Ho-Yeung Chan was conducting a recreational dive off the coast of Keelung, Taiwan, in the summer of 2019 when he spotted something extraordinary—a translucent creature barely the size of a grain of rice, with delicate black and yellow markings. He didn't realize in that moment that he'd discovered a species entirely new to science. It took a chance conversation on Facebook with sea slug expert Hsini Lin to reveal what Chan had found: Thecacera sesama, a nudibranch so tiny and elusive that its discovery speaks to just how much of the ocean's life remains hidden from human view.
The newly identified sea slug, measuring less than three millimeters in length, has been formally documented by researchers from National Taiwan Ocean University, National Museum of Natural Science, and National Taipei University of Education in the journal ZooKeys. Its name honors both the creature's diminutive size and the affectionate local nickname Taiwanese divers had already given it—"sesame"—a fitting tribute to something as small as a sesame seed.
The species' presence in Keelung waters is a reminder of Taiwan's remarkable marine biodiversity, but the actual research process reveals just how challenging it is to study such minuscule creatures. The waters off Keelung are notoriously volatile. Taiwan's northern coast experiences frequent typhoons during summer months and large waves during the winter monsoon season, with sea temperatures often dropping below 16 degrees Celsius. These conditions mean that meaningful diving expeditions for nudibranch research are only possible for about four months of the year, making each sighting of Thecacera sesama genuinely a matter of chance.
Despite their tiny size, nudibranchs like T. sesama play a vital role in marine ecosystems. The research team notes that these mollusks serve as key players in the marine food web, consuming bryozoans—minute aquatic invertebrates sometimes called "moss animals"—that form their entire habitat. The species exhibits remarkably focused behavior, with researchers observing just four primary activities: feeding, searching, mating, and laying eggs on bryozoans. Intriguingly, the specific bryozoan species that T. sesama depends on may itself be undescribed by science, suggesting that a single discovery can open windows onto entire unexplored relationships in nature.
What makes this discovery particularly significant is what it suggests about the ocean's remaining mysteries. Nudibranchs are renowned for their brilliant colors and striking appearance, yet many species are so diminutive that they remain invisible to the human eye even underwater. Researchers emphasize that T. sesama is "just the tip of the iceberg for Taiwanese marine biology." The small size and cryptic nature of countless marine organisms mean that many more species are almost certainly awaiting formal identification and study.
Chan's accidental discovery during his undergraduate years demonstrates how science still depends on curiosity and serendipity as much as institutional resources. A young diver's observant eye, combined with modern networking tools and expert consultation, transformed a chance encounter into a formal scientific contribution. As climate pressures mount on coastal ecosystems, discoveries like Thecacera sesama remind us that we may never fully know what we stand to lose—or what remains to be found in the waters we share.