Marine bivalves are at risk due to anthropogenic climate change. This poses a threat to commercial aquaculture, and the communities that depend on bivalves for food security. Ocean acidification, the lowering of seawater pH, is of particular concern for marine calcifiers, including bivalves. Ocean acidification occurs as carbon dioxide enters the atmosphere at a rate faster than natural geologic processes can buffer. It is energetically costly for marine calcifiers like clams, mussels, and oysters, to mineralize under acidic conditions, and low pH leads to the dissolution of calcium carbonate shells and skeletons. This problem warrants research on acidification adaptation strategies that will keep coastal populations healthy. Adding calcium carbonate shell hash to coastal sediments may mitigate negative effects of acidification on infaunal shellfish by altering the chemistry of pore fluids, thus providing a potential mitigation strategy for local populations, including species like the Pacific littleneck clam (Leukoma staminea).

Here, I present the first in-depth description of the shell mineralogy, microstructure, body size variability, and geochemical properties of modern L. staminea, a common eastern Pacific, shallow, infaunal bivalve. Additionally, I document the impact of ocean acidification on the shell growth and gene expression of L. staminea, and test whether adding shell hash to its infaunal environement can promote growth under acidified conditions. Juvenile clams were raised in four experimental conditions for 90 days: control seawater, acidified seawater, control seawater with shell hash, and acidified seawater with shell hash. Clam shell weight, soft tissue weight, and new shell growth were measured. The presence of shell hash increased the pH, alkalinity and aragonite saturation state of pore fluids across all treatments, and increased shell growth on average, although growth metrics were variable across and within treatments. Lastly, RNA-sequencing revealed the impacts of experimental acidification on juvenile clam gene expression. Clams grown in the presence of shell hash had gene expression profiles more similar to those grown in control (non-acidified) conditions. In all, this research suggests that adding shell hash to coastal sediments alters the chemistry of pore fluids and buffers against acidic conditions that deleteriously affects clam mineralization.