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Professor Robert Day, Zoology Department, the University of Melbourne, Parkville, Victoria, Australia. Tel: +61 3 83446262, email:

Dr Nerissa Hannink, Media Office, University of Melbourne. Tel: +61 3 8344 8151, mobile +61 430 588 055, email

An international study has predicted that the size and weight of shellfish and other marine animals will decline due to ocean acidity largely caused by burning of fossil fuels.

Ocean acidity makes it harder for marine animals to make shells and skeletons, so increased acidity is likely to impact these animals, reducing the food source for tropical seabirds, seals, large aquatic animals and humans.

The research was led by scientists from the British Antarctic Survey (BAS) and the National Oceanography Centre (NOC), with colleagues from the University of Melbourne, James Cook University, and the National University of Singapore and is published today [Monday 6 August] in the journal Global Change Biology. 

Ocean acidity currently varies naturally across the globe, but acidity is becoming more severe in most areas due to the increased carbon dioxide that humans are adding to the atmosphere said Professor Robert Day from the Department of Zoology at the University of Melbourne.

“The excess carbon dioxide dissolves in the ocean and reacts with water to produce an acid. This makes it harder for shellfish to extract calcium carbonate from the water- which shellfish use to make their shells- and therefore reduces their ability to build a skeleton as they grow,” Professor Day said.

The team looked at the natural variation in shell thickness and skeletal size in clams, sea snails, lampshells and sea urchins living in 12 different environments from the tropics to the Polar Regions, to predict how they might respond to increasing human-induced ocean acidity.

“We found that as the availability of calcium carbonate decreases, the skeletons of all these animals get lighter and accounts for a smaller part of the animal’s weight. The fact that the same effect occurs consistently in all four major types of marine animals suggests the effect is widespread across marine species, and that as human-induced ocean acidification increases, it will progressively reduce the rate at which they can grow skeletons.

“This effect is strongest at low temperatures and the results showed polar species to have the smallest and lightest skeleton, suggesting that they may be more at risk in the coming decades as the oceans change.”

“To safeguard shellfish populations, and therefore species higher up the food chain, we would need to slow the burning of fossil fuels and hope that evolution allows species to adapt their shells to the more acidic oceans.”