As a result their ecology, taxonomy, population genetics and phylogenetic relationship have been intensely studied at the morphological and molecular level (see for a recent review), including transcriptomic approaches. Because of their filter feeding habits, they also hold a major function in coastal ecosystems and reef ecology. Bivalves are also being increasingly used to study the spread of cancer in natural environments, as a model of contagious tumours, and to assess gene expression of cancer-related genes. Many species of bivalves constitute an important food source and have a role in culture and folklore, or even in the medical and bioengineering fields. Bivalves thus adopt a multitude of different life modes, from detritivory to filter feeding, with extreme cases of photo- and chemosymbiosis to carnivory. Overall, our study, by using densely sampled transcriptomes, provides the best-resolved bivalve phylogeny to date.īivalvia is among the most diverse molluscan classes, totalling almost 10 000 described extant species (see ) inhabiting various aquatic environments, spanning freshwater, brackish and marine, as well as ranging from the shallow continental shelve to the deep sea, including hydrothermal vents and hydrocarbon seeps. Neoheterodontei is always fully supported, and consists of Sphaeriida, Myida and Venerida, with the latter now also containing Mactroidea, Ungulinoidea and Chamidae, a family particularly difficult to place in earlier work. Gastrochaenida is sister group to all remaining imparidentians, which divide into six orders. Lucinidae, Thyasiridae and Gastrochaenida represent three early branches. Within Imparidentia, resolution increases when analysing Imparidentia-specific matrices.
Likewise, the inter-relationships of the six main bivalve clades were fully supported. Our results support the overall backbone of the bivalve tree, the monophyly of Bivalvia and all its main nodes, although the monophyly of Protobranchia remains less clear. We designed specific data matrices to address the poorly resolved relationships within Imparidentia. Here we combine dense taxon sampling with 108 deep-sequenced Illumina-based transcriptomes to provide resolution in nodes that required additional study. Yet the first approach lacked decisive taxon sampling and the second failed to resolve many interfamilial relationships, especially within the diverse clade Imparidentia. Bivalvia has been the subject of extensive recent phylogenetic work to attempt resolving either the backbone of the bivalve tree using transcriptomic data, or the tips using morpho-anatomical data and up to five genetic markers.
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