According to reports, the biological community has been aware of the genus Thecacera for nearly 200 years. There are six previously known species in this genus, which are distributed in various marine habitats around the world. They are usually only about half an inch to an inch long, which is quite small. Recently, Taiwanese scientists discovered a seventh species of this genus, which is even less than the size of a grain of rice.
The discovery dates back to 2019. At that time, Chen Haoyang, an undergraduate student at Taiwan Ocean University and one of the first authors, first noticed this tiny marine creature during a recreational dive. Due to the extremely small size and strong camouflage, and because the northern waters of Taiwan are affected by summer typhoons and dangerous surges, the time suitable for diving is only about four months a year. It is obviously difficult to discover a new species that is only a few millimeters long in this environment. He contacted sea slug researcher Lin Siyu through Facebook. The latter judged that this is likely to be a brand new discovery in the world of sea slugs.
After seven years of research and identification, the tiny, translucent sea slug with black and yellow spots has finally been officially described and named Thecacera sesama. Researchers chose this name because its size and shape resemble sesame seeds, and Taiwanese divers often call it "sesame". The species is less than 3 millimeters long and is so small that the research team even had to use some of the collected specimens whole for DNA extraction.
Genetic analysis shows that there is a 14.17% genetic difference between Thecacera sesama and its closest relative, Thecacera picta, which is enough to support its identification as a new species. Morphologically, its most striking feature is that it is translucent white, covered with a large number of small round black spots, accompanied by fewer but larger yellow spots. Phylogenetic analysis also showed that it is a sister species to T. picta.
The researchers also observed that the sea slug's behavior is relatively simple, including foraging, finding a mate, mating, and laying eggs on a type of sea creature called a bryozoan. Even more concerning is that the bryozoan host it relies on may also be a new species, as the sample matches only 82% of known species.
The research team said that sea slugs play an important role in the marine food web. They are brightly colored and commonly found in coral reef ecosystems, but many species are extremely small and difficult to detect underwater with the naked eye. Researchers believe Thecacera sesama may be just one of many undiscovered marine molluscs in the western Pacific waters surrounding Taiwan. The research has been published in the journal ZooKeys.