As we reckon with the effects of climate change, so too must the other organisms that call Earth home. But what if you couldn’t move away from your dwelling to escape a threat? What if your shelter, your refuge, was a part of your body?    

Shellfish face this plight due to the excess carbon dioxide pumped into the atmosphere, which ultimately ends up in our oceans, changing the water chemistry and leading to ocean acidification.

“As the water becomes more acidic, it makes it more difficult for things that build mineralized shells and skeletons to do that process,” said Hannah Kempf, a doctoral candidate in the UC Davis Department of Earth and Planetary Sciences who’s studying the wide-ranging effects of climate change on shellfish.

NOAA estimates that in the Pacific Northwest alone the shellfish industry injects roughly $270 million annually into the region’s economy. In addition to being economically important, shellfish are like ecological stewards, filtering water to feed, which in turn removes excess nutrients from the water column and creates valuable habitats.  

But as oceans acidify, shellfish are struggling. The answer to this problem may lie in unifying modern scientific techniques with Indigenous shellfish management practices.  

Supported by an $80,000 California Sea Grant Graduate Research Fellowship, Kempf is doing just that. She’s documenting how California’s Pacific littleneck clam (Leukoma staminea) populations respond to ocean acidification and is exploring the science behind how Indigenous shellfish management practices promote healthy clam populations.  

“I want to identify shellfish species that are resilient in the face of climate change, and figure out how to have a sustainable source of aquaculture in the future,” said Kempf, who works in the labs of Associate Professor David Gold and Professor Emerita Sandra Carlson, both of the Department of Earth and Planetary Sciences.   

The role of ocean acidity in shell strength  

What’s in a shell? Calcium carbonate is the main ingredient for starters. Add a smidgen of protein and you’ve got the general makeup. As a mollusk grows, so too must its shell. But what does this process look like?   

A Scientific American explainer on the shell-building process states “mantle tissue that is located under and in contact with the shell secretes proteins and mineral extracellularly to form the shell. Think of laying down steel (protein) and pouring concrete (mineral) over it.” Increased ocean acidity hinders this process by slowing the growth of calcium carbonate structures and dissolving those structures faster than they can form, according to the United States Environmental Protection Agency.  

Using the Pacific littleneck clam as a model species, Kempf is investigating these processes by analyzing how ocean acidification changes shells at the structural and genetic levels. 

“An important part of this experiment, getting into that adaptation piece, is trying to figure out how we can locally alter their habitat to protect them against these acidic conditions,” Kempf said. 

To do this, Kempf is harnessing the knowledge of Indigenous frameworks for habitat restoration. Specifically, she’s studying the practice of adding shell hash (broken shell pieces) back into beach sediments, a practice that dates back millennia.    

“The idea is if you add the pieces of broken shell material into the sand under highly acidic conditions, those broken pieces of the shell will break down and release molecules that can buffer ocean acidification,” Kempf said. “It actually alters the seawater at a very local scale.” 

A traditional solution to a modern problem 

When Kempf arrived on the UC Davis campus for graduate school, she did not expect her doctoral journey would lead to her current research. Initially, she wanted to focus her graduate work on paleobiology, with the intent of studying ancient marine ecosystems. 

“As I was learning more and reading more and more papers, I decided early on that I wanted to shift my research to do something that felt more directly applied to climate change and coastal ecosystems,” Kempf said. “I want to do science that affects society in a positive way.”

A fellow graduate student introduced her to research about Indigenous sea gardening practices from the Pacific Northwest, including the practice of crushing shells from harvested clams and adding them back into the sand.  

“Indigenous peoples have been adding shell hash back into actively managed clam beds for millennia,” Kempf said. “Shell hash also physically changes the nature of the beach and can make it easier to dig into. Baby clams will sometimes settle more in an area where there’s more shell material.”   

Already, Kempf is unraveling the benefits of adding shell hash to beach sediment. Last year, she presented preliminary findings at the Geological Society of America’s Connects meeting. In a lab, Kempf grew juvenile Pacific littleneck clams in acidified seawater and control seawater for 90 days.  

It turned out that adding “shell hash increased the pH and alkalinity of the pore fluids” for both the acidified and control seawaters, according to the Geological Society of America. Added shell hash thus altered the chemistry of the pore fluids, helping to buffer against acidic conditions. 

This summer, Kempf and new graduate student Liyu Mekonnen are continuing that research thread by analyzing the genetic data collected during Kempf’s lab experiment.   

“This will tell us what genes were turning on and off as clams were exposed to acidification, as well as shell hash,” Kempf said “This can give us a better understanding of what biological processes are affected by acidification, and whether shell hash mitigates any negative effects.” 

Learn more about Hannah Kempf research on her website.  

Photos courtesy of Hannah Kempf.