A matter of metres

On a few nights each year, a healthy reef fills the water with drifting pink beads. Each one is a bundle of eggs and sperm, and each needs to find a partner before it is lost to the dark.

Corals can’t move to find a mate, and that simple fact is behind one of the most important questions in reef science: how close do colonies need to be to reproduce?

Professor Peter Mumby from The University of Queensland’s School of the Environment has spent years chasing the answer.

“Unlike fish, corals can’t move so if their nearest neighbour is too far away their reproductive effort is wasted,” Professor Mumby said.

A thinning reef

Most corals are broadcast spawners, releasing bundles of eggs and sperm during mass spawning events.

That means fertilisation depends on gametes meeting in open water, and the greater the distance between colonies, the harder that becomes.

Sperm from a colony almost never fertilise its own eggs, so they have to reach a neighbour.

Ecologists call this an Allee effect: when a population gets too sparse, its members struggle to reproduce.

The danger is that Allee effects can be sudden and hard to reverse. If bleaching or storms thin a reef past a certain point, reproduction could start to fail.

“Seeing that reefs are suffering so many insults spanning cyclones, crown-of-thorns starfish outbreaks, and coral bleaching, coral populations are thinning out – becoming less dense,” Professor Mumby said.

“This raises the prospect of reproductive failure at quite large scales, something that will severely undermine the long-term persistence of corals.”

A close up of a soft coral on a black background. The coral is a vibrant yellow color, and the background is a deep black.

UQ's Professor Peter Mumby

A plastic bag floating on top of a coral reef, with a bottle of water beside it. The bag appears to be made of a transparent material, likely plastic, and is likely used to store and transport the contents of the bottle. The coral reef is a vibrant green color, and the bottle is a bright pink.

Collecting device that captures the egg/sperm bundles of an individual coral.

A clown fish swimming in an anemone, its bright orange and white stripes standing out against the vibrant colors of the sea anemones.

Spawning by glow stick

The team ran experiments to confirm what they saw in the field, setting up corals in arrays that differed in spacing and in orientation to the currents that carry eggs and sperm.

“One of the challenging elements was to find an environment where we could be sure that the only corals of that species were the ones we put there,” Professor Mumby said of the experiments.

Hoods placed above each colony caught the bundles as they were released.

“People often confuse the spherical bundles as being eggs but in reality each bundle comprises thousands of tiny eggs and packets of even tinier sperm,” he said.

The bundles collected in a container with mesh walls, which traps the eggs but lets sperm pass through.

Once about a cubic centimetre had gathered, the container was sealed and released, and the buoyant eggs floated to the surface.

“Seeing as this happens at night, we attach a colour glow stick to follow them for about an hour,” Professor Mumby said.

“At that point we bring them into the lab and quantify the percentage of fertilised eggs.”

The team also took the approach to a wild population of table coral, Acropora hyacinthus, on a sheltered Palau reef.

Divers checked 210 colonies for eggs ahead of the full moon, then patrolled the 26 that were ready to spawn, using red lights so as not to disturb them.

Over two nights in March 2024, they tracked fertilisation in each colony.

To see where sperm were headed, the researchers also released fluorescent dye on matching tide and wind conditions. It followed almost exactly the path of the egg containers.

“It takes quite a big team but it’s great fun,” Professor Mumby said.

Ten metres

When colonies were less than half a metre apart, fertilisation averaged about 30 per cent.

At 10 metres, it fell below 10 per cent, and by 15 to 20 metres it was virtually zero.

Table corals are abundant and large-bodied, among the species least expected to show Allee effects, which makes the steep decline notable. Their fast growth also helps drive rapid reef recovery.

The distance to a colony’s nearest neighbour predicted its fertilisation success better than the number of colonies in the surrounding area. Mapping reefs from images, known as photogrammetry, could make that distance easier to measure.

Neighbours also spawned closer together in time, with timing drifting apart by about three minutes over 20 metres, which may reinforce the effect.

Weather mattered too. On the second night of spawning, when winds picked up, average fertilisation dropped from 34 per cent to 12 per cent, even though more colonies spawned heavily.

“We’re finding that corals have to be within about 10 metres to have a pretty good chance of successful reproduction. That’s pretty close!” Professor Mumby said.

The experiments confirmed the pattern seen in the field.

“The great thing about experiments is that you can infer causality about the role of distance between colonies in driving the fertilisation success of eggs.

“And our results were further confirmed by undertaking genetic parentage analysis. We found that most colonies were fertilised by individuals only 3–4 metres away.”

A satellite view of a large body of water with a lot of trees on it, and a map with text and numbers on it. The map appears to be a satellite image, providing a detailed view of the area.

Experimental design from Palau showing a dense cluster of corals at natural density and then a series of individual corals placed at different distances along radiating transects in multiple orientations to the current.

A close up of an anemone on a coral reef, with a blurred background. The coral is a vibrant green color, and its tentacles are spread out in a fan-like shape.

A vibrant underwater scene with a purple anemone and a clown fish swimming around it. The anemones are a deep purple color, while the clown fish is a bright yellow. The water is a deep blue, and the coral reef is visible in the background.

Rebuilding reproductive reefs

The researchers also developed a simple measure of fertilisation potential to help assess Allee effect risk on other reefs.

At the Palau site, the reef achieved 71 per cent of its potential.

The study suggests isolated reefs, where few larvae arrive from elsewhere, are likely to be most at risk.

The findings also give restoration a target, because planting out colonies can lift density and improve reproduction.

“One of the desirable aspects of this is that restoration can play a role in enhancing coral density to improve reproduction,” Professor Mumby said.

“Currently, we’re using our results to help design restoration strategies that can maintain reproductive populations. It’s nice to have a quantitative target for restoration activity.

“Ultimately, we want to find ways to avoid the local extinction of corals – something that our modelling suggests is quite feasible if we don’t act.”

Modelling how corals respond to climate change can be disheartening work.

“To me, this research on Allee effects has brought real joy to my research – maybe it’s something about spending so much time puzzling about the generation of new life on the reef,” Professor Mumby said.

“Spawning events are one of the great experiences of nature. It never gets old.”

Professor Peter Mumby is a Pew Fellow in Marine Conservation and Australian Research Council Laureate Fellow at The University of Queensland, where he leads the Marine Spatial Ecology Lab.

Read the observational study: Allee effects limit coral fertilization success, Proceedings of the National Academy of Sciences.

Read the experimental study: Proximity and current alignment drive fertilisation success in a broadcast-spawning coral (Acropora cf. hyacinthus), Communications Biology.

The research involved colleagues from CSIRO and the Palau International Coral Reef Center, and was funded by the McCusker Foundation with contributions from the Australian Government’s Reef Restoration and Adaptation Program.