Exaptation and the Case for Ecology in Understanding ocean acidification Impacts

Chair: Peter Thor

Waldbusser, G.G. (1)*, B. Hales (1), C.J. Langdon (2), M.W. Gray (2), I. Gimenez (1), E.L. Brunner (1), S.R. Smith (1), P. Schrader (2)

1 College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, OR, United States of America
2 Coastal Oregon Marine Experimental Station and Department of Fisheries and Wildlife, Hatfield Marine Science Center, Oregon State University, Newport OR, United States of America

Background
There are strong physiological frameworks for understanding ocean acidification impacts on marine organisms; ecology can however provide an important conceptual perspective. Bivalves play significant ecological and economic roles in most of the world’s coastal zones, and many species appear to be very sensitive to ocean acidification. Understanding the mechanisms for bivalve sensitivity to ocean acidification, and what traits will likely be acted upon through selection is crucial for developing a clearer picture of bivalves in a future high CO2 world.

Methods
To determine modes of acidification stress on larval bivalve we conducted fully factorial experiments that independently tested the effects of CO2 and aragonite saturation state during early shell development across four species of bivalve larvae, Crassostra gigas, Ostrea lurida, Mytilus galloprovincialis, and Mytilus californianus. By manipulating total alkalinity and dissolved inorganic carbon we were able to create a suite of treatments with four levels of CO2 and four levels of saturation state. We measured shell development and shell size of normally developed larvae.

Findings
For three of the four species we found early shell development and growth to be affected by mineral saturation state, not pH nor CO2. Nativity to the naturally elevated CO2 waters along the Oregon coast did not convey resistance to acidification stress. The lack of acute response in Ostrea lurida appears to be related to the rate of initial shell development and calcification of this species.

Conclusions
Slow shell development, as a result of brooding in O. lurida, appears to provide a trait conveying fitness to this species during the critical bottleneck of early shell development. We contend this trait was not developed to cope with elevated CO2, rather is an example of exaptation; a trait co—opted for fitness, but initially evolved for different reasons.