Shellfish pump out greenhouse gases

Clams and marine worms have been found to make potent contributions to greenhouse gas emissions, prompting concerns about the future of shellfish farming.

A team led by Stefano Bonaglia from Stockholm University set out to calculate emissions of methane and nitrous oxides from “macrofauna” living in the Baltic Sea.

In this instance, macrofauna were defined as underwater invertebrates larger than one millimetre long. The methane and nitrous oxides they emit are generated by bacteria living in their gut.

Bonaglia’s team deployed a mix of trace gas, isotope and molecular analyses to measure the gases coming from just two invertebrate groups – bivalve shellfish and marine worms – and found that they accounted for 9.5% of the emissions of the entire Baltic Sea.

The size of the contribution was a surprise. It is roughly equivalent to the emissions produced by 20,000 dairy cows. And while a herd that size accounts for a tiny fraction of the cows in the world, the Baltic bivalves’ output has the researchers worried.

“What is puzzling is that the Baltic Sea makes up only about 0.1% of Earth’s oceans, implying that globally, apparently harmless bivalve animals at the bottom of the world’s oceans may in fact be contributing ridiculous amounts of greenhouse gases to the atmosphere that is unaccounted for,” says co-author Ernest Chi Fru.

The team measured both the direct gas contribution from the invertebrates (their farts, if you like) and their indirect influence. In the latter, the researchers found that Baltic Sea sediments that contained the clams and worms gave off eight times more gas than sediments that did not.

“It sounds funny but small animals in the seafloor may act like cows in a stable, both groups being important contributors of methane due to the bacteria in their gut,” says Bonaglia.

“These small yet very abundant animals may play an important, but so far neglected, role in regulating the emissions of greenhouse gases in the sea.”

The study was published in the journal Nature.

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