Phylogeny and Diversification Patterns among Vesicomyid Bivalves

Abstract

Vesicomyid bivalves are among the most abundant and diverse symbiotic taxa in chemosynthetic-based ecosystems: more than 100 different vesicomyid species have been described so far. In the present study, we investigated the phylogenetic positioning of recently described vesicomyid species from the Gulf of Guinea and their western Atlantic and Pacific counterparts using mitochondrial DNA sequence data. The maximum-likelihood (ML) tree provided limited support for the recent taxonomic revision of vesicomyids based on morphological criteria; nevertheless, most of the newly sequenced specimens did not cluster with their morphological conspecifics. Moreover, the observed lack of geographic clustering suggests the occurrence of independent radiations followed by worldwide dispersal. Ancestral character state reconstruction showed a significant correlation between the characters “depth” and “habitat” and the reconstructed ML phylogeny suggesting possible recurrent events of ‘stepwise speciation’ from shallow to deep waters in different ocean basins. This is consistent with genus or species bathymetric segregation observed from recent taxonomic studies. Altogether, our results highlight the need for ongoing re-evaluation of the morphological characters used to identify vesicomyid bivalves.

The smaller vesicomyid bivalves in the genus Isorropodon (Bivalvia, Vesicomyidae, Pliocardiinae) also harbour chemoautotrophic symbionts

“…Indeed, because genera were initially poorly defined in Vesicomyidae, recent work has attempted to clarify the taxonomy of the family (see Krylova and Sahling 2010 for review). However the taxonomic and phylogenetic study of this group is still in progress, and the incongruence between clades and named genera is fairly evident from our, as well as from other authors' phylogenies (Peek et al 1997;Goffredi et al 2003;Kojima et al 2004;Decker et al 2012). …”

“…Genome sequencing of symbionts has emphasized several traits typical of maternallytransmitted bacteria, including genome size reduction coupled with the loss of certain genes (Kim et al 1995;Harada et al 2009;Kuwahara et al 2008;Stewart et al 2008). The systematics and evolution of Pliocardiinae hosts have also been studied but their taxonomy is still under debate as genera are typically found non-monophyletic in phylogenetic reconstructions (Kojima et al 1995;Baco et al 1999;Osborn 2001;Goffredi et al 2003;Kojima et al 2004Kojima et al , 2006Decker et al 2012). Yet, apart from large-sized Pliocardiinae vesicomyids, there are in this subfamily smaller-sized species which have been overlooked, and for which very little data is available, both for hosts and associated symbionts.…”

“…Recently, we recovered a few specimens of three Isorropodon morphospecies from the Eastern Mediterranean, Gulf of Cadiz and Gulf of Guinea cold seeps. In order to verify the monophyly of Isorropodon, we sequenced cytochrome oxidase I (COI), a molecular marker previously used in vesicomyid studies (Baco et al 1999;Peek et al 1997;Goffredi et al 2003;Decker et al 2012). In order to characterize potential bacterial partners associated with Isorropodon species, we used comparative 16S rRNA gene sequence analysis and fluorescence in situ hybridization (FISH).…”

Phylogeny and origins of chemosynthetic vesicomyid clams

“…Molecular studies also revealed synonymous species designations that obscured broad trans-oceanic distributions for several clams (Audzijonyte et al, 2012;Kojima, Fujikura, & Okutani, 2004). Phylogenetic analyses of COI sequences from 51 OTUs suggests a recurrent pattern of speciation, leading from shallow to deep habitats in separate ocean basins (Decker et al, 2012). Although these molecular studies have supported some of the morphology-based assignments to genera, examinations of COI sequences alone do not adequately resolve the deeper phylogenetic nodes that might help to define genera.…”

“…It is unsurprising, therefore, that molecular systematic studies of extant species have revealed a number of cryptic species complexes and do not support monophyly of many groups currently treated as genera (Audzijonyte, Krylova, Sahling, & Vrijenhoek, 2012;Decker, Olu, Cunha, & Arnaud-Haond, 2012;Goffredi, Hurtado, Hallam, & Vrijenhoek, 2003;Peek et al, 1997Peek et al, , 2000Vald es et al, 2012;Vrijenhoek, Schutz, Gustafson, & Lutz, 1994). Though molecular traits have generally not been incorporated, recent taxonomic descriptions that included soft anatomical characters have greatly improved the diagnoses of species and genera (Cosel & Olu, 2009;Cosel & Salas, 2001;Krylova & Cosel, 2011;Krylova & Sahling, 2006;Krylova, Sahling, & Janssen, 2010).…”