Deciphering environmental forcings in the distribution of meiofauna and nematodes in mangroves of the Atlantic-Caribbean-East Pacific and Indo-West Pacific regions.

Mangroves develop under environmental conditions and anthropogenic pressures whose impact on benthic meiofauna remains poorly understood. It is unclear how meiofauna communities are structured according to local sedimentary conditions. This study was designed to characterize the community structure of meiofauna and nematodes (dominant taxa) and the associated environmental forcings in intertidal mangrove sediments from Mayotte (Indo-West-Pacific), Martinique and Guadeloupe (Caribbean). Sediment cores were sampled at the end of the dry season at low tide on adult mangrove stands with similar immersion time. In each sediment layer, we analyzed redox potential, pH, porewater salinity, grain size, organic matter, metals, organic contaminants, prokaryotes and meiofauna. Our results show that sediments far from cities and agricultural fields trapped site-specific contaminants due to local water transport processes. Some metals, PAHs or pesticides exceeded toxicity thresholds in most of the studied stations, thus being harmful to benthic fauna. The sedimentary environment acts as a filter selecting specific meiofauna communities at station scale only in the Caribbean. In Mayotte, horizontal homogeneity contrasts with vertical heterogeneity of the sedimentary environment and the meiofauna. Nematode genera showed particular distribution patterns horizontally and vertically, suggesting the presence of sediment patches suitable for a restricted pool of genera on each island. Results in the Caribbean are consistent with nested diversity patterns due to environmental filtering. Conversely, horizontal homogeneity at Mayotte would reflect greater dispersal between stations or more spatially homogeneous anthropogenic pressures. The nematode genera present at depth may not be the most specialized, but the most versatile, capable of thriving in different conditions. Terschellingia and Daptonema showed contrasted responses to environmental forcing, likely due to their versatility, while Desmodora showed uniform responses between study areas, except when toxicity thresholds were exceeded. Our results emphasize that a given genus of nematode may respond differently to sedimentary conditions depending on sites.

Nematode's morphometric shifts related to changing environmental conditions in the Mekong estuaries Ba Lai and Ham Luong, Vietnam.

Damming has been reported to give many short-term benefits for local people but also to result in long-term negative impacts on the aquatic ecosystem. The ecological impact of the Ba Lai dam was studied by investigating environmental differences and the response of associated aquatic nematode communities (structure and morphometric characteristics) in both downstream and upstream sections of its estuary in comparison to an adjacent dam-free estuary Ham Luong, both belonging to the Mekong delta in Vietnam. Depleted dissolved oxygen, elevated methane and sulfide concentrations and increased accumulation of contaminants, including total suspended solid, heavy metals, and nutrients in the dammed estuary and its upstream section, indicated an impact of the dam on the environment. The dammed estuary showed differences in the nematode communities inhabiting the subtidal sediments from the reference estuary such as a higher nematode individual biomass with smaller length/width ratio as a consequence of the larger body width. The absence of long/thin nematodes in the dammed estuary, but high abundance of a slender nematode morphotype, a group with a higher efficiency of obtaining dissolved oxygen as a consequence of their comparatively large surface/volume ratio, might represent an adaptation of those communities to live in poor oxygen condition. In the dammed estuary, the small L/W ratio of nematode communities was potentially driven by the interaction effects of enrichment of both total suspended solid and ammonium linked to dissolved oxygen depletion. These findings support the potential use of nematode communities as bioindicators in ecological quality assessment although plausible; it is not possible to ascertain if only the dam causes the changes in the nematode communities.

Potential impacts of polymetallic nodule removal on deep-sea meiofauna.

Deep seabed mining is potentially imminent in the Clarion Clipperton Fracture Zone (CCFZ; northeast Pacific). Seabed collectors will remove polymetallic nodules and the surrounding surface sediments, both inhabited by meiofauna, along their path. To determine potential impacts of polymetallic nodule removal, we investigated the importance of nodule presence for the abundance, composition and diversity of sediment meiofauna, and evaluated the existence and composition of nodule crevice meiofauna in the Global Sea Mineral Resources (GSR) exploration contract area. Nodule-free and nodule-rich sediments displayed high biodiversity with many singletons and doubletons, potentially representing rare taxa. Nodule presence negatively influenced sediment meiofaunal abundances but did not markedly affect taxonomic composition or diversity. This is the first report on CCFZ nodule crevice meiofauna, whose abundance related positively to nodule dimensions. Though dominated by the same taxa, nodules and sediments differed regarding the taxonomic and trophic composition of the meio- and nematofauna. Nevertheless, there were no taxa endemic to the nodule crevices and nodule crevice meiofauna added only little to total small-scale (~ cm) meiofaunal abundance and diversity. We formulated environmental management recommendations at the contract area and regional (CCFZ) scale related to sampling effort, set-aside preservation and monitoring areas, and potential rehabilitation measures.

Description and distribution of Erebussau nom. nov. pro Erebus Bussau, 1993 nec Erebus Latreille, 1810 with description of a new specie, and of Odetenema gesarae gen. nov., sp. nov. (Nematoda: Desmoscolecida) from nodule-bearing abyssal sediments in the Pacific.

Erebusssau nom. nov. was originally described by Bussau, (1993) in his PhD thesis "Taxonomische und ökologische Untersuchungen an Nematoden des Peru-Beckens" but the name assigned was already in use, becoming an invalid name. Based on male specimens recovered from the Clarion-Clipperton Fracture Zone, we propose a new replacement nomen for the genus and describe a new species, also adding information about juveniles. Erebussau nom. nov. differs from all other Meyliidae genera by its peculiar offset head. Erebussau profundus sp. nov. differs from Erebussau tenebricosus nom. nov., comb. nov. by its larger size, the presence of pre- and post-cloacal supplements and the shape of the spicules. We also describe a new genus of the family Desmoscolecidae, Odetenema gesarae gen. nov., sp. nov. with an updated key to the genera of the subfamily Tricominae. Odetenema gen. nov. differs from the described Desmoscolecidae genera mainly in terms of the cuticle annulation pattern and the unique end ring with two tubular outlets. Erebussau tenebricosus, E. profundus sp. nov. and Odetenema gesarae gen. nov., sp. nov. have so far only been reported from deep-sea areas where polymetallic nodules are present.

Metabarcoding free-living marine nematodes using curated 18S and CO1 reference sequence databases for species-level taxonomic assignments.

High-throughput sequencing has the potential to describe biological communities with high efficiency yet comprehensive assessment of diversity with species-level resolution remains one of the most challenging aspects of metabarcoding studies. We investigated the utility of curated ribosomal and mitochondrial nematode reference sequence databases for determining phylum-specific species-level clustering thresholds. We compiled 438 ribosomal and 290 mitochondrial sequences which identified 99% and 94% as the species delineation clustering threshold, respectively. These thresholds were evaluated in HTS data from mock communities containing 39 nematode species as well as environmental samples from Vietnam. We compared the taxonomic description of the mocks generated by two read-merging and two clustering algorithms and the cluster-free Dada2 pipeline. Taxonomic assignment with the RDP classifier was assessed under different training sets. Our results showed that 36/39 mock nematode species were identified across the molecular markers (18S: 32, JB2: 19, JB3: 21) in UClust_ref OTUs at their respective clustering thresholds, outperforming UParse_denovo and the commonly used 97% similarity. Dada2 generated the most realistic number of ASVs (18S: 83, JB2: 75, JB3: 82), collectively identifying 30/39 mock species. The ribosomal marker outperformed the mitochondrial markers in terms of species and genus-level detections for both OTUs and ASVs. The number of taxonomic assignments of OTUs/ASVs was highest when the smallest reference database containing only nematode sequences was used and when sequences were truncated to the respective amplicon length. Overall, OTUs generated more species-level detections, which were, however, associated with higher error rates compared to ASVs. Genus-level assignments using ASVs exhibited higher accuracy and lower error rates compared to species-level assignments, suggesting that this is the most reliable pipeline for rapid assessment of alpha diversity from environmental samples.

Benthic-pelagic coupling: effects on nematode communities along southern European continental margins.

Along a west-to-east axis spanning the Galicia Bank region (Iberian margin) and the Mediterranean basin, a reduction in surface primary productivity and in seafloor flux of particulate organic carbon was mirrored in the in situ organic matter quantity and quality within the underlying deep-sea sediments at different water depths (1200, 1900 and 3000 m). Nematode standing stock (abundance and biomass) and genus and trophic composition were investigated to evaluate downward benthic-pelagic coupling. The longitudinal decline in seafloor particulate organic carbon flux was reflected by a reduction in benthic phytopigment concentrations and nematode standing stock. An exception was the station sampled at the Galicia Bank seamount, where despite the maximal particulate organic carbon flux estimate, we observed reduced pigment levels and nematode standing stock. The strong hydrodynamic forcing at this station was believed to be the main cause of the local decoupling between pelagic and benthic processes. Besides a longitudinal cline in nematode standing stock, we noticed a west-to-east gradient in nematode genus and feeding type composition (owing to an increasing importance of predatory/scavenging nematodes with longitude) governed by potential proxies for food availability (percentage of nitrogen, organic carbon, and total organic matter). Within-station variability in generic composition was elevated in sediments with lower phytopigment concentrations. Standing stock appeared to be regulated by sedimentation rates and benthic environmental variables, whereas genus composition covaried only with benthic environmental variables. The coupling between deep-sea nematode assemblages and surface water processes evidenced in the present study suggests that it is likely that climate change will affect the composition and function of deep-sea nematodes.