How Crushed Shells Protect Against Ocean Acidification

UC Davis New research from the University of California, Davis reveals why adding crushed shells to beach sand helps protect shell-making animals - such as clams, oysters and mussels - from the detrimental effects of ocean acidification. In a study appearing in Biogeosciences , the researchers designed an experiment at the UC Davis Bodega Marine Laboratory in which they mixed crushed shells, or shell hash, into sand where baby Pacific littleneck clams burrowed and grew.

They then monitored the clams' growth and the water's chemistry over a 90-day period. Adding shell hash, the researchers found, altered the chemistry of the water that fills the tiny spaces between sand particles, decreasing its overall acidity and creating a better environment for shell growth.

"We found that, on average, clams grew larger shells when shell hash was present in their treatment," said study first author Hannah Kempf , an environmental scientist at the California State Water Board who graduated from UC Davis with a Ph.D. in earth and planetary sciences in 2024.

"Interestingly, we also found that, on a genetic level, the 'acidification' signal was dramatically minimized when shell hash was present." The findings suggest that shell hash can buffer, on a very local level, the impacts of acidification and promote clam growth. In the study, the researchers added crushed shells, pictured here, to sand to better understand how the practice can protect shell-making animals from the negative effects of ocean acidification. (Courtesy Hannah Kempf/UC Davis) Indigenous communities have long added shell hash to sands to keep coastal ecosystems healthy.

The research illuminates the biology underlying why this method is effective. "This was the first time the question was studied from a genetic perspective," said study corresponding author David Gold , an associate professor of earth and planetary sciences at UC Davis. Why does ocean acidification hurt shell-making organisms?

Ocean acidification occurs when seawater absorbs excess carbon dioxide from the atmosphere. As the water becomes more acidic, Kempf said, it becomes more energetically costly and difficult for shell-making animals to build their shells. Acidic seawater can even dissolve shells.

Larvae and juveniles are thought to be especially vulnerable to its negative effects. Raising groups of Pacific littleneck clams in four different conditions, the researchers monitored their growth and the water's chemistry over a 90-day period. (Courtesy Hannah Kempf/UC Davis) To understand why adding shell hash acts a buffer against ocean acidification, the researchers raised baby Pacific littleneck clams in four different conditions: seawater with sediment, acidified seawater with sediment, control seawater with sediment plus shell hash, and acidified seawater with sediment plus shell hash.

Over 90 days, the researchers sampled the seawaters three times a week, monitoring acidity, salinity, temperature and dissolved oxygen. They also tracked the clams' shell growth and after 90 days, they sampled the clams' mantle tissue for follow-up gene sequencing. Not only were the clams raised in shell hash-rich sediments larger on average, but the gene expression profiles of those raised in acidified seawater with shell hash were largely indistinguishable from those raised in non-acidified waters.

"Adding crushed shells removed the negative genetic changes that occur when young clams are raised in acidified water," said Gold. Lessons from Indigenous science Study co-author Tsim Schneider , an associate professor of anthropology at UC Santa Cruz , said there is significant evidence from traditional ecological knowledge that shell hash is beneficial to the health of shell-making animals.

In the Pacific Northwest, he said, some Indigenous communities create large-scale structures that enlarge intertidal spaces, promoting habitats for invertebrates and other marine organisms. "For me, one of the interesting aspects of this research is the potential to engage more fully with tribes on designing projects that promote their long-standing expertise and Indigenous science," said Schneider, who is also affiliated with the National Science Foundation's Center for Braiding Indigenous Knowledges and Science .

"If there's a history of adding shell hash to clam beds in California, for example, western science might be able to better support communities interested in reawakening dormant cultural knowledge and coastal stewardship practices." The research was supported by a California Sea Grant and the Russell J. and Dorothy S.

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