A piece of cotton fabric and four steel rods: that’s the prescription for corals suffering from Terpios Hoshinota infection, say scientists from the Pondicherry University in a new study published in PloS One.
Terpios is an aggressively growing sponge that forms black mats on coral surfaces. By 2010, 8 years after Terpios infection was first noticed in the South China Sea, nearly 70% of corals were killed due to Terpios outbreak. Similar accounts poured in from the Philippines, Japan and Indonesia. Buried in these reports were signs of a more pressing danger closer home.
Terpios was moving westward towards the Indian Ocean
At a rate of 11.5−23 cm/month, Terpios can cover squares of kilometres within a short time-span. This prompted a team of scientists from the Pondicherry University to investigate the matter. “Our motivation was clear – study Terpios invasion to manage reef protection,” says Thangadurai Thinesh, one of the authors of this study and postdoctoral Scholar at the Florida International University, Miami.
In 2013, researchers began examining Palk Bay, an unprotected shallow reef system in India for Terpios infection. They scraped sponge fragments sticking to coral surfaces for analysis. Both microscopy and genetic material extracted from the samples confirmed Terpios invasion in the area.
For the next two years, Thinesh and his colleagues carried routine surveys to understand Terpios’s growth patterns. Their field inventory shows that while Terpios grew by 9 folds, during this time 76% of the corals in The Palk Bay were destroyed.
“At that time, a research published by Chaolun Allen Chen of the National Taiwan University showed that Terpios accumulates large amounts of cyanobacteria at its edges, when invading corals” recounts Thinesh.
Cyanobacteria use sunlight to produce food, much like plants. The researchers posited that this extra bolus of energy from the bacteria could be fuelling the sponge’s expansion. If that were true blocking photosynthesis should stop Terpios in its track.
To test their idea, the researchers placed cotton hoods to filter the sunshine over five infected coral colonies and monitored sponge growth. Initially, there was no visible difference in Terpios growth in naked and covered corals, But after 10 days, in corals shielded from the sun, the Terpios started losing its colour. Tests show that amount of chlorophyll a – the pigment that helps cyanobacteria make food- was halved in sponge samples retrieved from covered corals. This can account for the loss in sponge colour. Scientists also noticed a visible growth retardation of Terpios in covered colonies Unable to perform photosynthesis within the hoods, a huge fraction of cyanobacteria let go of Terpios’s surface. With its nutrient making factories gone, the sponge could no longer sustain its onslaught explains Thinesh.
The growth halting effect of cotton hoods was effective for an entire year. By May 2016, when scientists performed their last field study, the corals kept under a hood for 10 days showed no additional Terpios growth even when the hoods were removed. Additionally, the chlorophyll content in the existing sponge mats was also reduced by ten times.
However, Jih-Terng Wang, a member from Chen’s lab expresses concern. “It is technically impossible to build the shading facility on the reef,” says Wang. “This method might work when Terpios infection just begins and the size of the patch is small”.
Thinesh agrees: “Shading is not feasible on a large scale, but it could be effective to conserve critically endangered species if we find the Terpios invasion before it is established,” he adds.
Thinesh and his team are now looking at other bacteria and their role in Terpios expansion “Understanding Terpios larvae ecology and its spreading dynamics will help in managing the infection in the long term”.