The area occupied by soft cauliflower corals shrank by almost 70% between 2011 and 2019, according to researchers who say that extreme flooding in Australia in March wiped out even more of these endangered marine creatures.

In a study published April 16 in Estuarine, Coastal and Shelf Science, researchers mapped the distribution of Dendronephthya australis, a species of soft coral found only in Australia, mainly in the Port Stephens estuary on the central coast of New South Wales.

“These soft corals are such a key and unique part of our environment here,” said lead author Meryl Larkin, a marine science Ph.D. candidate at Southern Cross University. “They’re like purple cauliflowers, beautiful branching little purple trees with white tufts on the ends. They come out of the sand and provide an amazing habitat for a range of critters.”

Soft cauliflower corals grow in aggregations, forming forests that provide shelter for invertebrates and fishes, including the endangered White’s seahorse. So when Larkin and other researchers noticed that the purple aggregations seemed to be dwindling, they set out to map the corals' decline and look for ways to help them recover.

The researchers mapped the distribution of D. australis by capturing footage of the seafloor with a video camera towed behind a boat. They compared their maps to a survey from 2011 that used the same methods to estimate that the coral covered an area of 28,600 square meters, equivalent to about four and a half soccer fields. Larkin and her colleagues found that the coral’s distribution had shrunk to an area of about one and a half soccer fields, or 9,300 square meters, in just eight years.

To examine potential drivers of these declines, the team looked at changes in environmental variables in the estuary from 2011-19. Their analysis suggested that coral losses were associated with reduced current flow, likely because these animals rely on tidal currents to transport the plankton that they feed on. Another factor linked with their decline was sand movement in the estuary, which the researchers suggest is due to sediment smothering the animals.

But the reasons for changes in sand depth are difficult to quantify, Larkin explained.

“It’s quite complex,” she said. “There are natural sand movements occurring in the estuary, but there are other factors at play, such as beach replenishment that happens when local councils move sand to bring beaches back for people that want to have beachfront properties. So there’s a lot of anthropogenic, non-natural stuff that’s happening in the estuary, too.”

Other factors such as pollution and damage from boat anchors, which weren’t evaluated in this study, could also harm D. australis, Larkin said. 

Like many other corals around the world, this species is also threatened by rising ocean temperatures, extreme weather events and other effects of climate change.

In March, record flooding in New South Wales decimated much of the remaining population of D. australis, Larkin told The Academic Times.

“I went down to check on ones that I’ve been monitoring for two years, and [almost] every single one that I was tracking had disappeared,” she said. “I was so devastated.”

The researchers have not yet quantified how much of the endangered corals was lost due to the floods, but Larkin said it could be up to 90%. 

A recent analysis suggested that because many coral species are widely distributed, their risk of extinction is low. But D. australis may not be so lucky. 

“There is a risk that it could be added to the extinction list, which is heartbreaking because so many creatures are so reliant on it,” Larkin said. “We’re in the sixth mass extinction event, what they’re calling the Anthropocene. We hope that this coral doesn’t become a part of that story.”

But there’s still hope for D. australis, according to the researchers. As part of her Ph.D. research, Larkin is developing methods to help restore populations of this species by growing new colonies from coral fragments. Although she’s had some success, more work is needed to figure out how to repopulate the coral on a mass scale. 

According to Larkin, the study highlights a need to prevent further damage to coral habitat, including limiting boat anchoring or using boat moorings that don't damage the sea floor. She also hopes that councils will consider this endangered species when moving sand as part of beach-replenishment programs.

“I’m hopeful that [this coral] can recover,” Larkin said. “But there’s a lot of work that needs to be done, and it’s all pretty urgent now. It’s very close to extinction.”

The study, “The rapid decline of an endangered temperate soft coral species,” published April 16 in Estuarine, Coastal and Shelf Science, was authored by Meryl F. Larkin and Stephen D. A. Smith, Southern Cross University; Tom R. Davis, Southern Cross University and NSW Department of Primary Industries; David Harasti and Gwenael Cadiou, NSW Department of Primary Industries; and Davina E. Poulos, University of Technology Sydney.