Fish guts produce more than just poo...

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Ancient limestome, mud and tropical sediments may have a surprising origin according to scientists from Manchester Metropolitan Museum – they might come from the guts of fish.

Until this study the origin of tiny particles of calcium carbonate found in tropical seabeds has been difficult to establish. Lead author Professor Chris Perry, a sedimentologist at Manchester Metropolitan University is quoted on the NERC website: "If you look at carbonate muds in the tropics, until now we haven't known where a significant proportion of the material comes from. For example in the Bahamas, I'd say 40 to 50% of the mud was of unknown origin. We've now recognised a new mud source that's relevant throughout the area."

Fish produce these fine particles as a by-product of the metabolism of minerals from the water around them. The minerals crystallise in their guts and are excreted within pellets of mucus.

Professor Perry’s team took mud from various habitats in the tropics and concluded that particles excreted by fish account for about 14% of mud produced across the different habitats in the area, and more than 70% in certain environments, like mangrove-fringed coastal inlets.

The researchers found that the tiny carbonate deposits are produced in distinctive shapes resembling ellipses, bundles of straw, spheroids and even dumb-bells.

The team now plan to extend their study to other parts of the world to establish whether these carbonates are as important there as they are in the tropics. It's likely that the faster metabolic rates of tropical fish mean they produce more carbonate than those elsewhere.

Perry and his team will also examine how climate change might affect production rates of the carbonates as metabolic rates increase with warmer waters leading to greater carbonate production.

For more information see: Chris T. Perry, Michael A. Salter, Alastair R. Harborne, Stephen F. Crowley, Howard L. Jelks, and Rod W. Wilson: Fish as major carbonate mud producers and missing components of the tropical carbonate factory: PNAS 2011 : 1015895108v1-5.