Mussels, those clam-like, stationary, palm-sized creatures found in waters around the world, have for centuries been an important source of seafood for humans and a good source of vitamin B12 and omega-3 fatty acids.
Aside from providing habitats for organisms that attach to them, mussels filter considerable amounts of water while feeding, and, in the process, clean pollutants from the waters they live in. Without them, water quality would degrade and various other species of plants and animals would be at risk.
Because of their crucial role in marine ecosystems, biologists have been keen to study them, especially as our planet warms.
“[They] displayed high tolerance and capacity to recover from exposure to elevated temperatures through adjustments of physiological processes that enabled individuals to withstand energetically costly conditions,” the team of researchers from the Chinese University of Hong Kong (CUHK) wrote in an article published in the peer-reviewed journal Marine Biology in August.
Asian green mussels are abundant and widely distributed across the Indo-Pacific, a region projected to see the biggest jump in marine heatwaves over the coming decades compared to other latitudes, according to the study.
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Lead author of the study, Laura Falkenberg, a marine biologist and an adjunct assistant professor at the CUHK school of life sciences, said their experiments showed that mussels in Hong Kong might be able to cope with marine heatwaves.
“But in addition to heatwaves, they will also be dealing with a bunch of other changed conditions [such as] gradual warming, ocean acidification as well as any other local pollutants. That could modify their responses and [also] make them more susceptible to heatwaves,” said Falkenberg, who is also a lecturer at the University of South Australia.
After the mussels were placed in tanks, the researchers mimicked a heatwave by increasing the temperature of the tanks by around 3 degrees Celsius from the mean summer temperature to 30 degrees Celsius, warmer than the mussels would normally have experienced.
After a three-week artificial heatwave, the team attached sensors to the mussel shells to detect the electromagnetic waves produced by their heartbeats.
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They found that the mussels’ heart rates increased, but among the larger ones, about 5cm (2 inches) in length, there was also a significant increase in the clearance rate – the rate at which a certain volume of water is cleared of all particles.
The mussels then underwent a week of recovery at normal temperature. Their physiological traits, including heart rates, temperatures and clearance rates, returned to normal levels, indicating that their long-term functions were not affected.
Since mussels have limited strategies to regulate their core body temperatures, adjusting their heart rates and clearance rates may be their primary responses, according to the researchers.
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“They were able to survive the heatwave,” Falkenberg said.
But some coastal animals, such as oysters, marine snails and limpets, might not be able to recover from stress events, she said.
“Sometimes what we see in these kinds of experiments is that once they have survived a heatwave or a stress event, they then can show some kind of delayed effects and die during that recovery period, which we did not see here,” Falkenberg said.
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Falkenberg said future research could focus on mussels elsewhere in the world, and how other species might respond to such heatwaves, as well as what might happen if the characteristics of the heatwave changed.
“Heatwaves are going to become longer, hotter and more frequent in the future,” she said.
“If we were to use different heatwave characteristics, will the mussels still be able to persist through those, or is there some threshold that they just cannot survive past?”