Across Taiwan, rushing streams of water are filled with stones — serving a much bigger purpose. Hidden beneath the rocks are colorful, trapezoid-shaped creatures: freshwater crabs.

Taiwan has been home to freshwater crabs, specifically the Candidiopotamon and Geothelphusa genera, since its geological inception some 5 million years ago. In recent decades, scientists have worked to identify the different species of crabs belonging to each genus.

While experts have identified nearly 40 species and counting of Geothelphusa, they have only discovered one species of Candidiopotamon — until now, according to a new study published Sept. 11 in ZooKeys.

Researchers recently discovered a pseudocryptic species of Candidiopotamon, known as Candidiopotamon penglai. The discovery came after genetic testing revealed that what was long considered one species of Candidiopotamon, known as Candidiopotamon rathbuni, is actually two species.

The newly discovered species of crabs have a subtrapezoidal carapace or shell that is typically dark red, orange or purple, scientists said. Smaller adults can be dark brown with black spots and younger individuals are often more reddish. The creatures have a flat and rough exterior.

The crabs live beneath large stones in streams, but they sometimes move outside the water at night, experts said. They are found in the eastern side of the Central Mountain Range of Taiwan, which runs north to south in the center of the island.

Scientists said they first discovered the creatures in Ji-an, Hualien, which is about 70 miles south of Taipei. The species is also found in other parts of Hualien County, in Taitung County and on the Hengchun Peninsula.

C. penglai was identified based on its distinctly shaped back legs, known as gonopods, the study said. DNA analysis found that the new species had between approximately 11% and 14% genetic divergence from C. rathbuni.

The species was named after Penglai, which was an ancient name for Taiwan, the study said. Its name is also a nod to the Penglai Orogeny — a movement in Earth’s crust that likely formed the island of Taiwan — as Candidiopotamon likely colonized the island at the time of its creation.

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