Meet the New Sea Slug So Small It’s Named After a Sesame Seed

A sea creature so tiny it could sit on your fingertip and still leave room to spare — and it just became the seventh member of its entire family to be discovered in 30 years.

Divers off Keelung, in northern Taiwan, kept spotting a translucent, black-and-yellow speckled nudibranch smaller than a grain of rice. Locals had a nickname for it: “sesame.” Now it’s official — scientists have confirmed it as a brand-new species, and they kept the name. Meet *Thecacera sesama*.

The story behind the discovery is half the fun. Lead researcher Ho-Yeung Chan first spotted the slug back in 2019 during a casual recreational dive as an undergraduate student. He had no idea what he’d found — until he sent photos to a sea slug expert he knew through Facebook. That tip set off years of follow-up dives in Keelung’s Mother Rock Bay, a spot that’s only diveable part of the year thanks to typhoons and monsoon swells. It took the team until 2025 to gather enough specimens to confirm it.

DNA analysis sealed the deal: this wasn’t just a small version of a known species. It’s a new one, described in the journal *ZooKeys* in May, and the first addition to its genus in nearly 30 years. Researchers found it feeds exclusively on bryozoans — tiny “moss animal” colonies that carpet the reef.

Why it matters: Ocean biodiversity is often measured in whales and reefs, but stories like this are a reminder of how much life is hiding in plain sight, waiting for someone curious enough to notice. Scientists say there are likely many more species like *Thecacera sesama* still unnamed, right off coastlines people swim past every day.

Small in size. Big on proof that the ocean still has surprises left to give.

Sources: Pensoft Publisher