Clams and worms at the bottom of the ocean may be releasing “ridiculous amounts of greenhouse gases” that contribute to climate change, according to a new study.

Researchers behind the study, published in the journal Scientific Reports, found that 10% of the methane emissions from the Baltic Sea come from clams and worms. That’s roughly equivalent to 20,000 cows.

Methane is one of the most potent greenhouse gases, roughly 30 times more effective at trapping heat in the atmosphere than carbon dioxide. The study evaluated the Baltic Sea specifically, but if that effect also occurs the globe the impact of ocean clams and worms to global warming would be significant. Study co-author Stefano Bonaglia of Stockholm University said the “small animals in the seafloor may act like cows in a stable.”

The researchers said the findings should receive consideration as some try to expand shellfish farming across the ocean. “The Baltic Sea makes up only about 0.1% of Earth’s oceans,” said study co-author Ernest Chi Fru of Cardiff University’s School of Earth and Ocean Sciences in a press release. “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.”

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Write to Justin Worland at justin.worland@time.com.