Two new sea slug species found in Hainan

Two new species of sea slugs have been discovered in Hainan province.

Wan Yinglang, a professor at the College of Tropical Crops at Hainan University, and his team found elysia leucolegnote living in mangroves in the Dongzhai Port National Nature Reserve in Haikou, the provincial capital, and elysia tomentosa living on a coral reef in Yunlong Bay, Wenchang.

This is the first official record of the presence of these mollusks, which rely on photosynthesis to generate nutrients, on the Chinese mainland.

"Elysia leucolegnote are an intertidal benthic mollusk, and are only 5 to 8 millimeters long, smaller than the size of a button," Wan said. "They look like a leaf with a white skirt and have a protective color, yellowish-green or dark green."

Elysia leucolegnote are often found clustered in small tidal pools in mangrove forests. As the pools dry, they hide in crab holes or burrow into the mud to escape the heat. A single pool can hold up to 100 of them.

Elysia tomentosa are found on offshore coral reefs. They are olive green and covered in white hairs of different lengths, with a scattering of orange and black dots that create a protective camouflage that helps them blend into their habitats. As they age, they can grow to about 40 millimeters in size, Wan said.

The slugs usually have a pair of long antennae and "steal" chloroplasts from algae.

They store the chloroplasts in their cells for photosynthesis, which provides them with the energy and organic matter they need to survive, much like plants, Wan explained.

In larval form, they eat algae, puncturing the cell wall with their tongues and teeth and siphoning out chloroplasts, which are placed in the cells of the digestive tract, turning it from transparent to a green color.

"Depending on the species and the algae they feed on, their photosynthetic capacity can last from two to three weeks to as long as a year," Wan said.

During photosynthesis, the slugs do not need to feed but will occasionally do so on algae to replenish aging or depleted chloroplasts. Elysia leucolegnote can maintain photosynthesis over a long period, while elysia tomentosa have to eat from time to time.

"The discovery of the two species has provided new clues for understanding China's marine species and ecosystems. Their ability to photosynthesize presents new research possibilities for researchers in photosynthetic and evolutionary biology," Wan said.

The in-depth study of the slugs is expected to shed more light on the symbiosis process of chloroplasts, as well as on the origins and differentiations between animals and plants.

Most previous research on the species has been carried out along the North American coast, from Florida to the Canadian Atlantic. In the 1990s, Hong Kong scientists reported finding a species of elysia leucolegnote in mangrove mud pits in the coastal waters of China.

Wan saw plenty of photos of the species in the Hainan nature observation WeChat group.

In May 2020, he and his team visited the Dongzhai Port National Nature Reserve and found elysia leucolegnote in knee-deep mud between mangroves.

Through morphological comparison and molecular marker analysis, they confirmed this as the initial sighting of the species in the reserve.

In March 2021, Wan's team published their discovery in the Chinese Journal of Zoology. The following year, they explored the reserve to determine the species' distribution.

"Mangrove forests not only help protect coasts from tidal surges, they are also one of the best places for preserving biodiversity and genetic diversity," said Feng Erhui, a forestry engineer at the Dongzhai Port National Nature Reserve Administration.

He said that the discovery of the elysia leucolegnote has added to the list of species present in the reserve and is proof that its environmental conditions are improving.

chenbowen@chinadaily.com.cn