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Abstract
In order to invade and adapt to deep-sea environments, shallow-water organisms have to acquire tolerance to high hydrostatic pressure, low water temperature, toxic methane and hydrogen sulfide, and feeding strategies not relying on photosynthetic products. Our previous study showed that the “evolutionary stepping stone hypothe-sis”, which assumes that organic falls can act as stepping-stones to connect shallow sea with deep sea, was supported in Mytilidae. However, it is not known whether other bivalves constituting chemosynthetic communities experienced the same evolutionary process or different processes from mytilid mussels. Therefore, here, we performed phylogenetic analyses by sequencing the nuclear 18S rRNA and mitochondrial COI genes of solemyid and thyasirid bivalves. In Solemyidae, the two genera Solemya and Acharax formed each clade, the latter of which was divided into three subgroups. The Solemya clade and one of the Acharax subgroups diverged in the order of shallow-sea residents, whale-bone residents, and deep-sea vent/seep residents, which supported the “evolutionary stepping stone hypothesis”. Furthermore, in Thyasiridae, the two genera Thyasira and Maorithyas formed a paraphyletic group and the other genera, Adontorhina, Axinopsis, Axinulus, Leptaxinus, and Mendicula, formed a clade. The “evolu-tionary stepping stone hypothesis” was not seemingly supported in the other lineages of Solemyidae and Thyasiridae.
Highlights
Our previous study showed that the “evolutionary stepping stone hypothesis”, which assumes that organic falls can act as stepping-stones to connect shallow sea with deep sea, was supported in Mytilidae
In the NJ tree based on 18S rRNA sequences (1300 bp, 253 variable sites, and 119 informative sites), Acharax formed a paraphyletic group composed of three clades, Acharax 1, Acharax 2, and Acharax 3
The tree showed that the Solemya clade and Subgroup 3 in the Acharax clade diverged in the order of shallow-sea residents, whale-bone residents, and deep-sea vent/seep residents, this was not well supported by maximum parsimony (MP) bootstrap values and Bayesian posterior probabilities
Summary
Vents and seeps emit methane and hydrogen sulfide that are oxidized by chemosynthetic bacteria to produce energy for the maintenance of these communities. The organisms must cope with high hydrostatic pressure and low water temperature; they must circumvent toxic methane and hydrogen sulfide emitting from vents and seeps. The organisms have to refine their feeding strategies or develop novel techniques to acquire energy under deep-sea conditions which are nutritionally poor due to a lack of photosynthetic products. It is unclear how the organisms have adapted to deep-sea conditions and overcome the difficult environment
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