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Sharks' Number Two Function Keeps Coral Reefs Healthy

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Sharks are well known as apex predators, but according to a recent study, their role in the food chain may well start – quite literally – at the bottom.

A new paper published in the Proceedings of the Royal Society B, reports on the work of a team of scientists from Imperial College London, in partnership with the Zoological Society of London, which found that reef shark faeces account for a large proportion of nutrients that support the world's coral reefs.

The researchers monitored the movements of grey reef sharks ( Carcharhinus amblyrhynchos ) by fitting them with acoustic tags in the waters surrounding Palmyra Atoll, a remote reef to the south of Hawaii. Grey reef sharks are of particular interest, as although they are frequent visitors to coral reefs, a large portion of their prey is bony pelagic fish. They are therefore effectively redistributing nutrients from the open ocean by depositing their poop over the reef.

Combining their knowledge of the sharks' feeding habits with the new tracking data, the scientists found that the population of grey reef sharks in this environment – estimated to be around 8,300 individual animals – dumped approximately 9.54kg of nitrogen per day into the reef's ecosystem.

'This is a substantial amount of nutrients, which contribute to reef primary productivity, which in turn effectively act as a fertiliser for thousands of other species that call these reef environments home.' said Dr David Jacoby, of  ZSL's Institute of Zoology, senior co-author of the paper.

'While estimating quantities of shark poo may not sound like the most glamorous of pastimes,' said Dr Jacoby, 'the findings of this research have fascinating implications for our understanding not only of fragile coral reef ecosystems but also the ecological significance of grey reef sharks.' 

Although grey reef sharks are distributed worldwide and are a relatively common sight for scuba divers, their numbers are in decline and the species is currently classified as 'Near Threatened' by the IUCN Red List. 'Coupled with their better-known role as predators,' said Dr Jacoby, 'our study underlines another, less obvious role played by reef sharks in improving the resilience of these fragile habitats and again underlines the vital importance of conserving these and other wide-ranging predators.'

 

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