Post-larval and adult shell dimensions of Idas modiolaeformis

We describe the first reproductive features of a chemosynthetic mussel collected at cold seeps from the eastern Mediterranean Sea. Idas modiolaeformis (Bivalvia, Mytilidae) is a hermaphroditic species in which production of male and female gametes likely alternates, a feature regarded as an adaptation to patchy and ephemeral habitats. By using fluorescent in situ hybridization, we demonstrate that bacterial symbionts, while present within the gills, are absent within acini that enclose female gametes and male gametes. This supports the hypothesis of environmental acquisition of symbionts in chemosynthetic mytilids. Prodissoconch I (PI) is relatively small compared to prodissoconch II (PII), suggesting a planktotrophic larval stage. Diameters of the two larval shells are in the range of sizes reported for mytilids, with a PII size between that of the shallow Mytilus edulis and that of the cold-seep mussel “Bathymodiolus” childressi

Plasticity in reproduction and nutrition in wood-boring bivalves (Xylophaga atlantica) from the Mid-Atlantic Ridge

International audienceHigh densities of the wood-boring bivalve Xylophaga atlantica colonized pine wood cubes in colonization devices deployed at 2279 m depth for 414 days (14 July 2007–31 August 2008) near hydrothermal vents at the Rainbow site on the Mid-Atlantic Ridge (36°13.7454′N/33°54.0513′W). Histological and biometric observations on specimens with shell lengths (SLs) of 0.5–4.2 mm revealed three cohorts in this dioecious population. The first cohort was dominated by mature females, each with an estimated fecundity of ~450 oocytes with a mean diameter of 28.0 ± 3.9 µm (maximum diameter 40.0 µm); an intermediate cohort was a mix of males and females with SL at first maturity of ≤1.7 mm; the third cohort was exclusively morphologically distinct, mature, dwarf males, SL ~500 µm. These dwarf males were attached to the dorsal shell surfaces of females in the first cohort. The difference in the SL of Prodissoconch I (~60 µm) and Prodissoconch II (500–530 µm) confirmed planktotrophy. Based on their carbon and nitrogen stable isotope ratios, and a paedomorphic morphology suggesting they are ill-equipped to bore wood, it seems likely that the dwarf males are heterotrophic filter-feeders. Fluorescence in situ hybridization showed, however, that dwarf males hosted a few Gammaproteobacteria in their gills. The absence of a bacterial signal in the germ cells and developing oocytes of females implies that direct trans-ovarial inheritance of symbiotic bacteria does not occur in X. atlantica

Annelid polychaetes experience metabolic acceleration as other Lophotrochozoans: inferences on the life cycle of Arenicola marina with a Dynamic Energy Budget model

International audienceArenicola marina is a polychaete (Lophotrochozoan) displaying a complex bentho-pelagic life cycle with two larval dispersal phases, only partially described up to now. A Dynamic Energy Budget (DEB) model was applied to the species in order to reconstruct its life cycle and growth under in situ environmental conditions. Two types of DEB models are usually applied to other Lophotrochozoans displaying similar life cycles: the standard (std-) model, applied to polychaetes (5 entries among the 1524 of the Add-my-Pet database on the 18/10/2018), and the abj-model, which includes an acceleration of metabolism between birth and metamorphosis, and which has been applied to most molluscs (77 abj-entries out of the 80 mollusc entries) enabling better fit predictions for the early life stages. The parameter estimation was performed with both models to assess the suitability of an abj-model for A. marina. The zero-variate dataset consisted of length and age data at different life cycle stages, the lifespan, the maximum observed length, and the wet weight of an egg. The uni-variate dataset consisted of two growth experiments from the literature at two food levels and several temperatures, laboratory data of oxygen consumption at several temperatures, and fecundity for different lengths. The predictions of the abj-model fitted better to the data (SMSE = 0.29). The acceleration coefficient was ca 11, which is similar to mollusc values. The field growth curves and the scaled functional responses (as a proxy of food levels) were suitably reconstructed with the new parameter set. The reconstruction of the early life-stages chronology according to in situ environmental conditions of a temperate marine ecosystem indicated a first dispersal phase of 5 days followed by a 7 months temporary settlement before a second dispersal phase in spring, at the end of metamorphosis. We emphasize the need for using abj-models for polychaetes in future studies

Epsilonproteobacteria as gill epibionts of the hydrothermal vent gastropod Cyathermia 1 naticoides (North East-Pacific Rise)

International audienceMollusks, and particularly gastropods, are one of the major taxonomic groups at vents. In these ecosystems, devoid of light, chemoautotrophic bacteria are at the base of the food web and symbiotic association between metazoa and these bacteria is numerous. Nevertheless, apart few “large-size” well-known species, the “small-size” gastropods (shell <5 mm), although very abundant, remain poorly studied regarding symbioses. We investigated here Cyathermia naticoides (Warén and Bouchet in Zool Scr 18(1), 1989), a small coiled gastropod found in abundance on the East Pacific Rise among Riftia pachyptila tubes, and usually inferred to graze on tubeworm bacterial cover, and/or filter feeding. Among mollusks, symbioses are well known in large species and almost exclusively rely on sulfide or methane-oxidizing proteobacterial endosymbionts, occurring within the host tissues in gill epithelial bacteriocytes. Combining several approaches (molecular biology, microscopy, stable isotopes analyses), we described here an unusual symbiosis, where autotrophic filamentous Epsilonproteobacteria are located extracellularly, at the base of host gill filaments. Numerous endocytotic lysosome-like structures were observed in the gill epithelium of the animal suggesting bacteria may contribute to its nutrition through intracellular digestion by gill cells. Additional food source by non-symbiotic proteobacteria grazed on R. pachyptila tubes could complete the diet. The possible role of temperature in the selection of Epsilon- vs Gammaproteobacterial partners is discussed

Unravelling mechanisms behind population dynamics, biological traits and latitudinal distribution in two benthic ecosystem engineers: A modelling approach

The mechanistic approach consisting of coupling Dynamic Energy Budget (DEB) models to Individual-BasedModels (IBMs) allows simulating individual and population biological traits and their dynamics. This approachwas developed here to study population dynamics of two sympatric intertidal ecosystem engineers, Arenicolamarina and Arenicola defodiens (Annelida Polychaeta) occurring in the North-East Atlantic from Portugal toSweden. Latitudinal heterogeneity of the two species’ performances were investigated in terms of populationdynamics and biological traits using latitudinal differences in environmental forcing variables. The impactof the forcing variables on population dynamics processes (shore colonisation and migration, spawning andrecruitment, etc.) within a specific foreshore (mean values and seasonal patterns) was also assessed. PublishedDEB parameters were used for A. marina and a specific calibration was undertaken for A. defodiens, combiningliterature data and new laboratory experiments and field data. Our DEB-IBM simulated super-individuals’growth and reproduction while lugworms were colonising, migrating and dying over a simulated foreshore.Density rules affected population dynamics. Environmental forcings consisted in monthly values of chlorophylla(chl-a) concentrations and daily values of SST. Scenarios focusing on the two most contrasted of theseforcing variables time series were used to explore their relative effects over populations’ dynamics and onshoreprocesses were investigated at two sites displaying highly different simulated population abundances.Overall, northern sites with higher chl-a levels performed better displaying higher biomass, maximum lengthand reproductive outputs for both species. As expected, Sea Surface Temperature (SST) changes between sitesdid not impact greatly populations dynamics. Under favourable environmental conditions, intra- and interspecificcompetitions emerged from the model. Under non-favourable environmental conditions, A. defodiens’populations crashed and A. marina displayed atypical population processes, with rare spawning events barelyallowing the population’s renewal, and lower size at maturity. Further use and development of this model willlead to better insights on the lugworm populations’ evolution over the next decades

Microplastic leachates induce chemical disruption of chemotactic and chemokinetic responses of an ecosystem engineer (Mytilus edulis)

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Fickle or Faithful: The Roles of Host and Environmental Context in Determining Symbiont Composition in Two Bathymodioline Mussels

International audienceThe Mediterranean Sea and adjoining East Atlantic Ocean host a diverse array of small-sized mussels that predominantly live on sunken, decomposing organic remains. At least two of these, Idas modiolaeformis and Idas simpsoni, are known to engage in gill-associated symbioses; however, the composition, diversity and variability of these symbioses with changing habitat and location is poorly defined. The current study presents bacterial symbi-ont assemblage data, derived from 454 pyrosequencing carried out on replicate specimens of these two host species, collected across seven sample sites found in three oceanographic regions in the Mediterranean and East Atlantic. The presence of several bacterial OTUs in both the Mediterranean Sea and eastern Atlantic suggests that similar symbiont candidates occur on both sides of the Strait of Gibraltar. The results reveal markedly different symbiotic modes in the two species. Idas modiolaeformis displays high symbiont diversity and flexibility, with strong variation in symbiont composition from the East Mediter-ranean to the East Atlantic. Idas simpsoni displays low symbiont diversity but high symbiont fidelity, with a single dominant OTU occurring in all specimens analysed. These differences are argued to be a function of the host species, where subtle differences in host evolution, life-history and behaviour could partially explain the observed patterns. The variability in symbiont compositions, particularly in Idas modiolaeformis, is thought to be a function of the nature, context and location of the habitat from which symbiont candidates are sourced

Immune failure reveals vulnerability of populations exposed to pollution in the bioindicator species Hediste diversicolor

International audienceHuman activities on the shoreline generate a growing pollution, creating deleterious habitats in coastal zones. Some species nevertheless succeed in such harsh milieus, raising the question of their tolerance to environmental stress. The annelid Hediste diversicolor lives buried in the sediments, directly exposed to contaminants trapped in the mud. After verifying the similarity of their genetic contexts, we compared reproductive output and individual immune resistance measures of populations living in polluted vs. ‘clean’ sediments, and related these assessments with measures of phthalates and metal pollution, and associated toxicity indices. Chemical analyses predicted no toxicity to the local infauna, and phenological studies evidenced no direct cost of living in noxious habitats. However, populations exposed to pollutants showed a significantly reduced survival upon infection with a local pathogen. Surprisingly, physiological studies evidenced a basal overinflammatory state in the most exposed populations. This over-activated baseline immune phenotype likely generates self-damage leading to enhanced immune cell death rate and immune failure. Monitoring the immune status of individual worms living in anthropic areas could thus be used as a reliable source of information regarding the actual health of wild populations