For nearly 200 years, biologists felt they had a pretty good grasp of the nudibranch genus Thecacera. Consisting of six known species found in marine habitats around the world, these shell-less molluscs seemed as small as small could be, ranging from around half an inch to an inch long.
Then, scientists in Taiwan found a seventh species, one barely the size of a grain of rice.
It's taken seven years from first sighting by Ho-Yeung Chan (at the time an undergraduate at National Taiwan Ocean University) for this miniature marine animal to be officially described.
I live around 10 km (6 mi) north of where this new species was found near Keelung port, roughly a 30-minute drive from the capital city Taipei.
The coastline here is stunning – surf beaches, rocky coves, colorful fishing ports – with massive, deep-green mountains taking up space in every direction except eastward. There, you can only see ocean and the horizon line, interrupted by a few rocky green islands in the distance.
But it's also only diver-friendly for around four months a year, due to summer typhoons and dangerous swells. Great for surfing, not so good for diving, let alone discovering a well-camouflaged sea slug just a few millimeters in length.
So the chances of a diver finding a unique species of this size are pretty low. Chan reached out to a "sea slug expert" on Facebook, researcher Hsini Lin, who felt that this discovery was something entirely new in the nudibranch world.
Measuring less than 3 mm (0.12 in), this tiny, translucent sea slug with tiny black and yellow dots is now etched into the history books, joining its nudibranch relatives and earning its own spot as a unique species.
And the name? Thecacera sesama – because the researchers found that its size and shape resembled a sesame seed.
Of the six individuals collected, three played an important role in their classification – given their size, their entire bodies had to be used for DNA extraction. But through this work, the researchers found a 14.17% genetic divergence from the closest relative, Thecacera picta.
"Morphologically, T. sesama is uniquely distinguished by its translucent white body covered with numerous small, round, black pigment spots and fewer, larger yellow spots," the team described. "The phylogenetic analysis places T. sesama as a sister species to T. picta."
And this was within the range that warranted its classification as a new species of nudibranch.
“Taiwanese divers call it ‘sesame’ in Chinese, and it is also small like a sesame seed, hence the name,” the researchers noted.
Besides being adorable, T. sesama is basically a sea slug on a mission, showing only four behaviors: feeding, searching, mating, and laying eggs on mossy marine organisms called bryozoans.
Incidentally, the bryozoan that T. sesama favors may also be new, with sampling returning a mere 82% match with any known species.
“Nudibranchs are one of the key players in the marine food web, the team said. “They are extremely colorful and can be spotted on coral reef ecosystems. However, many nudibranchs are very small in size and are extremely difficult to spot underwater with the naked eye."
T. sesama is likely just the first of many undiscovered sea mollusc species living in the western Pacific marine waters around the island of Taiwan.
The study was published in ZooKeys.
Source: National Taiwan Ocean University via EurekAlert!
Fact-checked by Mike McRae
