The contrasted evolutionary fates of deep-sea chemosynthetic mussels (Bivalvia, Bathymodiolinae)

International audienceBathymodiolinae are giant mussels that were discovered at hydrothermal vents and harboring chemosynthetic symbionts. Due to their close phylogenetic relationship with seep species and tiny mussels from organic substrates, it was hypothesized that they gradually evolved from shallow to deeper environments, and specialized in decaying organic remains, then in seeps, and finally colonized deep-sea vents. Here, we present a multigene phylogeny that reveals that most of the genera are polyphyletic and/or paraphyletic. The robustness of the phylogeny allows us to revise the genus-level classification. Organic remains are robustly supported as the ancestral habitat for Bathymodiolinae. However, rather than a single step toward colonization of vents and seeps, recurrent habitat shifts from organic substrates to vents and seeps occurred during evolution, and never the reverse. This new phylogenetic framework challenges the gradualist scenarios from shallow to deep. Mussels from organic remains tolerate a large range of ecological conditions and display a spectacular species diversity contrary to vent mussels, although such habitats are yet underexplored compared to vents and seeps. Overall, our data suggest that for deep-sea mussels, the high specialization to vent habitats provides ecological success in this harsh habitat but also brings the lineage to a kind of evolutionary dead end

Integrative Biology of Idas iwaotakii (Habe, 1958), a 'Model Species' Associated with Sunken Organic Substrates

The giant bathymodioline mussels from vents have been studied as models to understand the adaptation of organisms to deep-sea chemosynthetic environments. These mussels are closely related to minute mussels associated to organic remains decaying on the deep-sea floor. Whereas biological data accumulate for the giant mussels, the small mussels remain poorly studied. Despite this lack of data for species living on organic remains it has been hypothesized that during evolution, contrary to their relatives from vents or seeps, they did not acquire highly specialized biological features. We aim at testing this hypothesis by providing new biological data for species associated with organic falls. Within Bathymodiolinae a close phylogenetic relationship was revealed between the Bathymodiolus sensu stricto lineage (i.e. "thermophilus'' lineage) which includes exclusively vent and seep species, and a diversified lineage of small mussels, attributed to the genus Idas, that includes mostly species from organic falls. We selected Idas iwaotakii (Habe, 1958) from this latter lineage to analyse population structure and to document biological features. Mitochondrial and nuclear markers reveal a north-south genetic structure at an oceanic scale in the Western Pacific but no structure was revealed at a regional scale or as correlated with the kind of substrate or depth. The morphology of larval shells suggests substantial dispersal abilities. Nutritional features were assessed by examining bacterial diversity coupled by a microscopic analysis of the digestive tract. Molecular data demonstrated the presence of sulphur-oxidizing bacteria resembling those identified in other Bathymodiolinae. In contrast with most Bathymodiolus s.s. species the digestive tract of I. iwaotakii is not reduced. Combining data from literature with the present data shows that most of the important biological features are shared between Bathymodiolus s.s. species and its sister-lineage. However Bathymodiolus s.s. species are ecologically more restricted and also display a lower species richness than Idas species

Scanning electron micrographs of the larval shell of a specimen of <i>Idas iwaotakii</i>.

<p>Pictures: (A) Dorsal view showing prodissoconch I (PI) and prodissoconch II (PII); the white arrow indicates the boundary between the dissoconch and prodissoconch, (B) Detail of PI; the grey arrow indicated of the boundary between PI and PII.</p

Bayesian tree displaying bacterial symbionts based on the analysis of the 16S rRNA.

<p>Phylotypes associated with <i>I. iwaotakii</i> are shown in bold. Posterior probabilities (PP) and bootstrap values obtained from ML analysis are given above and below branches respectively. PP and bootstrap values lower than 0.90 and 50%, respectively, are not shown. The scale bar represents 0.5% estimated base substitution.</p

Gill filaments of <i>Idas iwaotakii</i>.

<p>(A) TEM view of gill filament of the lateral zone from a specimen of <i>I. iwaotakii</i> collected on wood off Vanuatu. Bacteria (black arrows) are located extracellularly in contact with microvilli. BL: blood lacuna; Lm: basal lamina. (B) Electron micrograph of the extracellular symbionts. Symbiotic bacteria possess a double membrane (white arrow) typical of Gram negative bacteria.</p

Geographical localities, substrates, depths and sequences of specimens of <i>I. iwaotakii</i> used in this study.

<p>The number of sequences is indicated in brackets. The superscripts refer to the authors of sequences. The stars indicate the sequences obtained during this study.</p

Population structure of <i>I. iwaotakii</i>.

<p>Left part: K2P neighbour-joining tree of unique COI haplotypes of <i>I. iwaotakii</i>. The localities at which each haplotype have been sampled is figured by a coloured circle (see legend for correspondence between localities and colours), the number of individuals sharing the same COI haplotype at each locality is given in brackets. The grey rectangles underline the haplotypes shared by the two studied regions. The scale bar represents 0.02% estimated base substitutions. The mitochondrial haplotypes of <i>I. iwaotakii</i> are divided in two clades, A’ and A’’, represented by one panel with large dots and the other one with small dots, respectively. Right part: Median-joining networks (MJN) from COI mtDNA data drawn independently for each of the two studied regions (North Western and South Western haplotypes). Size of haplotype circles is proportional to the number of specimens. Colours used to figure the localities within each region are the same of that used for the NJ tree. A small black circle represents a hypothetical haplotype. The haplotypes shared by the two studied regions are underlined using grey circles over haplotype labels.</p

Dendrogram obtained from Bayesian analyses of the combined dataset of COI mtDNA and the 28S rRNA.

<p>Asterisks correspond to nodes with posterior probabilities (PP) obtained from BA analysis and bootstrap values obtained from ML analysis that are higher than 0.90 and 50%, respectively. (modified from <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0069680#pone.0069680-Lorion1" target="_blank">[7]</a>).</p

South Western Pacific map showing the sampling localities of mussels of <i>Idas iwaotakii</i>.

<p>Specimens from Malo and New Caledonia have already been studied by <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0069680#pone.0069680-Lorion1" target="_blank">[7]</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0069680#pone.0069680-Lorion3" target="_blank">[30]</a>.</p