Trade-offs in a high CO2 habitat on a subsea volcano: condition and reproductive features of a bathymodioline mussel

Northwest Eifuku submarine volcano (Mariana Volcanic Arc) emits very high concentrations of CO2 at a vent where the mussel Bathymodiolus septemdierum experiences pH as low as 5.2. We examined how this natural setting of high pCO2 influences shell, body, and reproductive condition. Calcification is highly compromised: at a given shell volume, shells from NW Eifuku weigh about half those from reference sites in the south Pacific, and dissolution of the inner shell is evident. However, the condition indices of some NW Eifuku mussels were equal to or higher than those from Lau back-arc basin and the New Hebrides Island Arc. NW Eifuku mussels in pH 5.2 fluids had the highest symbiont abundances in gill bacteriocytes, probably due to greater dissolved sulphide access. Excess energy demands imposed by high pCO2 conditions appears moderated by adequate food availability through symbiont chemosynthesis. In the sample with the lowest body condition, gametogenesis was lagging, although all mussels in high pCO2 had developing gonads and the complete gametogenic cycle was present in our samples. Gamete development is synchronous between sexes and is possibly periodic. While mussels are functionally dioecious, protogynous hermaphroditism can occur—a first record for the genus—which may be an adaptation to resource availability. B. septemdierum likely makes energy allocation trade-offs among calcification, body mass maintenance, reproduction and other processes to maximize fitness. We suggest that flexibility to divert energy from shell formation, combined with good food supply, can mitigate the manifestation of high CO2 stress on B. septemdierum.

Rossi G. S. & Tunnicliffe V., 2017. Trade-offs in a high CO2 habitat on a subsea volcano: condition and reproductive features of a bathymodioline mussel. Marine Ecology Progress Series 574: 49-64. Article (subscription required).

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