Cover Sam Yiu King-fung (left) and Qiu Jianwen (Photo: courtesy of Jocelyn Tam)

These previously undocumented coral species shed light on how the city’s marine environment may be more diverse, and yet more fragile, than we imagined, according to Qiu Jianwen and Sam Yiu King-fung

It started as just another summer’s day of analysing coral health in a Hong Kong Baptist University laboratory. Then, suddenly, Qiu Jianwen and his colleague Sam Yiu King-fung spotted something they had never seen before: the coral being eaten by some of the coral-eating nudibranchs they were studying was bright orange and violet. They had inadvertently discovered what would turn out to be three new species.

Hong Kong isn’t known for colourful corals; these are more commonly found in diving paradises like Tubbataha Reef in the Philippines or Australia’s Great Barrier Reef, where the warmer tropical waters and gentler underwater terrain enable the growth of vast, brightly coloured communities. Here in Hong Kong, where nearly 90 species have been identified, most hard coral is pale, greyish and monochrome, as the relatively colder subtropical water temperature in winter constrains coral growth.

At the sight of this anomaly, Qiu, who is the associate head of the department of biology, and Yiu, a researcher working with Qiu at the time, returned to the site where they collected the nudibranch specimens to conduct an in-depth underwater search. As they dived to ten metres below sea level, radiantly coloured corals—with polyps and branches spreading out like sparks from a firecracker—loomed in the shadows.

After cross-checking local catalogues and public DNA sequence databases, the team confirmed that what they were seeing were three undocumented species belonging to the sun coral or Tubastraea genus. Its common name is derived from the polyps’ golden yellow colour as seen in known sun coral species in the South Pacific. Qiu and Yiu named their species Tubastraea dendroida, Tubastraea violacea and Tubastraea chloromura.

“The discovery of three species in one go is particularly rare,” Qiu says. The first sun coral species was discovered in 1830 in the waters near Bora Bora island, northwest of Tahiti; the last discovery before the Hong Kong haul was in 1982 in the Galápagos Islands; these three bring the total number of identified sun coral species to only ten.

Qiu, who has been studying marine science since the 1990s, says that in Hong Kong, there has been very little study of sun corals. He explains, “Most universities which study corals focus more on the climatic effects on corals than on taxonomy. If you want to learn about ecology, you first have to know the species.” He believes that there should be more studies on these corals because of their ecological value to local marine life: the nudibranchs he and Yiu collected prey only on certain types of sun corals, without which the former would likely face extinction in local waters.

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Qiu and Yiu’s discovery hints at how much more diverse Hong Kong’s marine life may be than we realise, and how much of the region’s underwater world remains uncharted. For example, the reason these sun corals remained hidden for so long is that they live in deep waters between ten and 30 metres below sea level, making it unlikely that casual divers will spot them, Qiu says.

Compared to the species found in shallower waters, sun corals’ vibrant colours and interesting shapes make a visually stunning underwater “firework display”—a sight which gives them their local nickname, “firecracker corals”. Their Latin names, too, are based on their appearances: Tubastraea dendroida resembles a tree—the English “dendroid” derives from the Greek dendron, meaning branch; violacea denotes that species’ violet colour; while chloromura has an olive green skeletal wall, hence chloro (green) mura (wall). The first two are believed to be endemic to Hong Kong, whereas a yellow variation of the third one—with a highly similar DNA sequence—is found in New Caledonia in the South Pacific.

Hong Kong has a higher coral diversity than the Caribbean Sea, and the coral community at Hoi Ha Wan alone is home to more than 120 species of reef fish and marine invertebrates, according to the Agriculture, Fisheries and Conservation Department.

Qiu says that, as with coral globally, Hong Kong’s species are facing imminent threats from global warming and the resulting rise in water temperature. Following the heatwave in July this year, he observed the most severe coral bleaching he has seen so far; far more serious than that caused by the heatwaves in 2014 and 2017. “In some areas, you see 90 per cent of the corals becoming whiter,” he says. The biologist is also worried about the increasing frequency of extreme weather conditions, which can bring more freshwater to the ocean and alter the salinity of seawater that corals need to grow in.

Qiu says that Hong Kong’s corals have also been threatened by the exploding sea urchin population; the creatures are bioeroders that graze on and destroy reef structures. In 2015, there was an outbreak of long-spine sea urchins that led to the collapse of coral communities in Hoi Ha Wan and Port Island. Following his discovery of the new species, Qiu has been studying the relationship between corals and their prey, and the threat posed by bioeroders, believing that new data on the new species may help shed light on these “natural disasters”.

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The discovery of new coral species has also inspired Qiu to conduct biodiversity research in the waters between Lantau and Lamma islands, where the government has plans for reclamation commencing in 2026 under the Lantau Tomorrow Vision project. Reclamation would crush and bury the area’s coral communities. This is only one of the many manmade threats to corals in Hong Kong; others include chemical pollution, fishing trawlers and boat anchors being dropped. “I want to determine what coral species there are,” he says. “If they are rare species, what can we do to save them? There’s no one doing such a project now.

 

“I study corals because of their conservation value, and their unique, beautiful features,” Qiu says. Despite his decades in the field, he still feels a childlike wonder when he sees photos of corals that he’s never seen or heard of before, and knows there could be many more such revelations. “I wouldn’t be surprised if there were more new species waiting to be discovered.”

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