Ho-Yeung Chan was on a recreational dive off northeastern Taiwan in 2019 when he noticed something odd clinging to what looked like a clump of underwater moss — a speck of an animal, barely the size of a grain of rice, translucent white and freckled, with tiny black and yellow dots.
He took a photo. Then he messaged a sea slug expert on Facebook.
That message to researcher Hsini Lin was the moment Chan learned he might be looking at something science had never formally described. Years later, the animal would become Thecacera sesama, a nudibranch (a type of soft-bodied marine sea slug) less than three millimeters long, officially described in ZooKeys by researchers from National Taiwan Ocean University, the National Museum of Natural Science, and National Taipei University of Education.
“Taiwanese divers call it ‘sesame’ in Chinese, and it is also small like a sesame seed, hence the name,” the research team explained in a press release.
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How a Tiny Sea Slug Discovered in Taiwan Became a New Species
Thecacera species with bryozoan
(Image Credit: Ho-Yeung Chan et al., 2026/ CC BY)
Turning one underwater photo into a formally described species took six years, and the weather didn't help.
The northeastern Taiwanese coast is only diveable for about four months a year. Summer typhoons, winter swells, and water temperatures that regularly fall below 60 degrees Fahrenheit (16 degrees Celsius) make nudibranch research unpredictable and difficult.
Chan returned to the site and began formally collecting specimens in 2021, two years after that first sighting. By 2025, the team had gathered just six individuals across multiple dives.
The sea slugs were so small that extracting DNA required using the entire bodies of three specimens, leaving the remaining animals intact for museum preservation.
From those samples, researchers sequenced two mitochondrial genes and found a 14.17 percent genetic divergence from the closest known relative, Thecacera picta. That falls within the typical 10 to 15 percent range used to distinguish separate nudibranch species.
Before this discovery, the genus contained only six formally recognized species, and no new member had been described in nearly three decades.
The Rice-Sized Sea Slug Spends Nearly Its Whole Life on Bryozoans
The whole body of T. sesama, including its gills, tail, and sensory organs, is dusted with small black circles and slightly larger yellow ones, all against a see-through white background. In some individuals, you can faintly make out the animal's internal organs through its skin.
Researchers watched the slug do essentially four things: eat, search, mate, and lay eggs, all on the same type of bryozoan colony, a tiny aquatic invertebrate sometimes called a "moss animal."
And that bryozoan may itself be new to science. Genetic analysis of the prey returned only about 82 percent similarity to any known species in the database, suggesting the slug's only known food source has yet to be formally identified.
Taiwan’s Reefs May Hold Many More Undiscovered Nudibranchs
Taiwan sits at the northern edge of the Indo-Pacific Coral Triangle, one of the most biodiverse marine regions on Earth. At the same rocky site where Thecacera sesama was found, the team also spotted Thecacera pacifica, Thecacera picta, and at least five additional Thecacera species that still do not have scientific names.
The nudibranch fauna of Taiwan remains poorly studied compared to nearby regions, especially because many species are extraordinarily small and difficult to spot underwater.
“Nudibranchs are one of the key players in the marine food web,” the team stated. “They are extremely colorful and can be spotted on coral reef ecosystems. However, many nudibranchs are very small in size and are extremely difficult to spot underwater with the naked eye.”
In waters explored by countless divers, a species smaller than a sesame seed managed to go unnoticed until now.
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Article Sources
Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:
- This article references information from a study published in ZooKeys:
