It looked like a sesame seed, and that turned out to be a pretty good clue.
Researchers in Taiwan have identified a new species of sea slug, Thecacera sesama, after finding the tiny nudibranch off Keelung in northern Taiwan. The translucent animal, marked with black and yellow, measures less than 3 millimetres in length.
The name comes from its appearance. “Taiwanese divers call it ‘sesame’ in Chinese and it is also small like a sesame seed, hence the name,” the research team said.
Lead author Ho-Yeung Chan first spotted it in 2019 during a recreational dive while he was an undergraduate student. “During a recreational dive in the summer during the undergraduate study of HY Chan in 2019, he accidentally discovered Thecacera sesama sp. nov. in northern Taiwan waters,” the researchers said.
Chan did not initially realise he had found a species new to science. The team said he “never realized Thecacera sesama was a new species until he consulted the sea slug expert ‘Hsini Lin teacher’ on Facebook.”
Researchers said studying the species was difficult because of conditions along Taiwan’s northern coastline. Taiwan gets typhoons in summer, while winter monsoons bring large waves and colder water. Sea temperatures can fall below 16 degrees Celsius, limiting nudibranch research to about four months each year.
The team said finding animals this small often depends heavily on luck because they are so hard to spot underwater with the naked eye.
Researchers observed that T. sesama appears to focus on four activities, feeding, searching, mating and laying eggs. It was found living on bryozoans, aquatic invertebrates also known as moss animals.
The bryozoan species that serves as its habitat may also be unknown to science.
“Nudibranchs are one of the key players in the marine food web,” the research team said. “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.”
The study was published in the open access journal ZooKeys on May 11, 2026.
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