not of the first, inner endite of the thoracic limb and for this reason they are not homologues to the gnathobases of other cladocerans and should be called pseudognathobases.The same position of the "gnathobasic" processes is characteristic for other Onychopoda, which stresses the uniqueness of the representatives of the order among Cladocera and the whole class Branchiopoda. The abdomen of Bythotrephes was found to be three-segmented, but the third distal segment has proved to be transformed and fused with the postabdomen, participating in the formation of a long and massive caudal process. The comparison of morphological features of Bythotrephes and Leptodora clearly shows their independent homoplasic evolutionary development and distant phylogenetic position. According to new available data, the range of B. longimanus embraces large and deep lakes of Switzerland, Southern Germany, North Italy, Austria, and Great Britain, whereas B. cederströmiis. str. is certainly known only in the south of Sweden (probably also in Finland). All other B. cederströmii-like forms, supposed to be interspecific hybrids, are much more widely distributed in Eurasia from Northern Europe and Central European Russia to the Lower Volga River and Yakutia in Eastern Siberia. It is suggested that just these hybrids invaded and conquered the North American inland waters.

Diaphanosoma macedonicum sp. nov. is described from material collected from the ancient Lakes Dojran and Prespa, located in the central part of the Balkan Peninsula. It can be regarded as a member of the "D. mongolianum" species group. It is characterized by specific, but not readily visible features, such as the absence of a thorn near the posterior valve margins, as well as setules between setae of the ventral valve inflection, and the presence of more chitinized integument. The discovery of this new species previously identified as "Diaphanosoma brachyurum (Liévin)" highlights the necessity of more detailed investigations of the zooplankton of Balkan lakes potentially populated by greater numbers of endemic cladoceran species. A short overview of the ancient lakes in the Central Balkans is provided.

Limnosida frontosa Sars, 1862 is redescribed based on the material from over its entire range, from Norway and Finland in the west to Yakutia in eastern Siberia and the Lower Amur River basin (Far East of Russia) in the east. Morphological analysis of the species has revealed a combination of advanced and evolutionary primitive features, latter characterizing Limnosida as the most primitive member of the order Ctenopoda. Investigation of intra- and interpopulational morphological variability, in particular of the post abdominal claws, led to a tentative separation of the species into two main forms, occurring regionally in the north-west and east of Northern Eurasia. Their taxonomic status as yet remains unclear. Some overlap in geographical distribution of these forms and the presence of populations with intermediate morphological features probably reflects irregularity of the diversification process in L. frontosa s.l. within its range. This diversification may be the result of both geographic disjunction and adaptation to waters of different trophic status. The low level of intraspecies differentiation and comparatively limited geographic distribution of this species imply its young age while the genus Limnosida itself is presumed ancient.