Shell hash promotes growth in Pacific littleneck clams ( Leukoma staminea ) by altering pore water chemistry

Bivalves that build calcium carbonate skeletons are at risk from ocean acidification, and mitigation strategies may be needed to keep coastal populations healthy. It can be energetically costly for organisms like clams and mussels to build their shells under low pH conditions, and acidification can lead to shell dissolution. Adding crushed shells (shell hash) to beach sediments, a practice used by some Indigenous communities and aquaculturists, may mitigate the negative effects of ocean acidification by altering the chemistry of the pore fluids in which they live. We tested the hypothesis that mixing shell hash into the sediment improves the growth and physiology of infaunal Pacific littleneck clams ( Leukoma staminea ). Juvenile clams were raised for 90 d under four conditions: control seawater with sediment, acidified seawater with sediment, control seawater with sediment plus shell hash, and acidified seawater with sediment plus shell hash. Pore water and overlying seawater were sampled three times a week for pH, alkalinity, salinity, temperature, and dissolved oxygen. Clam shell weight, soft tissue weight, and new shell growth were measured, and mantle tissue RNA was collected for gene sequencing after three months. Our results demonstrate that the addition of shell hash increased the pH of pore water relative to the control, acidified water decreased animal growth, and the presence of shell hash partially mitigated the effect. Gene expression profiling suggests that animals in acidified seawater with shell hash were largely indistinguishable from animals in non-acidified water. Our experimental results suggest that adding shell hash to sediments alters the chemistry of pore fluids, buffering against acidic conditions that can negatively affect the growth of economically and culturally important shellfish like littleneck clams.