Ocean Acidification and Shellfish

The coastal waters of the Pacific Northwest are currently undergoing a chemical change that threatens the survival of one of the region’s most iconic species. Recent research confirms that falling pH levels are actively dissolving the shells of young Dungeness crabs. This is not a future projection; it is an observable reality happening right now along the coasts of Oregon, Washington, and British Columbia.

The Chemistry Behind the Crisis

Ocean acidification is often called the “other carbon problem.” While much of the conversation about carbon dioxide (CO2) focuses on the atmosphere, the ocean absorbs about 30% of the CO2 released by human activity. When CO2 mixes with seawater, it undergoes a chemical reaction that forms carbonic acid. This process lowers the pH of the water, making it more acidic.

For shellfish, this chemistry is a matter of life and death. Crabs, oysters, clams, and mussels rely on carbonate ions available in the water to build their calcium carbonate shells. As acidity rises, these carbonate ions become scarce.

In severe cases, the water becomes corrosive enough to dissolve existing shells. This is known as the saturation state. When the saturation state of aragonite (a form of calcium carbonate) drops too low, the water actively eats away at the mineral structures these animals work so hard to build.

Dungeness Crabs Under Siege

The Dungeness crab is the most valuable fishery on the U.S. West Coast, with an annual value often exceeding $200 million. However, a NOAA-led study published in the journal Science of the Total Environment revealed startling evidence regarding the health of these crabs.

Researchers examined larval Dungeness crabs, known as megalopae, collected from the wild off the coasts of Washington, Oregon, and British Columbia. The findings were concerning:

• Shell Dissolution: The study found that the carapaces (upper shells) of these larvae showed clear signs of corrosion. Under scanning electron microscopes, the shells appeared pitted and folded rather than smooth and rigid.
• Sensory Loss: The damage extends beyond the shell. The acidity damaged the tiny, hair-like structures on the crabs’ shells called setae. These neurally connected hairs act as the crab’s primary sensory organs.
• Survival Implications: A crab with damaged setae is effectively blind to its environment. It may struggle to navigate, find food, or detect predators. Additionally, crabs that spend too much energy repairing their corroding shells grow more slowly, making them vulnerable for longer periods.

Why the Pacific Northwest?

While ocean acidification is a global issue, the Pacific Northwest is a unique hotspot due to a phenomenon called coastal upwelling.

In the spring and summer, strong winds push surface water away from the coast. To replace it, deep, cold water rises from the bottom of the ocean to the surface. This deep water is naturally older and richer in CO2 from the respiration of marine life over decades. When this naturally acidic deep water combines with the additional CO2 absorbed from the atmosphere, it creates a “double punch” of acidity.

This upwelling season coincides exactly with the time when many shellfish, including Dungeness crabs, are in their vulnerable larval stages.

Broader Impacts on the Ecosystem

The dissolving shells of Dungeness crabs are a warning sign for the wider food web. The same acidic conditions affecting crabs are also devastating pteropods, often called “sea butterflies.” These tiny swimming snails are a primary food source for pink salmon, mackerel, and herring.

In the mid-2000s, the Whiskey Creek Shellfish Hatchery in Oregon experienced a near-total collapse of oyster larvae production. They discovered the intake water was too acidic for the larvae to form their first shells. Since then, hatcheries have had to install sophisticated monitoring systems and buffer the water with soda ash just to allow the oysters to survive their first few days of life.

Unlike hatcheries, however, the wild Dungeness crab population cannot be artificially buffered. They are fully exposed to the changing chemistry of the open ocean.

Research and Future Outlook

Institutions like the Pacific Northwest National Laboratory (PNNL) and the Southern California Coastal Water Research Project are currently working to map these acidity hotspots. The goal is to help fishery managers understand which areas are most dangerous for shellfish stocks.

Current data suggests that by 2050, roughly 70% of the Dungeness crab population in the coastal Pacific Northwest could be significantly affected by shell dissolution if carbon emissions continue at current rates. The focus of current science is now on adaptation and understanding if these species have any genetic resilience that can be encouraged.

Frequently Asked Questions

What is ocean acidification? Ocean acidification is the reduction in the pH of the ocean over an extended period, caused primarily by the uptake of carbon dioxide (CO2) from the atmosphere.

Why are Dungeness crabs specifically vulnerable? Dungeness crabs use calcium carbonate to build their shells. Their larval stage (megalopae) is particularly sensitive. If the water lacks enough carbonate ions, or is corrosive, the larvae cannot build strong shells and suffer damage to their sensory organs.

Does this affect the safety of eating crabs? Currently, there is no evidence that ocean acidification makes crab meat unsafe for human consumption. The issue is strictly regarding the survival rate and population numbers of the crabs, which impacts the supply and price of seafood.

Are other shellfish affected? Yes. Oysters, clams, mussels, and scallops all rely on calcium carbonate to build shells. The Pacific Northwest oyster industry has already faced significant financial losses due to larval die-offs linked to acidic water.

What is being done to fix this? On a local level, shellfish hatcheries treat their water to protect larvae. On a global level, the only way to halt the progression of acidification is to reduce the amount of CO2 entering the atmosphere. Researchers are also studying whether aquatic vegetation, like kelp forests, can help absorb local CO2 and create “refuge” zones for shellfish.