Floods reduce salt content in seawater and induce stress in bivalves – ScienceDaily

The world's oceans generally have a salt content (salinity) of about 3.5%, but this percentage varies more in shallow coastal waters affected by precipitation. The researchers studied three groups of Olympia oysters from different areas of the California coast where the influence of rainfall on seawater salinity varies. One group was from a large estuary – a mass of seawater near the mouth of a river – regularly exposed to freshwater floods caused by extreme rainfall , which reduced the salinity of the oyster surroundings. A second group lived in a small estuary less exposed to freshwater and a third group lived far away from the greater estuary where salinity was also higher and more stable.

All organisms, including oysters, exhibit higher expression of genes involved in DNA damage and protein development in response to extreme stress. Protein deployment is a process in which proteins lose their structure and become unstable. If they are not repaired, they eventually result in the death of the animal. The researchers are studying Olympia oysters as they constitute a "fundamental species", which means that the presence of oysters is a habitat for many other smaller species and creates a much healthier ecosystem. If oysters die, all associated species will die too. Because of oysters' vital role in coastal ecosystems, researchers want to know if oysters living in certain areas are more tolerant of low salinity and are better equipped to withstand climate change.

The research team exposed the three groups of oysters and their offspring to low salt seawater (approximately 0.5% salt) and measured their pattern of expression. gene. They found that oysters living closest to the greater estuary were more tolerant of a five-day exposure to low-salt seawater. "More frequent exposure to freshwater in this region has probably forced oysters to develop new methods of survival under low-salinity conditions," said Tyler Evans, Ph.D. State University of California at East Bay, and first author of the study.

This group has expressed much higher levels of mRNA – genetic material that tells the cells the new proteins to make – than less tolerant oysters accustomed to higher salinity. Proteins encoded by mRNA control the activity of oyster eyelashes (hair-like structures on the gills that come and go to circulate the fluid inside the shell of the oyster). Researchers predict that this extra movement of eyelashes increases survival by allowing oysters to keep their shells closed (and the salt-free seawater) for longer. However, climate change is a concern for the very survival of the Olympia oyster group the most tolerant due to the expected increase in the severity of extreme rainfall events that would expose oysters to even longer periods low salinity. "Even oysters having gained greater tolerance to low salinity through natural selection will be vulnerable to future flooding in freshwater," the research team wrote.

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