Migration helps corals survive climate change

Coral bleaching- a threat from climate change
ARC Centre says the key to preserving the extraordinary richness and beauty of the world’s coral reefs through the coming period of fragmentation caused by climate change lies in a better understanding of how newborn coral larvae disperse across the oceans to settle and grow on new reefs.

Research by scientists at the ARC Centre of Excellence for Coral Reef Studies is throwing new light on the survival and settlement rates of larvae from different coral species, as a basis for predicting how fast coral species might change where they live in response to climate change.

'Coral larvae are weak swimmers, and they don't feed. They are swept around the ocean by currents and have to find a place to settle and start to grow before they run out of energy,' says Professor Sean Connolly of CoECRS and James Cook University.

'Understanding these dispersal patterns, the rate at which the larvae perish, and the rate at which they lose the ability to settle when they do arrive at a reef, hold the key to predicting how quickly a coral species can move with its preferred environments as climate change renders unsuitable parts of its current habitat.'

This information will also help in the management of existing coral reefs, under siege from ocean warming, acidification and other human impacts, he adds. If managers know the where the young corals come from that renew a damaged reef, they can take steps to limit human pressures such as fishing, coral damage or polluted runoff accordingly.

The world’s coral scientists and managers often rely on mathematical models to predict the dispersal and settlement of coral larvae, as a key part of understanding how interconnected and resilient a reef may be. However, existing models do not take good account of the rates at which coral larvae in the wild die or lose competence to settle.

In a series of groundbreaking experiments, Dr Andrew Baird has measured both the survival rates and ability to settle of larvae for five different coral species. 'I noticed that the ability of the larvae to settle increased sharply after three to five days, peaked at about one-two weeks and then tailed off very gradually,' Dr Baird says. 'Some larvae lost the ability to settle very quickly, but others from the same batch could still settle even 100 days after hatching. This suggested that the current models for predicting coral dispersal were likely to be inaccurate.'

Armed with these new findings, Baird then approached Connolly to develop a new generation of models that could better capture this high variation in settlement ability.

Modelling these wide variations has helped to resolve an important scientific puzzle, Professor Connolly says - how coral species can show substantial genetic differences between nearby reefs, but still disperse far enough to be very geographically widespread.

'Our new models indicate that more larvae than we thought should settle very close to home, perhaps on the same reef or one next door. At the same time, a small proportion of stellar performers can survive for longer, and travel further, than previously thought,' says Connolly.

This, say the researchers, provides some hope that coral species may be able to 'migrate' with climate change - dispersing their larvae towards cooler areas when their current habitat gets too hot to survive.

Their paper 'Estimating dispersal potential for marine larvae: dynamic models applied to scleractinian corals' by Sean R Connolly and Andrew H Baird appears in the latest issue of the journal Ecology, No 91, 2010.




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