THE world's mussel population could be under threat as climate change causes oceans to become increasingly acidic, it is being warned.

Scientists say molluscs' shells become more brittle - and thus more vulnerable - when they are formed in acidic water where the environment makes it harder for them to make crystals of calcium carbonate to form their two-layer outer shell.

The world's oceans are becoming increasingly acidic as they absorb some of the atmospheric carbon dioxide which contributes to climate change and Glasgow University scientists said there research on common blue mussels suggested it was having an impact on the species.

Dr Susan Fitzer, of the university's School of Geographical and Earth Sciences, said: "What we've found in the lab is that increased levels of acidification in their habitats have a negative impact on mussels' ability to create their shells."

The paper in the Royal Society's journal Interface detailed how water reacts with the carbon dioxide to form carbonic acid, which is gradually lowering the pH (the scale which measures how acidic or alkaline a liquid is) of the oceans. The scientists expect the pH of the world's oceans to have dropped from 8 today to 7.7 with increasing acidity, by the end of the 21st century.

The research team housed common blue mussels in laboratory tanks, controlling and altering the temperature and pH levels of the water in the tanks to simulate four different types of ocean waters at levels of acidity projected to occur in the coming decades.

Ocean conditions were also simulated as closely as possible by changing light levels over time to mimic the changing of the seasons.

Mussels' shells are composites of calcium carbonate and organic material created by the mussels through a process known as biomineralisation.

Mussels draw bicarbonate ions from seawater and use proteins in their bodies to make crystals of calcium carbonate to form their two-layer shells. The shell's outer later is composed of calcite and the inner layer is made of aragonite. In more acidic water, there are less bicarbonate ions available for the mussels to make their shells.

Dr Fitzer added: "We worked with colleagues in our School of Engineering to examine the toughness of the shells of the mussels in the more acidic water against those in control conditions.

"What we found was that the calcite outer shells of the mussels past a certain threshold of acidity was stiffer and harder, making it more brittle and prone to fracture under pressure, and the aragonite inner shell became softer."

He said this could mean that mussels growing in the wild in the future could be more vulnerable to attack from predators, as well as from the effect of ocean forces. As blue mussels are commonly used for human consumption, it could also have an effect on the yields of mussels available for the fishing industry.

He added: "However, we also found that the effect on the mussels' shells was reduced when the temperature of the water was increased by 2C. This might suggest that the mussels are reverting to ancestral evolutionary mechanisms to mitigate the effects of increased acidity."

The Glasgow scientists are planning to continue their research in this area and expand its scope to look at the effects of more acidic water on the shells of other marine organisms including oysters and abalone.

The shellfish industry is worth more than £250 million a year to the UK economy with a large part accounted for by mussels and oysters, the UK Department for Environment, Food and Rural Affairs reported in 2012.

Mussels contribute around £7 million to the Scottish economy alone, with other shellfish accounting for £1.4 million, according to the Scottish Government's marine atlas, published in 2011.