Tropical sediment traced to fish intestines

Planet Earth Online: Tropical sediment traced to fish intestines
Tiny particles of calcium carbonate that help make up tropical seabeds are produced in the guts of fish, scientists have shown.

Until now, the origin of many of these particles has been difficult to explain. This new discovery may go some way towards explaining where some of the muds that form important ancient limestone deposits come from.

The discovery also gives us a much better idea of where some tropical sediments come from; the results may well apply in other climatic zones, but more research is needed to be sure.

Earlier work had established that fish produce these carbonates in their guts, but a new paper in Proceedings of the National Academy of Science is the first to characterise their sedimentary properties and to assess how important they are in supplying sediment in particular tropical environments.

'If you look at carbonate muds in the tropics, until now we haven't known where a significant proportion of the material comes from,' says Professor Chris Perry, a sedimentologist at Manchester Metropolitan University and lead author of the paper. 'For example in the Bahamas, I'd say 40 to 50 per cent of the mud was of unknown origin. We've now recognised a new mud source that's relevant throughout the area.'

The team analysed mud from various habitats all over the archipelago to conclude that particles excreted by fish account for about 14 per cent of mud produced across the different habitats in the area, and more than 70 per cent in certain environments, like mangrove-fringed coastal inlets.

The carbonates in question are extremely fine-grained particles. They're produced as a side-effect of the way fish metabolise the minerals they ingest from the water around them; they crystallise in their guts and are excreted within pellets of mucus.

Because the particles are so small, it's difficult to examine them, and Perry says this may account for the fact they haven't been recognised until now.

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

Perry now plans to do more work on how these carbonate deposits from fish are preserved in seabed sediments, as well as carrying out similar research in other parts of the world to find out if these carbonates are as important there as they are in the tropics. It's likely that tropical fish produce more carbonate than those elsewhere as warmer waters mean they have higher metabolic rates.

He argues that more investigation is also needed into how climate change could affect production of carbonates by fish. Warmer waters might be expected to increase metabolic rates and hence carbonate production, but much will depend on numbers of fish - if destruction of habitats like coral reefs and overfishing continue to reduce populations, the impact on this source of marine carbonate production could be serious.