Tiny 'Sesame' Sea Slug Found in Taiwan After 30 Years

A tiny sea slug smaller than your fingernail is making waves in marine biology, and it looks just like it's been sprinkled with sesame seeds.

Undergraduate student Ho-Yeung Chan spotted the miniature marvel during a recreational dive off the coast of Keelung in northern Taiwan back in 2019. At just three millimeters long, the black and yellow spotted creature caught his eye, but he had no idea he'd just discovered a species completely unknown to science.

After posting about his find on Facebook and consulting with experts, Chan realized this wasn't just a rare sighting. Between May 2021 and June 2025, researchers collected six specimens during carefully timed diving expeditions, working around Taiwan's challenging typhoon season that makes underwater exploration risky for much of the year.

DNA analysis and physical examination confirmed what Chan suspected: this was indeed a brand new species. The research team officially named it Thecacera sesama, after its distinctive sesame seed appearance, making it only the seventh species ever described in the Thecacera genus.

The translucent white sea slug stands out against its underwater home with its scattered black and yellow spots. While it resembles another species called Thecacera pennigera, which sports black and orange spots, T. sesama is significantly smaller and genetically distinct.

Scientists discovered that these tiny creatures live exclusively on bryozoans, small aquatic invertebrates nicknamed "moss animals" that cluster together in colonies. The sea slugs both make their homes on and feed from these organisms, carving out a specialized niche in Taiwan's coastal ecosystem.

The Ripple Effect

This discovery shines a spotlight on just how much marine life remains hidden in plain sight. The Western Pacific is recognized as a marine biodiversity hotspot, yet countless smaller organisms remain poorly documented simply because they're incredibly difficult to spot with the naked eye.

Taiwan's challenging research conditions make discoveries like this even more remarkable. Seasonal typhoons and cold water temperatures limit safe diving to just a few months each year, meaning scientists have narrow windows to explore these underwater worlds.

The researchers believe many more cryptic sea slug species are waiting to be discovered in the region's overlooked marine habitats. These colorful nudibranchs play key roles in marine food webs and coral reef ecosystems, yet their tiny size makes them nearly invisible to casual observers.

Sometimes the most stunning discoveries come in the smallest packages, reminding us that our oceans still have countless stories left to tell.