Shellfish at Risk in Acidifying Waters

Ocean acidification is changing the chemistry of seawater in ways that directly affect shellfish. These changes are measurable, well-documented, and already visible in coastal communities and commercial fisheries.

What’s Happening

Shellfish such as oysters, clams, scallops, mussels, crabs, and shrimp rely on calcium carbonate to build their shells and exoskeletons. As the ocean becomes more acidic, the minerals they need become harder to access.

Acidic water reduces the availability of carbonate ions, which are essential for shell formation. When these ions decline, shellfish must use more energy to grow and maintain their shells. In severe conditions, acidic water can dissolve shells faster than young shellfish can build them. Larvae are especially vulnerable, and many fail to develop properly when acidity increases.

Why It’s Happening

Ocean acidification is caused by excess carbon dioxide (CO2) entering the atmosphere from human activities. The ocean absorbs about a quarter of this CO2. When CO2 dissolves in seawater, it forms carbonic acid, which lowers pH and increases acidity.

This chemical shift reduces carbonate ion availability, making it more difficult for shell-building organisms to grow. The ocean is now more acidic than it has been at any point in the last several million years, and the rate of change is faster than many marine species can adapt to.

What the Future Holds

If CO2 emissions continue at current levels, shellfish will face increasing challenges such as the following:

• Slower growth and weaker shells

• High mortality in larvae and juveniles

• Reduced reproductive success

• Greater stress on commercial fisheries that depend on oysters, clams, scallops, and crabs.

• Shifts in marine food webs as shell-building species decline

Some species may adapt over time, but the pace of acidification is rapid compared to natural evolutionary processes.

How We Can Change It

Reducing CO2 emissions is the most direct way to slow ocean acidification. This includes transitioning to renewable energy, improving energy efficiency, and supporting policies that lower carbon output.

Local and regional actions can also help. Restoring oyster reefs, protecting seagrass beds, and reducing nutrient runoff can improve water quality and strengthen coastal ecosystems. Some fisheries and hatcheries are adapting by monitoring water chemistry, adjusting intake practices, and developing more resilient shellfish strains.

While acidifications is a global issue, local improvements can increase survival rates and support healthier marine environments.