Oysters threatened by ocean acidification


Oyster Larvae

Oyster larvae/
Credit: Smithsonian Environmental Research Center

Seems like there’s a lot of bad news out there with regards to the world’s oceans. This time, the bad news is that ocean acidification, caused by climate change, is having serious ramifications in the Pacific Ocean, far sooner than originally predicted by climate change computer models. For the past several years, Pacific oysters (Crassostrea gigas) in Washington State have failed to reproduce. The oysters, native to Japan, have been farmed and harvested in Washington's coastal estuaries since 1920. It’s a $1.1 million industry, producing a full one sixth of the nation’s oysters. But the industry's crashing.

In 2004, the oysters failed to successfully reproduce the first time. Although they spawned normally, the larvae failed to settle and attach to hard surfaces, typically other oyster shells. One bad year was worrisome enough, but then it happened again in 2005. And then 2006, 2007, and 2008. At first, the oysters that failed to reproduce could be supplemented with larvae from hatcheries, but soon the unfortunate situation spread to hatcheries in Oregon and Los Angeles that produce oyster larvae. Hatcheries use ocean water, so it appeared that the same factor affecting the estuarine oyster farms had started affecting the hatcheries. Initially people blamed an explosion of the larvae-killing bacteria Vibrio tubiashii, but they ruled that out after hatchery oyster larvae failed to spawn even with after killing Vibrio with UV light.

After years of uncertainty, scientists think they finally have the real culprit: an increase in carbon dioxide (CO2) has caused ocean acidification, which the oysters don't like. In other words, scientists believe the chemistry of the Pacific Ocean has shifted due to the increase in greenhouse gases. Whitman Miller and colleagues from Smithsonian Environmental Research Center recently published a study in the scientific journal PloS One that states that humans have increased the atmospheric carbon dioxide by 36% in the last 200 years. A full third of that gets absorbed by the ocean and the result has been a reduction in pH; that CO2 makes the oceans more acidic. 

Miller’s study showed increased acidity in ocean water led to different outcomes among two closely related oyster species, the Eastern oyster (C. virginica) and the Suminoe oyster (C. ariakensis), both closely related to the Pacific oyster. Eastern oyster larvae, native to the Western Atlantic, were very sensitive to elevated CO2 (reduced pH, or more acidic water). The larvae grew slower and their tiny shells calcified at lower rates under high CO2 conditions. On the other hand, Suminoe oysters, native to the Western Pacific, displayed no such changes – and were apparently not affected by changes in CO2 levels.

“These findings suggest that biological responses to ocean acidification are species-specific, and therefore more difficult to predict than previously thought,” says Miller. Perhaps they should start farming Suminoe oysters instead?  Not so fast. A proposal to do so was floated for the Chesapeake Bay, but the Army Corps of Engineers, which regulates the introduction of non-native species, recently ruled no-go to an issue that has been debated for over 15 years. Introducing exotic species is an extremely imprecise and unpredictable science, and history shows that more harm than good usually comes.

Coastal estuaries – the ecosystems along the coast which mix oceanic saltwater and freshwater spilling over from creeks and rivers and where the oysters are framed – may get the brunt of the early effects of climate change. Because they’re shallower and naturally less salty and more acidic, they are not buffered against ocean chemistry changes as much as the vast, deep ocean. And besides their ecological importance, estuaries are spawning grounds for many commercially and recreationally important fish and shellfish species. 

The changes in ocean acidity were predicted by scientific computer models pretty much exactly as it’s playing out, but scientists predicted the effects would not be seen for another decade. The decline of the West Pacific oyster industry may be the first of many changes to come. Acidic, corrosive ocean water kills many shellfish and fish larvae, which provide the base of the oceanic food chain. Salmon, pollock, and other commercially important seafood eat shellfish larvae during their early years. And the bacteria Vibrio thrives in acidic waters, and can contribute to the ocean ecosystem getting out of whack.

A new study by The Nature Conservancy, Shellfish Reefs at Risk, shows that 85% of the world’s natural oyster reefs have been lost, but mainly due to dredging and destructive fishing practices. Non-native shellfish have led to declines of native species, and it looks like ocean acidification is just going to make things worse around the world for farmed oysters and native oyster beds.

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