Marine researchers have identified a new species of nudibranch, named Thecacera sesama, discovered in the coastal waters of northern Taiwan.

The discovery highlights how citizen science and recreational diving can lead to significant biological finds, expanding the known biodiversity of the region's marine ecosystems.

The specimen was first observed during a recreational dive in early 2025 [1]. Following the initial sighting, a sea-slug expert and other researchers conducted a detailed investigation to determine the animal's identity [2]. The formal description of the new species was released on May 26, 2026 [1].

Located in the waters off Keelung, northern Taiwan [3], the creature is remarkably small. Its maximum body length is approximately three mm [4]. While some reports compare its size to a grain of rice, other researchers describe it as being smaller than a sesame seed [1].

Scientists used a combination of morphological analysis and DNA sequencing to confirm the discovery [2]. The genetic data showed the animal did not match any known nudibranch species, which prompted the team to officially describe it as a new taxon [2]. The species is characterized by a body covered in spots resembling sesame seeds.

This process of identification, moving from a casual observation to a formal scientific description, underscores the importance of molecular tools in modern taxonomy. Because the animal is so small, visual identification alone was insufficient to distinguish it from similar species. The use of DNA analysis provided the definitive evidence required for the new species designation [2].

“Thecacera sesama is smaller than a sesame seed.”

The identification of Thecacera sesama demonstrates the critical role of DNA barcoding in discovering 'cryptic species'—animals that look nearly identical to known species but are genetically distinct. As researchers continue to catalog the microscopic life in Taiwan's coastal waters, these findings may reveal a much higher level of marine biodiversity than previously estimated using traditional visual surveys.