SEM INVESTIGATION OF EXPERIMENTAL OCEAN ACIDIFICATION ON THE SHELL MICROSTRUCTURE OF THE COQUINA CLAM (DONAX VARIABILIS)

Anthropogenic CO2 emissions have increased exponentially since the Industrial Revolution. Since 1750, oceans have absorbed roughly 28% of anthropogenic CO2. The uptake of excess CO2 impacts the ocean’s chemistry by forming carbonic acid (H2CO3), and decreasing ocean pH which is known as ocean acidification (OA). As pH decreases, it is more difficult for calcifying organisms to build and maintain their skeletons. The coquina clam (Donax variabilis) is a small bivalve found along the coast of the southeastern US and is integral in the coastal ecosystem as a prey species. This study examines the effect of experimental OA on the shell microstructure of coquinas. A control and 2 experimental groups (EGs) were maintained in a laboratory setting for 8 days. OA conditions were replicated by subjecting EGs to a lowered pH with EG1 receiving 10mL of an acid every 24 hours and EG2 receiving 5mL. After death, shells were dried and prepared for SEM analysis to image the shell and periostracum microstructures of treated and non-treated coquinas. We report here initial results from this in progress study. In EG1 a number of significant observations of the SEM imagery have been made. 1) We see the periostracal fringe starting to delaminate from the upper facets of the aragonite prismatic shell fabric. 2) The periostracum (outer organic protective coating of the shell) was deformed along the delamination, and 3) portions of the periostracum appear to show evidence of fraying along the edge. These same observations have not yet been observed in the same manner on the control shells. At this point, no obvious evidence of dissolution was observed in our shells. However, dissolution has been observed in other molluscs exposed to longer-term treatments. The significance of our short-term study is that deterioration of the periostracum may be an initial step before actual dissolution can progress to the carbonate layers of the shell. Deterioration of the periostracum may also impact the tubules that run through the shell. Studies of other clams highlight the role these play in providing access for pathogens to the clam's interior. Thus OA may impact these pathways by increasing the rate of dissolution, decreasing the integrity of the shell, and all while making the clams more susceptible to disease and potentially increased mortality rates.