UC Davis study reveals how crushed shells mitigate ocean acidification

A study led by researchers at the University of California, Davis reveals why mixing crushed shells, or shell hash, into beach sand protects shell-building marine animals from ocean acidification

Published in Biogeosciences, the 90-day experimental study demonstrates that shell hash alters the chemistry of interstitial pore water in sediment, reducing local acidity and suppressing the metabolic and genetic stress signals typically triggered in juvenile shellfish exposed to acidified seawater.

Pore water chemistry and genetic suppression

Ocean acidification occurs as seawater absorbs excess atmospheric carbon dioxide, lowering pH levels and making calcium carbonate shell synthesis energetically costly for bivalves. Juvenile and larval shellfish are particularly vulnerable to shell dissolution and suppressed growth rates.

To test the protective mechanisms of shell hash, the UC Davis research team raised juvenile Pacific littleneck clams (Leukoma staminea) across four controlled sediment and seawater conditions over a 90-day period:

• Chemical buffering:

  • Dissolving shell hash within the sediment altered the chemistry of the water filling tiny gaps between sand particles (pore water), creating a localised alkaline buffer that offset ambient water acidity.

• Morphological growth:

  • Clams grown in sediment enriched with shell hash developed significantly larger shells on average compared to control groups in standard sand.

• Transcriptomic normalisation:

  • Gene sequencing of mantle tissue revealed that clams raised in acidified water with shell hash exhibited gene expression profiles virtually identical to those raised in non-acidified conditions, effectively neutralising the genetic stress response induced by low-pH environments.

Validation of Indigenous ecological knowledge

The findings provide a scientific framework for traditional ecological knowledge practised by Indigenous communities throughout the Pacific Northwest. Indigenous coastal management strategies—such as the construction of clam gardens and intertidal bed enhancements—have long utilised shell debris to maintain productive habitats for marine invertebrates.

Co-author Dr Tsim Schneider highlighted that integrating transcriptomic and geochemical data with long-standing Indigenous stewardship offers opportunities to reawaken dormant cultural management practices and support tribe-led coastal restoration initiatives.