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      Low genetic but high morphological variation over more than 1000 km coastline refutes omnipresence of cryptic diversity in marine nematodes


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          The resilience of ecosystems to negative impacts is generally higher when high gene flow, species diversity and genetic diversity are present. Population genetic studies are suitable to investigate genetic diversity and estimate gene flow between populations. Seaweed beds form a dynamic shallow water ecosystem influenced by climate change and human exploitation, as such, seaweed beds are a particularly powerful model to investigate ecosystem resilience in coastal areas. We studied the population genetic structure of the new nematode species Paracanthonchus gynodiporata associated with seaweeds in northeastern Brazil. Nematodes are generally believed to have a limited dispersal capacity because of the lack of planktonic larvae. Yet, they can drift on seaweeds, and water currents might be a natural barrier for their dispersal. Populations of P. gynodiporata were sampled over more than 1000 km coastline in regions across major oceanic currents with and without historical exploitation of seaweed.


          P. gynodiporata is described in an integrative way using mitochondrial and nuclear sequences and morphological data. The 3D model of the head region shows for the first time a detailed view of the ventrosublateral teeth, a character often overlooked in older taxonomic studies of the genus. A total of 17 mitochondrial COI haplotypes were found with one haplotype representing 63 to 83% of the frequencies in each population. AMOVA showed overall little population genetic structure ( F ST  = 0.05204), and no genetic subdivision between the populations under the influence of the two different water currents were found. Effects of historical seaweed exploitation on population genetic diversity were not detected. In contrast, significant differences between populations were found in morphometric characters. This discrepancy in genetic and morphological differentiation between populations across 1000 km of coastline is surprising in view of the frequently observed presence of several cryptic species at small geographical scale in other macroalgal associated nematodes.


          Our results show that cryptic species are not omnipresent in marine nematode species, suggesting that nematodes associated with seaweeds have been able to disperse over large distances across well-known biogeographic barriers.

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          Cryptic species as a window on diversity and conservation.

          The taxonomic challenge posed by cryptic species (two or more distinct species classified as a single species) has been recognized for nearly 300 years, but the advent of relatively inexpensive and rapid DNA sequencing has given biologists a new tool for detecting and differentiating morphologically similar species. Here, we synthesize the literature on cryptic and sibling species and discuss trends in their discovery. However, a lack of systematic studies leaves many questions open, such as whether cryptic species are more common in particular habitats, latitudes or taxonomic groups. The discovery of cryptic species is likely to be non-random with regard to taxon and biome and, hence, could have profound implications for evolutionary theory, biogeography and conservation planning.
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            Connectivity of marine populations: open or closed?

            Most marine populations are thought to be well connected via long-distance dispersal of larval stages. Eulerian and Lagrangian flow models, coupled with linear mortality estimates, were used to examine this assumption. The findings show that when simple advection models are used, larval exchange rates may be overestimated; such simplistic models fail to account for a decrease of up to nine orders of magnitude in larval concentrations resulting from diffusion and mortality. The alternative process of larval retention near local populations is shown to exist and may be of great importance in the maintenance of marine population structure and management of coastal marine resources.
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                Author and article information

                BMC Evol Biol
                BMC Evol. Biol
                BMC Evolutionary Biology
                BioMed Central (London )
                7 March 2017
                7 March 2017
                : 17
                : 71
                [1 ]ISNI 0000 0001 2069 7798, GRID grid.5342.0, Faculty of Science, Department of Biology, , Ghent University, ; K.L. Ledeganckstraat 35, 9000 Ghent, Belgium
                [2 ]ISNI 0000 0001 2171 9581, GRID grid.20478.39, , Royal Belgian Institute of Natural Sciences, OD Taxonomy and Phylogeny, ; Vautierstraat 29, 1000 Brussels, Belgium
                [3 ]ISNI 0000 0001 2069 7798, GRID grid.5342.0, Faculty of Science, Department of Biology, Marine Biology, , Ghent University, ; Krijgslaan 281 (S8), 9000 Ghent, Belgium
                [4 ]ISNI 0000 0001 0670 7996, GRID grid.411227.3, Department of Biology, , Federal University of Pernambuco, ; Av. Professor Morais Rego, 1235 - Cidade Universitária, CEP 50670-901, Recife Brazil
                © The Author(s). 2017

                Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License ( http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver ( http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.

                : 4 August 2016
                : 8 February 2017
                Funded by: FundRef http://dx.doi.org/10.13039/501100006338, Vlaamse Interuniversitaire Raad;
                Research Article
                Custom metadata
                © The Author(s) 2017

                Evolutionary Biology
                coi,connectivity,morphometry,population genetics
                Evolutionary Biology
                coi, connectivity, morphometry, population genetics


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