A recently discovered species of sea slug, measuring less than three millimeters, has been identified off the coast of Keelung in northern Taiwan. The translucent marine creature, marked with distinctive black and yellow patterns, has been officially named Thecacera sesama by researchers from National Taiwan Ocean University, the National Museum of Natural Science, and National Taipei University of Education.
The species name derives from its visual similarity to sesame seeds. “Taiwanese divers refer to it as ‘sesame’ in Chinese due to its small size and appearance,” explained the research team. The discovery occurred in 2019 when undergraduate student Ho-Yeung Chan stumbled upon the organism during a recreational dive.
Initial identification proved challenging, as Chan initially mistook the find for a known species. Breakthrough confirmation came after consulting sea slug expert Hsini Lin through social media. “Chan never realized Thecacera sesama was a new species until discussing it with ‘Hsini Lin teacher’ on Facebook,” researchers noted.
Studying this minute nudibranch presents significant logistical hurdles. Taiwan’s northern coastline experiences intense seasonal variability, including summer typhoons and winter monsoons with cold temperatures. With water temperatures falling below 16°C, researchers can only conduct systematic nudibranch surveys for approximately four months annually.
Lifestyle observations reveal Thecacera sesama engages in four primary activities: feeding, foraging, mating, and egg-laying. The species inhabits bryozoans—microscopic aquatic invertebrates also known as “moss animals”—which themselves may represent an undocumented species.
Nudibranchs serve crucial roles in marine ecosystems as key components of coastal food webs. Despite their ecological importance and vibrant coloration, many species are too small for easy underwater detection. This discovery suggests Taiwan’s waters may harbor numerous undiscovered marine organisms.
The research was published in the open-access journal ZooKeys on May 11, 2026.
