In the coastal waters off Keelung, Taiwan, marine researcher Ho-Yeung Chan was on a recreational dive in 2019 when he spotted something extraordinary in its ordinariness: a creature so small it could fit on the head of a pin, its translucent white body scattered with black and yellow spots that resembled sesame seeds sprinkled across a surface. The undersea explorer had no idea at the time that he was looking at an entirely unknown species—one that would take six years, multiple diving expeditions, and genetic analysis to formally describe and name.
That tiny sea slug is now Thecacera sesama, a species that belongs to the family of nudibranchs, or marine snails without shells, and it represents something far larger than its three-millimeter frame suggests. It is the first member of its genus to be formally named in nearly three decades, a marker of how much remains hidden in the world's oceans, even as we believe we have catalogued most of what lives there. The discovery also underscores a deeper truth: the planet's marine biodiversity is far richer than we know, concentrated in hotspots like the Western Pacific that remain poorly explored and documented.
When Chan first spotted T. sesama, he was still an undergraduate student. Rather than keeping the discovery to himself, he reached out to an expert on Facebook—a digital bridge between curious observer and scientific knowledge that proved decisive. The consultation led to a formal research effort. Between May 2021 and June 2025, scientists conducted diving expeditions to collect six specimens of the creature. The work was painstaking and constrained: the researchers had to navigate a narrow window of opportunity each year, because typhoons and cold water temperatures make diving dangerous and difficult for most of the year. From May through September, the waters off Taiwan become too treacherous for safe underwater research.
Back in the laboratory, the team examined the specimens' physical structure and appearance, then analyzed their DNA to confirm that T. sesama was genuinely new to science. The results revealed a species that is visually striking despite its diminutive size. The translucent white body and its distinctive sesame-like markings distinguish it from a similar species, Thecacera pennigera, which has black and orange spots and is notably larger. More importantly, genetic analysis confirmed that T. sesama is fundamentally different.
The new species feeds exclusively on bryozoans—small aquatic invertebrates sometimes called "moss animals" that live together in colonies. This specialization hints at the intricate relationships that structure marine ecosystems, even at scales invisible to the naked eye.
What makes this discovery particularly significant is what it suggests about what we have yet to find. The researchers noted that many other cryptic species of nudibranch likely remain undiscovered in the region's overlooked habitats. These creatures, the team emphasized, "are one of the key players in the marine food web," yet their small size and camouflage make them extraordinarily difficult to spot underwater. The Western Pacific, already recognized as a marine biodiversity hotspot, may hold far more secrets than we realize. The discovery of T. sesama is not just a win for taxonomy—it is an invitation to keep looking, to dive deeper, and to recognize that wonder still waits in the world's overlooked corners.