The key to preserving the extraordinary richness and beauty of the world’s coral reefs, as they face an uncertain future, lies in gaining 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 headquartered in Queensland, Australia, is throwing new light on the survival and settlement rates of larvae from different coral species, as a basis for predicting how fast species of coral might change where they live in response to climate change.
“Coral larvae are weak swimmers, and they don’t feed,” said Professor Sean Connolly of CoECRS and James Cook University. “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.”
“Understanding these dispersal patterns, including 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 within its preferred environment, as climate change renders areas of current habitat unsuitable,” he said.
This information will also help in the management of existing coral reefs, under siege from ocean warming, acidification and other human impacts. If scientists know where the young corals come from, in terms of renewing 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 adequately account for the rates at which coral larvae in the wild die or cannot settle.
In a series of groundbreaking experiments, Dr. Andrew Baird has measured both the survival rates, and ability to settle, for larvae of 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 to two weeks and then tailed off very gradually,” he said. “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.
According to Connolly, modeling these wide differences has helped to resolve an important scientific puzzle — 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,” Connolly said. “At the same time, a small proportion of stellar performers can survive for longer, and travel further than previously thought.”
This shows that some coral species may be able to “migrate” if faced with climate change, by dispersing their larvae towards cooler areas when their current habitat becomes too hot to survive.
The paper “Estimating dispersal potential for marine larvae: dynamic models applied to scleractinian corals” by Sean R. Connolly and Andrew H. Baird appears in the journal Ecology, number 91, 2010.