An Ancient Indigenous Practice Could Help Marine Life Survive a More Acidic Ocean

Indigenous communities have been doing it for thousands of years.

For thousands of years, Indigenous communities along North America’s Pacific coast have practiced a remarkably simple form of marine stewardship: they crush old seashells and mix them into the sand where new clams grow.

It’s a practice rooted in generations of observation and experience. By tending these coastal habitats, communities helped sustain productive clam beds that provided food year after year.

Now, scientists have discovered that this ancient practice may offer a surprising defense against one of the ocean’s most pressing modern threats: acidification.

The ocean has a chemistry problem

The ocean absorbs roughly a quarter of the carbon dioxide released by human activities each year. In a way, this is good, because it slows down atmospheric warming.

But there’s a cost.

When carbon dioxide dissolves in seawater, it triggers reactions that make the water more acidic. In turn, this reduces the availability of carbonate ions. These ions are essential building blocks for animals that construct shells and skeletons from calcium carbonate.

If the water becomes too acidic, seawater can begin dissolving these protective structures. But even before that happens, building and maintaining shells becomes more difficult and energetically demanding. That leaves less energy available for growth and other essential functions. Young shellfish are particularly vulnerable because their shells are still developing.

Since the Industrial Revolution, the acidity of surface ocean waters has increased by approximately 30%, and it’s getting worse every year.

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Yet the chemistry these animals experience isn’t determined solely by the seawater above them. For clams buried beneath the sand, the water trapped between sediment grains can be even more important.

And that’s where crushed shells come in.

An ancient approach to healthier clam beds

Many Indigenous communities along the Pacific Northwest coast have maintained what are known as clam gardens.

These are deliberately managed coastal habitats that can include carefully constructed rock walls, modified beach slopes, and the addition of broken shells to sediment. Together, these practices create productive environments for harvesting shellfish. Some clam gardens have been maintained for thousands of years.

Archaeological evidence from California’s Bodega Bay region documents clam-bed management by the Coast Miwok.

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But these traditions were disrupted.

In the 1930s, federally imposed conservation restrictions brought traditional clam gardening in the region to an end. As the researchers note, the policies were based partly on the assumption that harvesting depleted clam populations, overlooking the role Indigenous management played in maintaining them.

Previous studies have found that clam gardens can support more abundant shellfish populations and greater biodiversity.

Now, researchers at the University of California, Davis, wanted to investigate one small but potentially important piece of this larger tradition.

Could crushed shells actually change the chemistry surrounding young clams, making them more resilient to acidic conditions?

Putting an ancient practice to the test

The team raised juvenile Pacific littleneck clams (Leukoma staminea) in laboratory containers for 90 days.

They created four experimental conditions: normal seawater with sand, normal seawater with crushed shells, acidified seawater with sand, and acidified seawater with crushed shells.

The shell material, known as shell hash, consisted of fragments measuring approximately 2.5 to 5 millimeters across. It made up 10% of the sediment mixture by weight in the treated containers.

The researchers monitored the chemistry of the water trapped in the sediment, measured the clams’ growth, and analyzed gene activity in their mantle tissue — the part of the animal responsible for producing its shell.

The first finding was straightforward. Crushed shells made the water between sediment grains less acidic.

Adding crushed shells consistently increased the pH of water within the sediment, making conditions less acidic. It also improved the water’s saturation state for aragonite, the calcium carbonate mineral that forms the clams’ shells.

The clams appeared to benefit, too.

Under acidified conditions, clams without shell hash recorded average new shell growth of 4.7%, compared with 6.4% for those living among crushed shells.

The genetic results, however, revealed something even more intriguing.

The surprise was in their genes

When researchers examined gene expression (the process by which cells activate or suppress genetic instructions) they found that clams raised in acidic water without crushed shells were distinctly different from the other groups.

Across the experiment, the researchers identified 284 genes whose expression differed substantially between experimental conditions.

The clams exposed to acidified seawater without shell hash had a distinctive genetic profile. But when crushed shells were added to the same acidic conditions, that pattern largely disappeared. Instead, their gene-expression profiles closely resembled those of animals raised in ordinary seawater.

Instead, their gene-expression profiles closely resembled those of clams raised without experimental acidification. In other words, the crushed shells appeared to soften the biological response to acidic seawater, not just change the surrounding water chemistry.

Scientists are still unsure what many of these genes do. Some identified sequences resembled genes involved in DNA repair, while others may be associated with shell formation or environmental tolerance.

Nevertheless, the molecular evidence provides a new clue about why shell-rich sediments might benefit developing clams.

Can crushed shells help save coastal ecosystems?

The discovery doesn’t mean sprinkling crushed shells across beaches will solve ocean acidification. That would require addressing the carbon emissions driving the problem in the first place.

The researchers suggest that shell hash may be most useful for creating localized refuges (small areas where shellfish can grow under more favorable conditions) rather than treating ocean acidification across entire coastlines.

There are still important unanswered questions. This was a controlled laboratory experiment, and real beaches are much more complicated. Tides, sediment composition, microbial activity, and differences between shellfish species could all influence how well the technique works. Previous field studies have produced mixed results.

Any practical application would also need to account for local ecosystems and be developed in partnership with Indigenous communities whose expertise helped inspire the research.

But perhaps, that’s the most valuable lesson here.

As the ocean changes faster than many marine organisms can adapt, scientists are searching for increasingly sophisticated ways to protect vulnerable ecosystems. But some promising approaches may already exist in the accumulated knowledge of communities that have managed those ecosystems for millennia.

The science is beginning to explain something Indigenous coastal stewards have long understood: healthy clam beds don’t necessarily thrive when humans leave them alone. Sometimes, they thrive because people take care of them.

And sometimes, something as simple as a handful of broken shells can make a difference.

The study was published in Biogeosciences, 23, 7627–7642.