Stress caused by hurricane rainfall overwhelms sea anemones

Two Virginia high school students have discovered how sea anemones respond to salinity changes caused by hurricane rainfall. As hurricanes worsen due to climate change, this important information could help conservation efforts and inspire advocacy efforts. The research was shared in a paper published in Ocean-Land-Atmosphere Research on October 7.

“In coastal zones, tropical cyclones can cause significant damage to crucial marine ecosystems. Coral reefs, which house over 25% of marine species, are some of the most biodiverse ecosystems on the planet and are essential to several ecological processes. We sought to understand how coral reefs near hurricane impact zones are affected physiologically by heavier rainfall,” said Nishka Shah, a student at Thomas Jefferson High School for Science and Technology in Alexandria, VA.

As ocean temperatures continue to rise, there will be more intense hurricanes that produce significant rainfall in coastal regions. When freshwater from rainfall and runoff enters the ocean, it can significantly reduce the salinity of the seawater and cause mild to severe hyposalinity. Hyposalinity causes osmotic stress, which leads to bleaching and even death for entire coral reef systems. Understanding how coral reefs are affected by hyposalinity stress is essential for developing conservation strategies, and not all of the effects of hyposalinity fluctuations caused by hurricanes have been fully understood.

“Hurricanes bring not only direct consequences like heavy winds but also several indirect consequences including heavy rainfall. Often, the latter group of consequences are considered more ‘indirect’ and are overlooked; however, they can have a large impact on coral reefs, spread over a wider area than just the body of the hurricane and often persisting long before and after the brunt of the storm has passed,” said Shah.

Researchers sought to understand how salinity fluctuations affect coral ecosystems and their thermotolerance by studying the sea anemone Exaiptasia diaphana. They used 50 E. diaphana and randomly assigned them to 5 different groups with varying levels of salinity fluctuations. The groups were cycled through these fluctuations for 5 weeks and then one anemone from each group was randomly selected for extensive testing. The rest were tested for Symbiodiniaceae, an algal symbiote that is important for maintaining photosynthetic efficiency. Reduced amounts of Symbiodinaceae can cause bleaching and starvation of anemones. After these initial tests, half of the anemones were then put through heat stress tests. The growth and health of the anemones was recorded at regular intervals throughout the experiment.

The results showed that there were significant differences in the health of the anemones between the groups, depending on the severity of the hyposalinity they were exposed to. There were significant fluctuations in the amount of Symbiodinaceae between the mild and severe fluctuation groups. Growth differences were also observed, with the least amount of growth found in the severe and moderate fluctuation groups. The highest mortality and worst health rates were also found in the severe fluctuation groups.

To a certain point, exposure to hyposalinity fluctuations activated protective mechanisms that maintained and even improved the health of the anemones. However, once a specific salinity threshold was breached at 20 parts per thousand, these protective mechanism adaptations became overwhelmed. Higher Hsp70 expression was an indicator of thermotolerance between the mild and moderate groups and seemed to indicate an improved stress response. These findings indicate that E. diaphana can adapt to moderate fluctuations in salinity brought on by rainfall, but when the fluctuations are severe and more frequent and the hyposalinity threshold is reached, the anemone can no longer adapt to the osmotic stress.

Looking ahead, researchers would like to complete real-world studies to confirm the conclusions found in the lab. “Eventually, we would like to apply this research to develop protective mechanisms for corals from more under looked consequences of hurricanes and motivate more scientific inquiry to explore other effects of hurricanes, such as acidification or wind, by looking at the synergistic and cumulative effects of these factors on corals. Having a more holistic and comprehensive understanding of the interactions between reefs and the environment can result in more informed and effective conservation efforts,” said Shah.