A new computer simulation conducted at the University of Bristol (UB) and University of Miami (UM) Rosenstiel School of Marine¹ & Atmospheric² Science has revealed the epic³, ocean-spanning journeys travelled by millimetre-sized coral larvae⁴ through the world's seas. The study, published in Global Ecology and Biogeography, is the first to recreate the oceanic paths along which corals disperse⁵ globally, and will eventually aid predictions of how coral reef distributions may shift with climate change.

 

Coral reefs are under increasing threat from the combined pressures of human activity, natural disturbances⁶ and climate change. It has been suggested that coral may respond to these changing conditions by shifting to more favourable⁷ refuges, but their ability to do this will depend on the ocean currents.

 

Sally Wood, a Ph.D. candidate at UB, explains: "Dispersal is an extremely important process for corals. As they are attached to the seafloor as adults, the only way they can escape harmful conditions or replenish⁸（补充） damaged reefs is by releasing their young to the mercy of the ocean currents."

 

Where these intrepid⁹ explorers end up is therefore an important question for coral reef conservation. However, tracking the movement of such tiny larvae in the vast oceans is an impossible task. "This is where computer simulation comes in," adds Wood.

 

Collaborating¹⁰ across the pond, Wood used the Connectivity Modeling System (CMS) developed by Dr. Claire Paris, associate professor of Applied¹¹ Marine Physics at UM to identify the billions of paths taken. This larval migration¹² model had been tested in a previous study against the reef-building coral Montastraea annularis in the Caribbean, where consensus¹³ between modeled estimates of genetic¹⁴ structure were found.

 

"Simulating an unprecedented¹⁵ number of mass spawning¹⁶ events from all known shallow reefs in the global ocean proved essential to identifying critical long dispersal distance events that promote the establishment of new coral colonies. What we found using the CMS are rare long distance dispersers that are thought to contribute to species persistence¹⁷ in isolated¹⁸ coral reefs, and to geographic¹⁹ range shifts during environmental changes," said Paris.

 

Some of the results yielded by the team were surprising. While the majority of simulated larvae settled close to home, others travelled as far as 9,000 km., almost the entire width of the Pacific Ocean. When considered over multiple generations, this means that corals are able to cross entire ocean basins, using islands and coastlines as 'stepping stones.' However, a few places proved too distant for all but the hardiest²⁰ of larvae: Coral in the tropical eastern Pacific are almost entirely²¹ cut off from those on islands of the central Pacific by a daunting²² 5000 km of open ocean. Geographically²³ isolated reefs such as these may be particularly vulnerable, as they are not stocked with external recruits as frequently.

 

The model captured the start of the coral larvae's journey to its survival, and further work is ongoing²⁴ to complete the story. Even after overcoming the trials of the open ocean, coral larvae arriving at a suitable location must first negotiate a 'wall of mouths' to settle on the reef face, and then compete fiercely for the space to thrive and grow.

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