1.6 Vent mussels and associated fauna are bathed in hydrothermal fluids at the Wideawake vent field on the Mid‐Atlantic Ridge. Photo taken by the remotely operated, deep‐sea submersible MARUM‐QUEST.
Source: From Orphan & Hoehler (2011). Reproduced with permission from Nature Publishing Group.
Although these mussels were initially regarded as archaic, molecular age estimates and fossil records suggest that most of the modern vent and seep animals appeared during a short time interval between the Late Mesozoic and the Early Cenozoic, less than 100 mya (Van Dover et al. 2002). The evolutionary history of deep‐sea symbiotic mussels has been investigated over the past two decades using a range of mitochondrial and nuclear DNA markers. Distel et al. (2000) were the first to propose the stepping stone hypothesis (i.e. that vent and seep mussels derived from ancestors associated with wood and whale fall ecosystems). Results from subsequent phylogenetic studies have strongly supported this proposal (Samadi et al. 2007; Miyazaki et al. 2010; Lorion et al. 2013; Thubaut et al. 2013). Rather than a single step toward colonisation of vents and seeps, there were multiple habitat shifts from organic substrates to vents and cold seeps, always in the one direction (Thubaut et al. 2013). Cold seeps seem to be an intermediate habitat for the colonisation of deep vents (Thubaut et al. 2013). Ancestors of Bathymodiolus mussels were shallow species that acquired the ability to associate with bacteria, most likely sulphur oxidisers (Duperron 2010). For mussels, the acquisition of such symbionts is seen as a prerequisite for their adaptation to, and successful radiation within, chemosynthetic environments (Lorion et al. 2013). Subsequent acquisition of methanotrophic symbionts allowed the colonisation of new niches within the vent and seep habitats, resulting in a second wave of diversification (Lorion et al. 2013). To date, there is no general consensus on the phylogenetic relationships of deep‐sea Bathymodiolus mussels and their mytilid relatives (see Jones et al. 2006; Fujita et al. 2009; Miyazaki et al. 2010; Thubaut et al. 2013; Oliver 2015).
Notes
1 1 A system of classification based on the phylogenetic relationships and evolutionary history of groups of organisms, rather than purely on shared features.
2 2 A monophyletic group of organisms believed to comprise all the evolutionary descendants of a common ancestor.
3 3 The classification of organisms and the evolutionary relationships among them.
4 4 A polyphyletic taxon is defined as one that does not include the common ancestor of all members of the taxon.
5 5 A paraphyletic taxon is one that includes the most recent common ancestor but not all of its descendants.
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