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Species of the genus Daphnia O.F. Müller, 1785 (Cladocera: Daphniidae) have become very important models in evolutionary biology research. Previous morphological and genetic evidence suggests that numerous closely related "species groups" exist within the subgenus Daphnia (Ctenodaphnia) Dybowski & Grochowski, 1895, containing both described and undescribed species. The Daphnia similis group is among these species groups. The aim of the present paper is to revise the taxonomy of the Daphnia (Ctenodaphnia) similis group in the Old World with both morphological and genetic evidence (based on mitochondrial COI and 12S rRNA genes). We found that there are at least four species in the Old World D. similis species group: D. similis Claus, 1876; D. sinensis Gu, Xu, Li, Dumont et Han, 2013; D. similoides Hudec, 1991 and D. inopinata sp. nov. These four taxa of the similis-group, confused previously with D. similis, have different distributional ranges in the Old World, from extremely wide, spanning several biogegraphic regions (as D. sinensis), to regional endemics (D. similoides) and even species known so far from a single locality (D. inopinata sp. nov.). The Daphnia similis group provides another example in the cladocerans whereby the study of males yields more valuable characters for taxonomy than the study of parthenogenetic females.

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The name Diaphanosoma leuchtenbergianum Fischer, 1854 is included in the synonymy of the species Diaphanosoma brachyurum (Liévin, 1848) and cannot be used as a valid name for the American populations of the genus. The populations described under this name have been identified as a new species, Diaphanosoma birgei n.sp., and populations from Europe, Asia, and Africa with conspecific characters as Diaphanosoma birgei ssp. lacustris n.ssp. A comparison with other species ocuring within the area of its distribution is given.

Abst)ract I t the fish ponds under study the authors defined several types of plankton which have been frequently found during the season (April -October). These types are: Early-spring maximum of phytoplankton, Depression of phytoplankton, Bloom of Apbnizomenon, Maximum of Chlorococcales.The periods of "depression" seem to be typical for the managed carp ponds in the spring. They are characterized by the low density of rapidly reproducing algal populations (e. g. Cryptomonas) and by the dense populations of large cladocerans of the genus Dqhnia. Chlorophyll in phytoplankton is leas than 5 pg/l, transparency is higher than 2 meters.Periods of the spring depression may be followed by the maxima of either Aphnizomenon or Chlorococcales, with concentrations of chlorophyll increasing to 100 pg/l and more. The change from the phase of "depression" to the "maximum of Chlorococcales" is accompanied by decrease in numbers of Daphnia and increase in numbers of the small cladoceran species, but all the mechanisms responsible for the transition are not yet fully understood.

Although systematics of the cladoceran genus Daphnia (Cladocera: Daphniidae) has been intensively investigated for decades using both morphological and genetic approaches, new lineages are being discovered on all continents, including in well-studied regions. Among Holarctic daphnids, Daphnia curvirostris Eylmann, 1887 held an interesting position, sharing some morphological characters of both the D. pulex and D. longispina groups. Recently, additional species of the D. curvirostris complex have been discovered in the Eastern Palaearctic. Here, we describe a new species in this complex from Central Europe, D. hrbaceki sp. nov. It was discovered in small, newly created fishless pools in the Czech Republic, and an additional sample of apparently the same taxon was collected in 1951 in Slovakia. D. hrbaceki is the closest yet known relative of D. curvirostris, but remains genetically divergent from all members of the complex (based on the sequences of three mitochondrial genes: 12S, COI, and ND2). In general, adult females of this species are morphologically very similar to D. curvirostris. Unlike the latter species, D. hrbaceki may develop a specific hump-shaped dorsal outline of the carapace, presumably an inducible defence against invertebrate predators. Juveniles of the new species occasionally form neckteeth, which may also be retained in adult individuals. The species also shows substantial variation in the size of spines in the middle pecten of the postabdominal claw, similarly as in the Japanese member of the species complex, D. tanakai Ishida, Kotov & Taylor, 2006. This variable character of spine size in the postabdominal middle pecten (a transition from the pulex to the longispina group character), as well as a bent and heavily setulated terminal seta on the male 2nd endopodite (considered as the pulex group character), are typical for the new species. D. hrbaceki also differs from D. curvirostris as well as other members of the complex in the ephippial surface ultrastructure. Our study demonstrates the utility of such ultrastructural characters in Daphnia taxonomical studies.

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