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

Abstract. Bivalves that build calcium carbonate skeletons are at particular risk from ocean acidification, and mitigation strategies will 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 they live in. 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 (pre-sexual maturity) were raised for 90 days 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 porewater relative to the control, and animals exposed to acidified water plus shell hash grew larger than animals exposed to acidified water alone. 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, thus buffering against acidic conditions that can negatively affect the growth of economically and culturally important shellfish like littleneck clams.