For 90 days at UC Davis’s Bodega Marine Laboratory in California, baby Pacific littleneck clams grew in sand mixed with crushed shells—and the added shell material helped protect them from acidified water.
The experiment offers a close look at a long-standing Indigenous practice: adding crushed shells, sometimes called shell hash, to coastal sands. Ocean acidification happens when seawater absorbs excess carbon dioxide from the air. More acidic water makes it harder and more costly for animals such as clams, oysters and mussels to build their shells, and can even dissolve them. Young animals are thought to be especially at risk.
Researchers raised baby clams under four conditions: regular seawater with sediment; acidified seawater with sediment; regular seawater with sediment and shell hash; and acidified seawater with sediment and shell hash. They sampled the water three times a week, checking acidity, salinity, temperature and dissolved oxygen. The team also measured shell growth and, after 90 days, examined the clams’ mantle tissue, which is involved in shell production, to study gene activity.
Shell hash changed the water chemistry in the tiny spaces between grains of sand. It lowered the water’s acidity, creating conditions better suited to shell growth. On average, clams raised in sediment with crushed shells grew larger shells than those without the added material.
The genetic results were striking, too. In acidified water, clams raised with shell hash showed gene activity largely indistinguishable from clams raised in nonacidified water. The crushed shells appeared to prevent many of the genetic changes linked to acidification. “Adding crushed shells removed the negative genetic changes that occur when young clams are raised in acidified water,” said David Gold, the study’s corresponding author and an associate professor of earth and planetary sciences at UC Davis.
The findings, published in the journal Biogeosciences, suggest shell hash can soften acidification’s effects on a very local scale. They also help explain the biology behind a practice Indigenous communities have used for generations to support coastal ecosystems. The study’s first author, Hannah Kempf, an environmental scientist at the California State Water Board who earned her Ph.D. at UC Davis in 2024, said the clams grew larger when shell hash was present and that the acidification signal in their genes was “dramatically minimized.”
Co-author Tsim Schneider, an associate professor of anthropology at UC Santa Cruz, pointed to Indigenous knowledge of shell hash’s benefits. In the Pacific Northwest, some Indigenous communities build large structures that expand intertidal areas and create habitat for invertebrates and other marine life. Schneider said the research could help scientists work more closely with tribes on projects grounded in Indigenous expertise.
For communities interested in restoring coastal stewardship practices, the study offers evidence that traditional knowledge can also open useful questions for laboratory research. The next step, Schneider suggested, is to engage tribes in shaping projects that support their long-standing expertise and help bring dormant cultural knowledge back into practice.