Overlooked for a long time, wood falls on the deep oceanic floor are now recognized as extreme, reducing ecosystems based on chemosynthesis, in the same way that hydrothermal vent, cold seeps and whale falls with which they share important physicochemical and faunistic similarities. In these ecosystems, bacterial chemosynthesis using reduced compounds (H2S, CH4) and/or digestion of refractory organic compounds (cellulose and lignin in wood, organic matrix of bone) by heterotrophic bacteria play an essential trophic role, and appear to promote establishment of mutualistic symbioses between bacteria and metazoans for the exploitation of environmental resources. Beyond the description of the taxonomic composition of fauna associated with these sites, current studies aim to understand the functioning and evolutionary links of these ecosystems. Digestive and/or chemoautotrophic associations with microorganisms that could be the key to survive in these habitats have been described in a number of molluscs and annelids but only few crustaceans. The latter distinguish from the other groups by the fact they do not realize intracellular symbioses (endosymbioses) but mainly carry ectosymbioses on their integument (epidermis and cuticle) even in the digestive tract that is partly lined by epidermis (in stomodeum and proctodeum). Moreover, up to now, nothing was known about crustaceans from deep-sea wood falls, their feeding habits and associations with microorganisms. The present work focused on the crustaceans and especially 15 species of decapods from wood falls in the South Pacific Ocean. The specimens were recovered during several French cruises (Salomon2, BOA1, SantoBOA, SalomonBOA3) organized near the Vanuatu, Solomon Island and New Caledonia by the Muséum National dHistoire Naturelle of Paris. The investigation of the feeding biology and microbial associations of the decapod species combined three complementary approaches : 1) a morphological approach using light and electron microscopy to describe the external and gut structures as well as the gut content and microflora, 2) a trophic approach based on stable isotopes analyses (C and N) together with the nature of the gut content and 3) a molecular approach based on 16S rRNA gene analyses and FISH labeling to identify and locate the bacteria in the gut content (ingested bacteria) and on the gut lining (resident bacteria or symbionts). Taxonomic identification and morphological observations of the species provided a first non-exhaustive inventory of the best represented decapod species in wood accumulations in the deep South Pacific and point out three interesting informations. 1) Most decapods from the sunken woods belong to Reptantia. 2) Some species (e.g. Munidopsis spp.) belong to deep sea taxa and exhibit typical characteristics while others (e.g. Xylopagurus) much more resemble shallow-water species, indicating that the colonization of wood falls by decapods may have occurred from both deep and coastal habitats. 3) Association degree with sunken wood could be important for some species that appear endemic (e.g. Pylochelidae, Xylopagurus) or which probably have their complete life cycle on wood falls (most of the gravid females having big eggs with a direct larval development). From a trophic point of view, morphological observations of the digestive system, examinations of gut contents and stable isotope analyses carried out on 15 species, allow us to classify the decapod crustacean from deep-sea sunken woods into two major groups, detritivores and predators/scavengers, and four trophic guilds (trophic levels) depending on two primary food sources, the wood and the particulate organic matrix (marine snow). The four guilds were identified as bacteriovorous detritivores (M. nitida, M. pilosa, M. bispinoculata, Munidopsis sp.1), xylophagous detritivores (M. andamanica, R. amboinensis), omnivores (X. caledonicus) and predators/scavengers (pylochelid species, Munida spp, Axiidae sp.1, M. cylindrophthalma). A fifth guild could be represented by limivorous detritivores (C. acutirostella, Alpheidae sp.1). This breaks with the widespread idea that decapod crustaceans from wood falls are all scavengers or predators. In addition, these results have evidenced of some special dietary strategies and highlight the importance of xylophagous decapods in the ecosystem, by their mechanical degradation of the substrate and their production of feces which should enrich the sediment and have a significant impact on the composition of wood and sediment microbiota (bacteria and fungi). Microscopic observations also allowed us to identify potentially symbiotic associations with resident (and transient) microorganisms (bacteria and/or trichomycetes) in the hindgut of five detritivorous species. Owing that, three of these species have a wood-based diet (M. andamanica, M. nitida, R. amboinensis) and considering the location of these microflora, it is strongly suggested that the resident microorganisms are directly related to the xylophagous diet of the host and most particularly involved in the digestion of wood. More detailed in two xylophagous species, M. andamanica and R. amboinensis, not only these microflora can be distinguished on the basis of their morphology, location and probable genetic differences but also by their roles and involvement in the digestion of the woody substrate. The digestive bacteria of M. andamanica could act as trophic intermediates while those of R. amboinensis probably only help in its digestion. From the stable isotopes ratio, it is likely that R. amboinensis directly assimilates the wood digestion products while M. andamanica feeds on wood with a trophic intermediate. Cloning and sequencing 16S rRNA gene from the gut bacteria of M. andamanica revealed its resident hindgut microflora is largely dominated by two phylotypes (OTUs) of possibly symbiotic bacteria that belong to Firmicutes and Alphaproteobacteria. Surprisingly, these OTUs are very close to gut bacteria isolated from a coastal thalassinid shrimp N. californiensis and from the Chinese crab E. sinensis. The results raise the question of the development and evolution of digestive symbioses in decapods and crustaceans in relation to the diet and/or adaptation to a special habitat, owing that among galatheid crabs the symbiotic relationships appear closer in certain species (M. andamanica) than in others (M. nitida, M. bispinoculata) and that similar differences appear between taxonomic groups. On the other hand, the genetic proximity of bacteria from distant crustacean species raise the question of an eventual co-evolution of linked hosts and symbionts contrasting with the permanent re-acquisition of symbionts from the medium.

The Japanese knotweeds are invasive alien clonal species originating in Asia (Japan, Korea, Taiwan and Nord of China). They were introduced in Europe at the beginning of the 19th century. They are now some of the most troublesome invasive alien species in Europe and in the United States. In Belgium, a complex of four taxa and a hybrid represents them. All these taxa take part in the pattern of invasion and represent an excellent opportunity for studies of population biology in Western continental Europe. The presence from at least three taxa and a hybrid is confirmed by cytological, genetic and morphological studies. Vegetative reproduction is recognized as the main mode of reproduction and expansion of these taxa in the introduce range. However, interspecific hybridization events are observed confirming the restoration of the sexual reproduction by hybridization within this complex species in Belgium. Hybrid F. x bohemica with various ploidy levels from tetraploid to octoploid is observed. An increase in genotypic and morphological diversity is shown at the hybrid F. x bohemica which missed with the parental species. This could increase the potential of Japanese knotweeds to adapt to the new environment and contribute to the invasive success of these taxa in Belgium. Assignment test indicates a genetic pool differentiated at the hybrid F. x bohemica and not a mixture of the genetic pool of the parental species as expected for hybrid taxa. Hybrid F. x bohemica has always been considered as rare in Belgium and of horticultural origin. An analysis of the spatial distribution shows that hybrid F. x bohemica is widespread in Belgium and that its abundance depends on the areas. An analysis of extent of differentiation between groups of hybrid geographically distant could not provide proof of an independent evolution of hybrid populations under limited gene flow. An analysis of the sexual reproduction capacity and dispersal of seeds shows important production of viable seeds and consequent seed rain. Hybrid seeds may be dispersed beyond 16m, leading the possibility of founding new individuals and to contribute to the invasive success of these taxa. However, a trend towards decreasing germination rate is shown after a cold period. An analysis of the distribution at the landscape scale shows that the dynamics of colonization of habitats patches by Japanese knotweeds rising mainly from clonal propagation in spite of important pressure of propagule. The knotweeds prefer human disturbed habitats with a clear prevalence of communication routes. This leads the possibility of dispersing towards the adjacent habitats patches. A high dynamics of establishment of propagule was not observed at the hybrid plants compared with the parents plants in spite of the increase in genotypic diversity and the consequent pressure of propagule. Lastly, proposals for the integrated management of these taxa are proposed in the Belgium context. Their management will have first to identify hybrids and taxa involved. Emasculation and management of the existing clones represent solutions to prevent flowering and expansion of these taxa. An active management of disturbed habitats may represent alternative to prevent the invasion by Japanese knotweeds / Les renouées du Japon sont des espèces clonales non-indigènes originaires dAsie (Japon, Corée, Taiwan et Nord de la Chine). Elles ont été introduites en Europe au début du 19è siècle. Elles font désormais partie des espèces non-indigènes les plus invasives en Europe et aux Etats Unis. En Belgique, elles sont représentées par un complexe de quatre taxons et un hybride. Tous ces taxons participent à la dynamique dinvasion et représentent un modèle dintérêt pour les études de biologie des populations en Europe continentale occidentale. La présence dau moins trois taxons et un hybride est confirmée par des études cytologiques, génétiques et morphologiques. La reproduction végétative est reconnue comme le principal mode de reproduction et dexpansion de ces taxons dans la zone dintroduction. Toutefois, des phénomènes dhybridation interspécifique sont observés confirmant la restauration de la reproduction sexuée par hybridation au sein de ce complexe despèces en Belgique. Lhybride F. x bohemica avec différents niveaux de ploïdie, du tétraploïde à loctoploïde, est observé. Un accroissement de la diversité génotypique et morphologique qui manquait aux espèces parents est démontré chez lhybride F. x bohemica. Ceci pourrait augmenter le potentiel des renouées du Japon à sadapter au nouvel environnement et contribuer au succès invasif de ces taxons en Belgique. Un test dassignation indique un pool génétique différencié chez lhybride F. x bohemica et non un mélange du pool génétique des espèces parentales comme attendu dans le cadre de taxons hybrides. Lhybride F. x bohemica a toujours été considéré comme rare en Belgique et dorigine horticole. Une analyse de la répartition spatiale montre que lhybride F. x bohemica est très répandu en Belgique et que son abondance dépend des régions. Une analyse de létendue de la différenciation entre groupes dindividus hybrides géographiquement distants na pas pu fournir de preuve dune évolution indépendante des populations hybrides sous un flux de gènes limité. Une analyse de la capacité de reproduction sexuée et de dispersion des graines démontre une production importante de graines viables et une pluie de graines conséquente. Les graines hybrides sont capables de se disperser à plus de 16m, laissant la possibilité de fonder de nouveaux individus et de contribuer au succès invasif de ces taxons. Cependant, une tendance à la diminution du potentiel de germination est observée après une période de froid chez ces taxons. Une analyse de la distribution à léchelle du paysage a permis dinterpréter la dynamique de colonisation des taches dhabitats par les renouées du Japon comme relevant principalement de la propagation clonale malgré une pression de propagule importante. Les renouées du Japon préfèrent les habitats perturbés par lhomme avec une nette prédominance des réseaux linéaires de communications. Ceci laisse ensuite la possibilité de se disperser vers les taches dhabitats adjacents. Une dynamique détablissement de propagules plus importante na pas été observée chez les plants hybrides comparées aux plants parents malgré laccroissement de diversité génotypique et la pression de propagules considérable. Enfin, des pistes pour la gestion intégrée des renouées du Japon en Belgique sont proposées. Cette gestion devra en priorité identifier les hybrides et les taxons en présence. Des mesures démasculation et de gestion des clones existants pourraient constituer une solution pour empêcher la floraison et lexpansion de ces taxons. Une gestion active des habitats perturbés pourrait représenter une alternative pour prévenir linvasion par les renouées du Japon