The unusual name comes from the slug's resemblance to sesame seeds. "Taiwanese divers call it 'sesame' in Chinese and it is also small like a sesame seed, hence the name," the research team explained.

The tiny nudibranch measures less than three millimeters in length and was first spotted in 2019 by lead author Ho-Yeung Chan during a recreational dive.

Chance Discovery Led to New Species Identification

The discovery happened unexpectedly while Chan was still 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.

At first, Chan did not realize he had found a species unknown to science. The breakthrough came after he contacted a sea slug expert online. According to the team, he "never realized Thecacera sesama was a new species until he consulted the sea slug expert 'Hsini Lin teacher' on Facebook."

Harsh Conditions Made the Research Difficult

Studying the species proved challenging because of the unpredictable conditions along Taiwan's northern coastline. Researchers explained that strong seasonal weather patterns greatly limit opportunities for underwater surveys.

Taiwan regularly experiences typhoons during the summer, while winter monsoon seasons bring large waves and colder waters. Sea temperatures can drop below 16 degrees Celsius, making diving conditions difficult for much of the year. Because of this, researchers are only able to conduct nudibranch studies for roughly four months annually.

The scientists noted that locating such tiny marine animals often depends heavily on luck due to their size and the limited research window.

Tiny Sea Slug With a Simple Lifestyle

Researchers observed that T. sesama appears to spend its life focused on just four main activities: feeding, searching, mating, and laying eggs. The species was found living on bryozoans, tiny aquatic invertebrates commonly known as "moss animals."

Interestingly, the bryozoan species serving as the sea slug's habitat may itself be previously unknown to science.

Hidden Ocean Species Still Waiting To Be Found

Nudibranchs, despite their small size, play an important role in marine ecosystems. "Nudibranchs are one of the key players in the marine food web," the research team explained. "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 researchers believe the discovery of Thecacera sesama may represent only a small glimpse into Taiwan's hidden marine biodiversity. Because many ocean species are tiny and easily overlooked, scientists suspect there are many more still waiting to be discovered.

The study describing Thecacera sesama was published in the open access journal ZooKeys on May 11, 2026.