Oysters respond to stress by growing less meat on their shells

Oysters that are exposed to heat and low oxygen conditions early in life are more vulnerable to the same stressors later on, according to a new study from the Smithsonian Environmental Research Center (SERC).

The experts found that in response to repeated  environmental stress, oysters invest more energy in growing their shells than in growing their tissues. As a result, they have larger shells that are less meaty.

“What we all of course want to eat at the raw bar is the oyster tissue,” said study lead author Sarah Donelan. “Customers and restaurants might be less pleased if there’s less tissue in what looks to be a large oyster.”

In the Chesapeake Bay, oysters live mostly in shallow tributaries where oxygen levels can swing drastically during the hot summer months. The researchers explained that these oxygen levels can shift from perfectly healthy in the daytime to near zero at night. To save energy, some oysters react by focusing more on their shell growth. 

Low oxygen was found to have the strongest negative impact on growth. The experts also discovered that early exposure to low oxygen and heat stress led to a drastically sharper decline in tissue growth when the oysters were exposed to these conditions again later in life. 

“Low oxygen and warming waters are a real double whammy for marine organisms,” said SERC senior scientist Denise Breitburg. “Warmer water holds less oxygen and causes oxygen to decline faster. At the same time, cold-blooded animals like oysters and finfish require more oxygen at warmer temperatures.”

The researchers designed an experiment to investigate whether exposure to threats could shape oysters later in life. Nightly swings of low oxygen were used to put pressure on the shellfish.

“If it’s always bad, they can evolve over time to cope with those poor conditions,” said Donelan. “But especially for (immobile) organisms like oysters, these fluctuations can be very stressful.”

The study was focused on 3,600 young oysters around the age of three months old. The animals were exposed to four possible scenarios, including warmer water, swings of low oxygen, both conditions, or neither condition. 

When the oysters were given a break after 18 days, they were all about the same size regardless of their environments. At this point, there were no significant differences in shell or tissue size among the individuals. 

Two months later, when half of the oysters were returned to the experimental tanks, some effects started to show that may have been dormant.

The oysters that had suffered from both low oxygen and hotter water in the first phase of the experiment started showing signs of strain. Oysters that were facing the challenging conditions for the second time grew their shells more than their tissue. They had half the tissue-versus-shell growth ratio that was observed in oysters who escaped the early double exposure.

The findings raise a new question: why would the oysters not be toughened up by early exposure? Donelan suspects the combination of warming and low oxygen leaves a scar that does not easily heal. “I think that there’s likely a physiological change that’s irreversible.”

The key, said Donelan, is to protect oysters while they are still young. Oysters that were not exposed to the warm water and low oxygen conditions early in life fared much better when they faced the same stressors later on.

The study is published in the journal Ecological Applications.

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By Chrissy Sexton, Earth.com Staff Writer