An unlikely ally may have been found in the fight against the effects of climate change. Fish excretions seem to play a key role in maintaining the ocean's delicate pH balance, says a study that also reveals that there are 2 billion tonnes of fish in the world's oceans. Bony fish excrete lumps of calcium carbonate, known as "gut rocks" which are thought to dissolve in the upper layers of the ocean. A team led by Rod Wilsonof the University of Exeter in the UK has now shown that the sheer amount of gut rocks produced plays a key role in buffering the carbon dioxide that acidifies seawater.