The animals, also including worms and insect larvae, emitted nitrous oxide -- commonly known as laughing gas -- as a by-product of their digestion when nitrate was present in water. Nitrate is often used in fertilisers, whose use is rising.

"Aquatic animals have never (before) been shown to emit this greenhouse gas," the German and Danish experts wrote in the US journal Proceedings of the National Academy of Sciences, released on Monday.

They said that past studies have shown that earthworms and plants on land are sources of the gas, which is 310 times more powerful at trapping heat in the atmosphere than carbon dioxide, the main greenhouse gas from human activities.

The scientists estimated that nitrous oxide releases by animals were small compared to other natural emissions from underwater soils. They did not not estimate how much reached the atmosphere, where it could be a tiny natural factor affecting the climate.

"It's not something that we should overlook but we should not make a drama of it," Peter Stief, the lead author of the study at Denmark's University of Aarhus and the Max Planck Institute for Marine Microbiology in Germany.

"Carbon dioxide is still the main problem," he told Reuters of the study of seven freshwater and marine sites in Denmark.

Carbon dioxide is released by burning fossil fuels in cars, factories and power plants. The U.N. Climate Panel says that a buildup of greenhouse gases will bring more heatwaves, more powerful storms, droughts, floods and rising sea levels.

Amounts of nitrous oxide from underwater creatures were likely to rise because of widening use of fertilisers in tropical nations, according to the study. Nitrate fertilisers can be washed off farmland by rains into rivers and the sea.

"Growth rates...are much higher in the tropics than in temperate zones where already some measures are being taken to reduce nitrate use," Stief said.

He said that more study was needed of underwater nitrous oxide emissions. "It will needs more testing -- it's not known whether fish, for instance, release it too," Stief said.

On land, livestock such as sheep, cows and goats emit methane, another type of greenhouse gas, as part of their digestive system. Livestock account for about 3 percent of all greenhouse gas emissions from human activities.

Trees and other plants are a major store of carbon. Deforestation from the Amazon to the Congo accounts for about 20 percent of greenhouse emissions from human activities.

More than 190 nations aim to agree a new treaty to fight climate change at a meeting in Copenhagen in December. Many countries want measures to slow deforestation to be part of the agreement.