The freshwater pearl mussel (Margaritifera margaritifera) is best known for producing valuable pearls and is a long-lived species, living up to 250 years. These bivalves have complex life stages; they attach themselves to the gills of fish when they are young before settling in rivers. Although they once spanned a large portion of Europe, their range has reduced significantly over the past century as a result of several factors such as climate change causing increasing temperatures, habitat destruction and pollution, putting their populations at risk. Warmer water can be dangerous to these mussels, as extreme temperatures may push them past their limits and restrict their range of suitable habitats. However, little is known about how these higher temperatures impact these shellfish, although this knowledge could help us to protect their populations. Sebastian Wacker and colleagues at the Norwegian Institute for Nature Research and University of Bergen, Norway, sought to address this question by measuring their survival and growth over a range of temperatures to determine the conditions that the species can withstand.
The team collected young mussels in the lab after they had been released from brown trout gills and gently warmed the youngsters to high summer temperatures: one group to 23°C (a typical hot day in River Etna, Norway, where the mussels originated) and a second group to 26°C (an exceptionally hot day in the river), keeping the mussels in these conditions for 28 days. Then, they transferred molluscs from each group either to cooler water (18°C) or an extreme heatwave (29°C) for a further 12 days. The researchers then measured the mussel's survival and growth to determine how the high temperatures impacted them.
It turned out that the mussels were quite sturdy and endured the highest temperatures without any decline in survival. The scientists also found that there was no difference between the survival of mussels that were warmed up to 29°C for 12 days and that of muscles that were kept cool at 18°C during the entire 40 day period, suggesting that these mussels can at least tolerate short periods of extreme heat. The mussels also grew significantly faster when living in warmer water. These findings are significant and important for conservation and future research, because they highlight the resilience of mussels to temporary events such as heat waves, which should be investigated further.
However, the researchers explain that this was a short-term study, so the long-term consequences for mussels of living at higher temperatures as the molluscs grow older remains unknown. Adjusting to higher temperatures often costs energy, which can change how efficiently the mussels reproduce or find food, resulting in increased mortality. However, faster growth may also mean that you can defend yourself better against predators, which would be an advantage.
This study lays the foundation for understanding how mussels are affected by higher temperatures and addresses questions about long-term growth and survival that future studies can investigate. Climate change is expected to increase in intensity in the coming years, which means that short-term heat waves, and gradual increases in water temperature are expected to occur worldwide. By understanding both short- and long-term effects of warmer temperatures, conservation management can implement better policies to protect this endangered species.
