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Marine heatwaves are killing coral and denuding reefs of their colourful beauty – but in a world where everything is food for something else, these heatwaves also pose a major threat to biodiversity.
How? Lets start with coral spawning, which is always in the news around Christmas. A tropical phenomenon, blizzards of eggs and sperm — billions of sex cells disgorged by uncountable polyps.
Last year gave us a split spawning on the Great Barrier Reef (GBR), after full moons in October and November. Slicks of egg and sperm bundles washed over the reef and combined to produce tiny ‘planula’ larvae. Now settled, these planulae are new corals that will eventually replace reefs lost to storms, bleaching and predation.
Mass coral spawning increases the chances of fertilisation and reduces the likelihood that eggs will be eaten by the fish, crabs and starfish that come to this pre-Christmas feast, a phenomenon called predator saturation. But, of course, predators need to grow and reproduce, too.
“A lot of reef fish just fall on those egg and sperm bundles when they are released and on the coral larvae when they come in to settle,” says Professor Morgan Pratchett of James Cook University.
“The morning after the spawning event, damselfish are so full of coral spawn, they can barely swim — you can see their fat little stomachs.”
“They will literally start spawning within weeks after that, turning all that new energy into their own reproductive investment.
“Damselfish are plankton feeders, feeding up in the water column, living in a mutualistic relationship with corals [which] provide these tiny fish with places to hide. In return they help water flow through the coral as they swim and use their gills, and provide nutrients as they sleep inside.”
Morgan Pratchett
The GBR is much more than just corals and pretty fish. Add around 1300 known species of crustaceans (crabs, prawns, barnacles etc), 4000 species of molluscs (clams, oysters, squid, octopus, cuttlefish, nautilus, nudibranchs and snails etc), 630 species of echinoderms (starfish, sea urchins, etc), as well as turtles, birds and sea snakes — the list goes on.
“All the biomass of the Reef’s biodiversity is not stuff we’ve studied — it’s not coral or fish — it’s all these benthic invertebrates, the little crabs that live inside the corals, and they are definitely feeding on the spawn” says Pratchett.
Dr Kerry Cameron, of Reef Recruits, agrees — coral-dwelling crabs feed on emerging spawn, but it’s the zoea, the planktonic crab larvae, that she has observed having the greatest impact. Collecting coral slicks for study in the Philippines, 10 years ago, she collected “a big, beautiful slick, but it got smashed by zoea and was gone in two days.”
“Every coral has many different crustacean species living inside it, but on the outside all you see are a pair of gobies and a handful of damselfish,” says Pratchett. “Crustaceans are as important as the fish, if not more so”, he adds. “Given the numbers of crabs, the quantity of eggs eaten must be phenomenal.”
Much of what happens on reefs remains a mystery. Dr Rohan Brooker of AIMS is exploring relationships between fish, invertebrates and the reefs of Coral Bay, Ningaloo, W.A. “You get a really interesting suite of small invertebrates,” he says. This distinctive community raises many questions. “Does it change as the corals grow, and how do they relate to the fish that live amongst those corals? How stable are these communities? Do you see the same fish and shrimps and crabs if you come back a year later or do they turn over within a month?”
“All important to understand,” he adds, “because we know that these little crabs and shrimps play important roles in coral health — fending off coralivorous starfish, providing nutrients and picking off dead bits.”
Coral polyps host dinoflagellate algae, ‘Symbiodinium’, that provide enough photosynthetic energy — “like a solar panel for polyps”, says Cameron — to build the familiar limestone of the reef. “We wouldn’t have a reef without algae”, she adds.
But this delicate, mutualistic relationship is vulnerable to marine heatwaves. Under higher-than-normal temperatures, Symbiodinium start to produce toxic oxygen-free radicals instead of sugars, and are ejected by the polyps, leaving the corals colourless and vulnerable to disease and starvation.
What does that mean for spawning and that annual energy boost to the coral community? For the corals, as Cameron puts it — “It’s like getting hit by a bus — you’re not dead but you don’t have the energy to make a baby”.
Kerry Cameron
“The energy for that single egg and sperm bundle produced by each coral polyp takes about 9 months to accumulate” adds Dr Abbi Scott, of TropWATER at James Cook University, Cairns. “A stressful summer affects spawning the following year, because the corals must reacquire their algae, and egg production takes so long.”
“If we are getting marine heatwaves and other stressors continually year on year, then we’re really degrading the resilience of the coral reef system.” says Scott.
“There are degrees of severity, Cameron adds, “it’s mostly about how long it lasts.” A bleached coral may not be dead, but it’s a very unhappy. It depends how hot, but typically if they are bleached for a month, they’re unlikely to recover. And that’s when you get that mortality,” she says.
Our GBR is currently going through its fifth mass bleaching event in eight years.
“It’s hard to quantify, but I’m sure coral bleaching would have a big impact on the food web.” Scott adds.
“Corals dying during a massive bleaching event can be major problem”, says Brooker. “Losing that habitat structure can be critical. You might see massive declines in the number of fish on that reef, as fewer eggs and larvae are produced for the following year,” he adds.
And the knock-on effect spreads around the food web — everything is food for something else. ‘Obligate species’, those fish and crustaceans that depend on the reef, can’t survive the loss of habitat, and will disappear, only returning if corals recover, says Brooker. “Healthy coral has a distinctive assemblage compared to a bleaching, dying, or overgrown, dead, reef.”
“Once a coral dies, you might see a boost in more generalist species feeding on detritus and living in the rubble of the dead reef. But that rubble degrades, losing 3D complexity and habitat value”, he adds.
More frequent marine heatwaves are expected to further stress the Reef. “The reef will change, depending on which species survive and which we can use to help restoration,” says Scott.
“We’re a long way from the 5% coral cover they have in the Caribbean”, says Cameron. “The GBR is in trouble. Absolutely, it’s changing, but the complexity is different. But if you say it’s all over, then people don’t care. And that’s a real worry,” says Cameron.
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This article was originally published on Cosmos Magazine and was written by Richard Musgrove. Dr Richard Musgrove is a freelance science and business writer based in Far North Queensland. He was a Senior Researcher at the South Australian Research and Development Institute.
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