Sea Slug The Size Of A Sesame Seed Officially Named A New Species

A spotted sea slug no bigger than a sesame seed has been officially recognized as a new species, after crawling along reefs off northeastern Taiwan for years before scientists finally gave it a name.

Researchers working near New Taipei City have formally described Thecacera sesama, a miniature sea slug belonging to a group called nudibranchs, colorful shell-less relatives of snails and clams. Its see-through white body is covered in tiny black dots and larger yellow spots that reminded the team so much of scattered sesame seeds that they named the new species after the plant. At less than 3 millimeters long, about the length of a sesame seed, it is exceptionally small even by nudibranch standards.

Writing in the journal ZooKeys, the researchers noted that the discovery “highlights the rich, yet under-documented, marine biodiversity of Taiwan,” a region at the northern edge of one of the most species-dense ocean zones on Earth. Systematic surveys of nudibranchs in Taiwanese waters have historically been limited, leaving many animals without names or recognition. Thecacera sesama is also the first new member of its genus to be formally described in nearly three decades.

Scientists Found the New Sea Slug Species at a Single, Hard-to-Reach Reef

Six specimens were collected by scuba divers over multiple expeditions between May 2021 and June 2025 at a single reef site called Mother Rock Bay, at depths of 18 to 30 meters. Diving there is only possible during certain months, since cold temperatures and strong waves close the site from October through April, and typhoons disrupt trips during summer. That narrow window of access may partly explain why the species stayed off the scientific radar for so long.

Up close, the animals have see-through white bodies that let faint traces of internal organs show through the skin. Five feathery gills sit on each animal’s back, along with a pair of prominent finger-like projections, features arranged and colored in a way unique to this species.

Thecacera sesama Is Ten Times Smaller Than Its Closest-Looking Relative

Once researchers knew what to look for, telling this species apart from its relatives was not difficult. Thecacera pennigera, a well-known sea slug found across the world’s oceans, shares a spotted body pattern but can reach up to 25 millimeters in length, roughly ten times larger than the new species.

Another close relative, Thecacera picta, lives in the same habitat in Taiwan. That species carries black and orange markings on its sensory organs and body projections, while T. sesama is transparent white with yellow and black spots and no orange coloring. Juvenile T. picta look somewhat similar, which may have contributed to past confusion.

DNA Results Put the New Species on Its Own Branch of the Family Tree

Because the specimens were so small, the whole body had to be used to extract genetic material for testing. To preserve the remaining specimens in museum collections, only three of the six animals were used for DNA analysis. Scientists examined two stretches of genetic code and compared results against a global database of known organisms.

Both genes showed differences between T. sesama and T. picta well within the range scientists typically use to confirm two animals are separate species rather than geographic variations of the same one. Genetic variation among the six T. sesama specimens themselves was minimal, less than half a percent, suggesting a genetically unified species despite being collected over four years.

Evolutionary reconstructions using two different analytical methods placed T. sesama in its own distinct branch, closest to T. picta, confirming it as a genuinely new species rather than a regional variant of one already known.

A Picky Eater With a Potentially Unnamed Meal

Like its relatives, T. sesama appears to be a picky eater. Researchers found it feeding on bryozoans, small moss-like colonial animals that grow on reef surfaces and are easy to overlook. Specimens were consistently found living directly on or next to these colonies, and the researchers observed the slugs actively rasping away at the tissue.

What made the situation even more unusual is that the bryozoan being eaten could not be conclusively identified. When scientists sequenced its DNA, the closest database matches came in at only about 82 percent similarity, far too low to make a confident identification. The prey may itself be an undescribed species, meaning a newly named predator and a potentially unnamed meal have been sharing the same small patch of reef.

Other nudibranch species, including T. pacifica and T. picta, were also observed feeding and laying eggs on the same bryozoan at the site, a corner of the ocean floor far more biodiverse than it appears at a glance.

Thecacera sesama brings the total formally recognized species in its genus to seven. Its naming is a reminder that entire species can exist just beyond the reach of scientific notice, living out their lives on reefs researchers have visited many times before. With so much of Taiwan’s coastal marine life still undocumented, and given that this new species may well range beyond Taiwan’s waters, there are almost certainly more animals like it still waiting to be found.

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