Background
With respect to the effects of ocean acidification on various forms of marine life, the authors write that "experiments provide insight into instantaneous responses of organisms to increased CO2 levels, but information from populations that have acclimatized to such conditions may better inform models of future responses." And it was just such information that their research was targeted to acquire.

What was done
Using remotely-operated vehicles to collect mussel specimens, water samples and imagery on the northwest side of Eifuku volcano on the Mariana Arc, Tunnicliffe et al. examined shells of the mussel Bathymodiolus brevior - "a vent-obligate species that relies partly on symbiotic sulphide-oxidizing bacteria for nutrition (von Cosel and Metivier, 1994)," which they say "occurs at many sites in the western Pacific Ocean, where it occupies habitats of low hydrothermal fluid flux."

What was learned
The six scientists found pH values to be in the range 5.36-7.29 at NW Eifuku, which enabled them to calculate saturation ratios for calcite (Ωcalc) ranging from 0.01 to 0.61, with an average value of only 0.18. Nevertheless, they discovered "a dense mussel population, along with many other associated species (Limen and Juniper, 2006), on NW Eifuku, where chemosynthetic symbiosis provides an energetic benefit to living in a corrosive, low-pH environment."

What it means
In discussing their findings, Tunnicliffe et al. say they attest to "the extent to which long-term adaptation can develop tolerance to extreme conditions." In fact, they report discovering four-decade-old mussels living at the sites they visited, which led them to state that "the mussels' ability to precipitate shells in such low-pH conditions is remarkable."

Yes, it is remarkable ... and true! Earth's animal life appears to have the capacity to eke out a living in places where even scientists believe that such would not be possible.

References
Limen, H. and Juniper, S.K. 2006. Habitat controls on vent food webs at NW Eifuku Volcano, Mariana Arc. Cahiers de Biologie Marine 47: 449-455.

von Cosel, R. and Metivier, B. 1994. Three new species of Bathymodiolus (Bivalvia: Mytilidae) from hydrothermal vents in the Lau Basin and the North Fiji Basin, Western Pacific, and the Snake Pit area, Mid-Atlantic Ridge. The Veliger 37: 374-392.