A sea slug about the length of a pen, so rare and sensitive that it disappears when water quality deteriorates, has been spotted off Wuzhizhou Island in Sanya, Hainan province, offering evidence that a coral reef once damaged by overuse and development is recovering, researchers said.

Divers monitoring the island's marine ranch recorded the roughly 10-centimeter panda sea bunny — scientifically known as Jorunna funebris and also called the reticulated sea slug — along with a reticulated nudibranch and a ribbon eel last month, according to the Wuzhizhou Island Tourist Area.

It was the second time the area has recorded multiple uncommon marine organisms, the tourist area said.

The panda sea bunny is a typical tropical coral reef indicator species that is highly sensitive to environmental changes. It inhabits clean waters with intact reef structures and abundant food; such fragile mollusks grow slowly, reaching maturity only where the marine environment remains stable over the long term and the food chain is complete.

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"These small marine organisms are extremely sensitive to environmental change. If water quality or reef ecology fluctuates, they quietly disappear," said underwater photographer Jiang Guangfeng, who discovered the creatures.

Wang Aimin, a professor at Hainan University and chief scientist at the Hainan modern marine ranching engineering research center, said the sighting of a relatively large adult sea slug indicates more than good water quality.

"From a marine biology perspective, a sea slug reaching a relatively large adult size means it has gone through a relatively complete growth process," Wang said. "It needs not only 'good water quality' but also a suitable benthic habitat, a stable microenvironment and a continuous food source."

Sea slugs lack the long-distance swimming ability of fish and are sensitive to local habitat changes, Wang said, adding that they can quickly disappear from an area if water turbidity, sediment disturbance, pollution, declining food resources or reef degradation occurs.

The appearance of the less common reef-dwelling species follows more than a decade of ecological restoration efforts.

Since 2010, with support from university research teams, the Wuzhizhou Island Tourist Area has carried out systematic and sustained ecological restoration, deploying artificial reefs and sunken ship structures to create three-dimensional underwater habitats while transplanting and nurturing coral seedlings one by one.

To date, restoration work has covered more than 63,000 square meters, with over 70,000 coral fragments transplanted.

Fishery resources in the waters around Wuzhizhou Island have doubled compared with before the restoration, according to the tourist area.

Scientific surveys have recorded 265 species of reef fish, while more than 8.21 million fish, shrimp and shellfish seedlings have been released.

Wild dolphins and blacktip reef sharks have returned to the area year after year, while whale sharks, sea turtles and spotted seals have also been spotted.

"Large animals show us the 'bustle' of an ecosystem, while small benthic organisms often reveal its 'details'. Only when species across different trophic levels and ecological functions coexist can we more reliably assess whether a coral reef ecosystem is recovering toward completeness and stability," Wang said.

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However, he cautioned that long-term, fixed-point and quantitative monitoring is needed to determine whether the changes are sustained, and that a single species cannot be used to assess the health of an entire reef ecosystem.

Wang also called for standardized scientific surveys, underwater photographers' image records and citizen science to be combined to build a long-term biodiversity database for Hainan's coastal reefs.

"A beautiful sea slug photo has value for public outreach, but if it carries standardized marine ecological information, it can also become a scientific record," he said.

Contact the writers at chenbowen@chinadaily.com.cn