A tiny marine creature, smaller than a fingernail, has made waves in the waters off Wuzhizhou Island in Sanya, Hainan.

Ecological photographer Lin Yinyue recently captured clear images of a rare “leaf sheep” sea slug while diving near Wuzhizhou Island. The sighting marks the first officially recognized clear image record of the species in waters off the Chinese mainland.

Photo by Lin Yinyue

The sea slug belongs to the genus Costasiella, a group of sacoglossan sea slugs known for their striking appearance. Often called the “leaf sheep” or “sheep sea slug,” even in adulthood, the creature is usually less than one centimeter long. With its tiny black eyes and leaf-like structures on its back, it looks almost like a miniature sheep grazing on the seafloor.

Photo by Gong Qiya

What makes the species even more unusual is its ability to perform kleptoplasty. By feeding on algae, the sea slug can ingest chloroplasts and retain them within its body, allowing it to use sunlight in a process similar to photosynthesis. Because of this, the species depends heavily on clean water, healthy algae communities, and a stable marine environment.

Photo by Kim Eun Jong

Its appearance in the waters off Wuzhizhou Island is therefore more than a rare photographic moment. It is also a sign of the area’s healthy marine ecosystem and rich biodiversity. The waters surrounding the island are home to China’s first tropical marine ranch. Crystalline shallows, thriving coral communities, and abundant marine life have made it a popular destination for divers, underwater photographers and marine researchers. The discovery of the leaf sheep sea slug adds another vivid detail to the island’s growing ecological story.

Lin said the waters around Wuzhizhou Island left a strong impression on him. “This area is very special,” he said. “There are many kinds of coral, and a lot of them. When diving, you can also see schools of small fish, which is really fascinating. I enjoy quietly observing marine life. Real images are the most moving, and they are also what make people fall in love with the ocean and want to protect it.”

The island’s underwater world has also impressed international photographers. Jerome, an ecological photographer from South Korea, was visiting Chinese waters for the first time and said he was surprised by both the marine environment and the level of ecological protection there.

“I have dived in many countries, but the waters and conservation efforts around Wuzhizhou Island really surprised me,” he said. “With the advantages of the Hainan Free Trade Port, as well as policies such as visa-free entry for travelers from 59 countries and island-departure duty-free shopping, Hainan is becoming more convenient and welcoming for international visitors. I will share the beauty of Hainan with more friends, so the world can see the charm of the South China Sea.”

From a tiny sea slug less than one centimeter long to the coral reefs it calls home, this discovery offers a close-up look at the ecological richness of Wuzhizhou Island. It also highlights the pulling power of Hainan’s clear tropical waters, which continue to attract photographers, divers and nature lovers from around the world.