Diversification of mitochondrial genome of Daphnia galeata (Cladocera, Crustacea): Comparison with phylogenetic consideration of the complete sequences of clones isolated from five lakes in Japan.

To characterize genetic diversity and gene flow among Daphnia galeata populations, the complete nucleotide (nt) sequences of the mitochondrial (mt) DNAs of D. galeata clones isolated from five lakes in Japan (Lakes Shirakaba, Suwa, Kizaki, Kasumigaura, and Biwa) were determined. Comparison of non-synonymous (amino acid altering) substitution rates with synonymous substitution rates of D. galeata mt protein-coding genes demonstrated that ATPase8 and COI genes were the most and least susceptible, respectively, to the evolutional forces selecting the aa substitutions. Several non-synonymous substitutions were found in ATPase8 and ATPase6 even in the comparison that no synonymous substitution was found. Comparison of the total number of nt variations among the mt DNAs suggested the phylogenetic relationship ((((Shirakaba/Suwa, Kizaki), Kasumigaura), Biwa), D. pulex). Maximum-likelihood analysis using the total nt sequences of mt protein-coding genes confirmed this relationship with bootstrap values higher than 98%. All the mtDNAs of the analyzed Japanese D. galeata clones contained a control region of essentially the same structure that is distinct from those of the previously reported European Daphnia species of the D. longispina complex. The two control regions of different structures spread among mtDNAs of the Japanese and European Daphnia species, respectively, probably after the divergence of the Japanese D. galeata under different selection pressures associated with their habitats.

Effects of polycyclic aromatic hydrocarbons (PAHs) on an aquatic ecosystem: acute toxicity and community-level toxic impact tests of benzo[a]pyrene using lake zooplankton community.

We estimated acute toxicity of benzo[a]pyrene (B[a]P) using two cladoceran species, Ceriodaphnia reticulata and Daphnia magna, and also analyzed its impact on zooplankton community throughout an exposure experiment using small-scale mesocosms. LC(50) of B[a]P for C. reticulata and D. magna was 4.3 and 4.7 µg/l, respectively. However, individuals fed with Chlorella showed higher LC(50), 6.1 µg/l for C. reticulata and 8.0 µg/l for D. magna. In the exposure experiment, we examined the impact of B[a]P on zooplankton community using conceivable concentrations in the environment (5 and 10 µg/l) using typical zooplankton community in eutrophicated systems. Despite the residence time of B[a]P in the water column was short as < 4 days, application of B[a]P induced decrease of zooplankton abundance. However, the recovery pattern was different among cladocerans and rotifers. Consequently, B[a]P showed insecticide-like impacts, suppressing cladoceran populations and inducing the dominance of rotifers particularly under high concentration (10 µg/l). Results have suggested that, even such short duration of B[a]P in the water body can have impact on zooplankton abundance and community structure. Since B[a]P easily precipitate to the bottom and rapidly disappears from the water body, careful monitoring and further assessment of the potential toxicity of polycyclic aromatic hydrocarbons are necessary.

Impact of an insecticide on persistence of inherent antipredator morphology of a small cladoceran, Bosmina.

Pesticides often modify predatory interactions by enhancing (or inhibiting) the development of antipredator morphologies of freshwater cladocerans. In the present study, we assessed the impacts of an insecticide, carbaryl, on the life-history parameters of a polymorphic cladoceran, Bosmina longirostris, and on Bosmina-copepod interaction. In this Bosmina species, all juvenile individuals have the defensive morphology irrespective of presence of predators in juvenile stages, and they lose or maintain such morph compliance with the presence/absence of the predators in adult stages. In the present study, individual somatic growth and population growth rate decreased due to applied carbaryl. Moreover, the animals lost the defensive morphology when their body size was smaller than with no-carbaryl treatment even in the presence of predators, indicating that the insecticide inhibited persistence of the inherent antipredator morphology. Such a chemical disturbance will increase the predation risk to individuals and, in turn, influence the population dynamics of the bosminids through increased mortality.

Zooplankton community structure driven by vertebrate and invertebrate predators.

A zooplankton community was established in outdoor experimental ponds, into which a vertebrate predator (topmouth gudgeon: Pseudorasbora parva) and/or an invertebrate predator (phantom midge larva: Chaoborus flavicans) were introduced and their predation effects on the zooplankton community structure were evaluated. In the ponds which had Chaoborus but not fish, small- and medium-sized cladocerans and calanoid copepods were eliminated while rotifers became abundant. A large-sized cladoceran Daphnia longispina, whose juveniles had high helmets and long tailspines as anti-predator devices, escaped from Chaoborus predation and increased. In the ponds which had fish but not Chaoborus, the large-sized Daphnia was selectively predated by the fish while small-and medium-sized cladocerans and calanoid copepods predominated. In the ponds containing both Chaoborus and fish, the fish reduced the late instar larvae (III and IV) of Chaoborus but increased the early instar larvae (I and II). Small- and large-sized cladocerans were scarcely found. The former might have been eliminated by predation of the early instar larvae of Chaoborus, while the latter was probably predated by fish. Consequently, the medium-sized cladocerans, which may have succeeded in escaping from both types of predator, appeared abundantly. The results suggest that various combinations of vertebrate and invertebrate predators are able to drive various kinds of zooplankton community structure.