Mangrove microbiota along the urban-to-rural gradient of the Cayenne estuary (French Guiana, South America): Drivers and potential bioindicators.

The microbial communities inhabiting the Atlantic-East Pacific (AEP) mangroves have been poorly studied, and mostly comprise chronically polluted mangroves. In this study, we characterized changes in the structure and diversity of microbial communities of mangroves along the urban-to-rural gradient of the Cayenne estuary (French Guiana, South America) that experience low human impact. The microbial communities were assigned into 50 phyla. Proteobacteria, Chloroflexi, Acidobacteria, Bacteroidetes, and Planctomycetes were the most abundant taxa. The environmental determinants found to significantly correlated to the microbial communities at these mangroves were granulometry, dieldrin concentration, pH, and total carbon (TC) content. Furthermore, a precise analysis of the sediment highlights the existence of three types of anthropogenic pressure among the stations: (i) organic matter (OM) enrichment due to the proximity to the city and its wastewater treatment plant, (ii) dieldrin contamination, and (iii) naphthalene contamination. These forms of weak anthropogenic pressure seemed to impact the bacterial population size and microbial assemblages. A decrease in Bathyarchaeota, "Candidatus Nitrosopumilus", and Nitrospira genera was observed in mangroves subjected to OM enrichment. Mangroves polluted with organic contaminants were enriched in Desulfobacteraceae, Desulfarculaceae, and Acanthopleuribacteraceae (with dieldrin or polychlorobiphenyl contamination), and Chitinophagaceae and Geobacteraceae (with naphthalene contamination). These findings provide insights into the main environmental factors shaping microbial communities of mangroves in the AEP that experience low human impact and allow for the identification of several potential microbial bioindicators of weak anthropogenic pressure.

Spatial gradients of introgressed ancestry reveal cryptic connectivity patterns in a high gene flow marine fish.

Assessing genetic connectivity among populations in high gene flow species is sometimes insufficient to evaluate demographic connectivity. Genetic differentiation quickly becomes zero as soon as a few dozen migrants are exchanged per generation. This provides little information to determine whether migration can ensure demographic coupling. The resulting difficulties in delineating conservation units for the management of commercially exploited marine fish species are well illustrated in the case of the European sea bass (Dicentrarchus labrax). Previous attempts to assess connectivity patterns in the northeast Atlantic have been hampered by a lack of spatial genetic structure. In contrast, mark-recapture data suggested low migration rates between regional spawning areas. Here, we show how a spatial gradient of introgressed Mediterranean ancestry across the northeast Atlantic reflects cryptic patterns of genetic and demographic connectivity. Using a 1K SNP chip data set in 827 individuals sampled from Portugal to the North Sea, we found null overall genetic differentiation across the northeast Atlantic. We however detected a subtle latitudinal admixture gradient originating at the edge of the contact zone with the Mediterranean sea bass lineage. Two significant breaks in the ancestry gradient at the tip of Galicia and northern Brittany indicated barriers to effective dispersal between demographically distinct units. Moreover, a northward expansion signal in Irish and North Seas was revealed by the surfing of rare Mediterranean alleles at the edge of the species range. Our results show that introgressed ancestry gradients offer a powerful alternative to assess genetic and demographic connectivity when the neutral migration-drift balance is not informative.

Evolution of diadromy in fish: insights from a tropical genus (Kuhlia species).

Diadromous species undergo regular migration between fresh and marine waters. This behavior is found in many species, including fish, mollusks, and crustaceans, some of which are commercially valuable species. Several attempts to trace the evolution of this behavior have been made in Salmonidae and Galaxiidae, but ambiguous phylogenies and multiple character state changes prevented unequivocal conclusions. The Kuhliidae family consists of 12 fish species that inhabit tropical islands in the Indo-Pacific region. The species have marine, partially catadromous, or fully catadromous life histories (i.e., they migrate from rivers to the sea to reproduce). The evolution of migratory behavior was traced on a well-resolved phylogeny. Catadromous Kuhlia species were basal, and partially catadromous and marine species formed derived monophyletic groups. This is, to our knowledge, the first time that a clear origin and polarity for the diadromous character has been demonstrated. We propose that the relative lack of resources in tropical, inshore, marine habitats and the ephemeral and isolated nature of freshwater environments of tropical islands, combined with phenotypic plasticity of migratory traits, play key roles in driving the evolution of diadromy in the Kuhliidae and probably in other groups. This work is an important starting point to understand the role of diadromy in speciation and adaptation in unstable habitats.

Sublethal effects of exposure to chemical compounds: a cause for the decline in Atlantic eels?

Because of their unusual life cycle, American and European eels (Anguilla rostrata Lesueur and A. anguilla L.), are particularly exposed to pollutant effects. Because silver eels fast when they leave the freshwater system, the transoceanic migration forces them to constitute energy reserves in the form of muscle lipids, that are needed for successful spawning. Using species biological data, toxicological and ecotoxicological information, hypotheses are given to assess the contribution of pollution from freshwater sources to the recorded decline in the American and European eels fisheries since the 1980s. This paper first describes the lipid storage problems and the relative migratory capacities. Then several studies on the accumulation of xenobiotics in various anatomical compartments, on the biological half-lives of these compounds, and on their sublethal toxicity, are reviewed. During migration, lipid mobilization returns persistent lipophilic pollutants back into circulation, these being concentrated particularly in gonads at the crucial time of gametogenesis. Extrapolation of toxicological analysis (individual physiology) to the population level (spawning success) suggests that the quality of future spawners leaving freshwaters is one of the prime factors for the conservation of this threatened species.