Mussels face the heat

Collecting mussels in Njarakal near Kochi. Ocean acidification is threatening the formation of its shells.

MUSSELS, a delicacy in many parts of the world, including south India, are at an increasing risk from climate change. Ocean acidification, caused by additional uptake of atmospheric carbon dioxide by seawaters, makes it difficult for mussels to form their hard calcareous shells that protect them from enemies and adverse environmental conditions.

German scientists led by the ecophysiologist Frank Melzner at the GEOMAR Helmholtz Centre for Ocean Research Kiel found that the bivalve creatures found in tidal regions of coastal zones are very sensitive to ocean acidification, which leads to reduced calcification and shell formation.

During their early life stages—between the first and second day of life—mussel larvae form a calcified shell. But scientists who tracked the process of calcium carbonate deposition in living larvae using specialised equipment found that the mussel larvae failed to form calcium carbonate as previously thought, said Kirti Ramesh, doctoral student of Melzner and the first author of the study published in the journal Nature Communications.

Subsequently, through laboratory experiments, the scientists showed that when the pH value (pH value corresponds to acidic or alkaline nature of liquids such as water, and a lower pH value of a liquid indicates its acidic nature and vice versa) was high, larvae were able to increase calcium carbonate production, leading to higher calcification. With increasing ocean acidification, the pH values below the shell decrease, which leads to reduced calcification rates and, at very high carbon dioxide concentrations, shell dissolution and increased mortality occur, according to Melzner.

The scientists said the results established that there was a direct relationship between the calcification rate of mussels and the carbonate chemistry of seawater.

As the next step, the scientists plan to identify proteins that play a role in the transport of calcium and carbonate, and which organic substances in the shell increase resistance to shell dissolution. According to them, there are some mussel populations in the Baltic Sea which are more tolerant to ocean acidification, and studying them will help identify organic shell constituents that resist dissolution. Understanding them will probably help marine scientists to breed mussels which can withstand ocean acidification.

T.V. Jayan