Capelin larvae (Mallotus villosus) and community structure of zooplankton off the coast of northern Norway

Capelin larvae (Mallotus villosus) and mesozooplankton werecollected in May 2001 in the southern part of the Barents Sea.During the period of hatching and first feeding, capelin larvaeexperience high mortality rates. In order to understand theunderlying mechanisms influencing larval survival, we monitoreda selected site for 10 days using a variety of different samplingapproaches. Hydrological parameters conductivity–temperature–depth–fluorescence(CTDF) were recorded and zooplankton samples obtained simultaneouslyusing MOCNESS at three time periods. The mesozooplankton consistedessentially of copepods (85), dominated by Calanus finmarchicus.The zooplankton community consisted of different assemblagesderived from different areas (i.e. different water masses; Atlanticand coastal). Capelin larvae were found in surficial coastalwaters. Copepod nauplii and C. finmarchicus were less abundantin coastal waters where small copepodites (such as Acartia spp.and Temora longicornis) were found together with early stagesof coastal invertebrates that are possible prey for capelinlarvae. We suggest that capelin larvae base their foraging onsmall prey organisms frequently found among the coastal zooplanktonand that these larvae are less dependent on the propagationof the recruiting generation of C. finmarchicus present fromApril to June.     

Demersal fish assemblages and spatial diversity patterns in the Arctic-Atlantic transition zone in the Barents Sea

Direct and indirect effects of global warming are expected to be pronounced and fast in the Arctic, impacting terrestrial, freshwater and marine ecosystems. The Barents Sea is a high latitude shelf Sea and a boundary area between arctic and boreal faunas. These faunas are likely to respond differently to changes in climate. In addition, the Barents Sea is highly impacted by fisheries and other human activities. This strong human presence places great demands on scientific investigation and advisory capacity. In order to identify basic community structures against which future climate related or other human induced changes could be evaluated, we analyzed species composition and diversity of demersal fish in the Barents Sea. We found six main assemblages that were separated along depth and temperature gradients. There are indications that climate driven changes have already taken place, since boreal species were found in large parts of the Barents Sea shelf, including also the northern Arctic area. When modelling diversity as a function of depth and temperature, we found that two of the assemblages in the eastern Barents Sea showed lower diversity than expected from their depth and temperature. This is probably caused by low habitat complexity and the distance to the pool of boreal species in the western Barents Sea. In contrast coastal assemblages in south western Barents Sea and along Novaya Zemlya archipelago in the Eastern Barents Sea can be described as diversity "hotspots"; the South-western area had high density of species, abundance and biomass, and here some species have their northern distribution limit, whereas the Novaya Zemlya area has unique fauna of Arctic, coastal demersal fish. (see Information S1 for abstract in Russian).     

Nested Evolution of a tRNALeu(UAA) Group I Intron by both Horizontal Intron Transfer and Recombination of the Entire tRNA Locus

The origin and evolution of bacterial introns are still controversial issues. Here we present data on the distribution and evolution of a recently discovered divergent tRNA(Leu)(UAA) intron. The intron shows a higher sequence affiliation with introns in tRNA(Ile)(CAU) and tRNA(Arg)(CCU) genes in alpha- and beta-proteobacteria, respectively, than with other cyanobacterial tRNA(Leu)(UAA) group I introns. The divergent tRNA(Leu)(UAA) intron is sporadically distributed both within the Nostoc and the Microcystis radiations. The complete tRNA gene, including flanking regions and intron from Microcystis aeruginosa strain NIVA-CYA 57, was sequenced in order to elucidate the evolutionary pattern of this intron. Phylogenetic reconstruction gave statistical evidence for different phylogenies for the intron and exon sequences, supporting an evolutionary model involving horizontal intron transfer. The distribution of the tRNA gene, its flanking regions, and the introns were addressed by Southern hybridization and PCR amplification. The tRNA gene, including the flanking regions, were absent in the intronless stains but present in the intron-containing strains. This suggests that the sporadic distribution of this intron within the Microcystis genus cannot be attributed to intron mobility but rather to an instability of the entire tRNA(Leu)(UAA) intron-containing genome region. Taken together, the complete data set for the evolution of this intron can best be explained by a model involving a nested evolution of the intron, i.e., wherein the intron has been transferred horizontally (probably through a single or a few events) to a tRNA(Leu)(UAA) gene which is located within a unstable genome region.     

Climate change alters the structure of arctic marine food webs due to poleward shifts of boreal generalists

Climate-driven poleward shifts, leading to changes in species composition and relative abundances, have been recently documented in the Arctic. Among the fastest moving species are boreal generalist fish which are expected to affect arctic marine food web structure and ecosystem functioning substantially. Here, we address structural changes at the food web level induced by poleward shifts via topological network analysis of highly resolved boreal and arctic food webs of the Barents Sea. We detected considerable differences in structural properties and link configuration between the boreal and the arctic food webs, the latter being more modular and less connected. We found that a main characteristic of the boreal fish moving poleward into the arctic region of the Barents Sea is high generalism, a property that increases connectance and reduces modularity in the arctic marine food web. Our results reveal that habitats form natural boundaries for food web modules, and that generalists play an important functional role in coupling pelagic and benthic modules. We posit that these habitat couplers have the potential to promote the transfer of energy and matter between habitats, but also the spread of pertubations, thereby changing arctic marine food web structure considerably with implications for ecosystem dynamics and functioning.     

Genetic divergence and phylogeographic relationships among european perch (Perca fluviatilis) populations reflect glacial refugia and postglacial colonization

We used the widely distributed freshwater fish, perch (Perca fluviatilis), to investigate the postglacial colonization routes of freshwater fishes in Europe. Genetic variability within and among drainages was assessed using mitochondrial DNA (mtDNA) D-loop sequencing and RAPD markers from 55 populations all over Europe as well as one Siberian population. High level of structuring for both markers was observed among drainages and regions, while little differentiation was seen within drainages and regions. Phylogeographic relationships among European perch were determined from the distribution of 35 mtDNA haplotypes detected in the samples. In addition to a distinct southern European group, which includes a Greek and a southern Danubian population, three major groups of perch are observed: the western European drainages, the eastern European drainages including the Siberian population, and Norwegian populations from northern Norway, and western side of Oslofjord. Our data suggest that present perch populations in western and northern Europe were colonized from three main refugia, located in southeastern, northeastern and western Europe. In support of this, nested cladistic analysis of mtDNA clade and nested clade distances suggested historical range expansion as the main factor determining geographical distribution of haplotypes. The Baltic Sea has been colonized from all three refugia, and northeastern Europe harbours descendants from both eastern European refugia. In the upper part of the Danube lineages from the western European and the southern European refugia meet. The southern European refugium probably did not contribute to the recolonization of other western and northern European drainages after the last glaciation. However, phylogenetic analyses suggest that the southern European mtDNA lineage is the most ancient, and therefore likely to be the founder of all present perch lineages. The colonization routes used by perch probably also apply to other freshwater species with similar distribution patterns.     

Restriction cutting independent method for cloning genomic DNA segments outside the boundaries of known sequences

We present a simple method for cloning genomic DNA segments outside the boundaries of known sequences, which is not dependent on restriction cutting or mapping. In the first step of the method, a library of single-stranded flanking sequences is generated by linear amplification with one primer in the known region. A homooligomeric cytosine tail is added to each of the single-stranded fragments by a terminal transferase catalyzed reaction. The tailed fragments are then amplified by PCR with a nested primer in the known region and a poly-guanine primer complementary to the cytosine tail in the unknown region. Finally, the different fragments are separated by cloning and characterized by sequencing. The method was used to clone both the upstream (5') and the downstream (3') genomic regions of an intron-interrupted tRNA(Leu)(UAA) gene from three cyanobacteria belonging to the genus Microcystis.     

FliC,a Flagellin Protein,Is Essential for the Growth and Virulence of Fish Pathogen Edwardsiella tarda

Edwardsiella tarda is a flagellated Gram-negative bacterium which causes edwardsiellosis in fish. FliC, as a flagellar filament structural protein, is hypothesized to be involved in the pathogenesis of infection. In this study, a fliC in-frame deletion mutant of a virulent isolate of E. tarda was constructed through double crossover allelic exchange by means of the suicide vector pRE112, and its virulence-associated phenotypes and pathogenicity were tested. It was found that the deletion of fliC significantly decreased the diameter of flagella filaments. In addition, the mutant showed reduced pathogenicity to fish by increasing the LD₅₀ value for 100-fold compared to the wild-type strain, as well as showed impaired bacterial growth, reduced motility, decreased biofilm formation and reduced levels of virulence-associated protein secretion involved in the type III secretion system (TTSS). The phenotypic characteristics of the fliC deletion mutant uncovered in this investigation suggest that fliC plays an essential role in normal flagellum function, bacterial growth, protein secretion by TTSS and bacterial virulence.