Scientists have discovered a species of nudibranch barely the size of a grain of rice in Taiwan’s waters
By Khadeejah Masud
In the coastal waters of Keelung, Taiwan, a new species of nudibranch has been discovered by a group of scientists from National Taiwan Ocean University, National Museum of Natural Science and National Taipei University of Education.
Nudibranchs are soft-bodied marine molluscs that lack external shells. This newly-discovered nudibranch in Taiwan is a transparent organism with a distinct black and yellow speckled pattern resembling sprinkled sesame seeds, where it gets its name – Thecacera sesama. The species’ rhinophores — the rod-like antennae on its head that detect chemical signals in the water — emerge from flared sheaths, appearing like two spirals, and the frilly gills on its back are flanked by two fin-like appendages.
Measuring less than three millimetres in length, it was first spotted by lead author of the study Ho-Yeung Chan during a recreational dive in 2019.
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Specimens were collected in Mother Rock Bay, a convergence point for varying ocean currents, supporting an estimated 14 per cent of all nudibranch species found worldwide.
Typhoons between October and April pummel this coastline, so dives must be carried out between May and September. Researchers dive whenever the weather permits. It took them four years, with multiple dives between May 2021 and June 2025, to collect six specimens.
For marine biologists, this marks a milestone in the laborious process, as the western Pacific region – including the waters around Taiwan – represent a biodiversity hotspot for marine gastropods. The nudibranchs and fauna of this region remains poorly documented compared to other marine provinces.
From a broader ecological perspective, these vibrant molluscs play a vital role in the marine environment. ‘Nudibranchs are one of the key players in the marine food web’, the team stated. ‘They are extremely colourful 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.’
How have others encouraged faster nudibranch discovery?
Elsewhere in the world, nudibranch discovery is a rapidly expanding area of research.
Senior curator at the California Academy of Sciences Terrence Gosliner has emphasised the importance of community efforts in assisting with nudibranch discovery. Specialising in Anilao, the Philippines, he highlights the magnitude of this kind of research. He and other scientists along this coastline have enlisted groups of divers as citizen scientists to assist in the challenge of identifying new species of nudibranch.
‘Having citizen scientists identify species adds critically important biodiversity data for establishing priority areas for conservation,’ he said. Evidently, it is crucial to incorporate initiatives between scientists and the public to accelerate the rate of discoveries of such small species.
Back in Taiwan, the researchers believe that the discovery of T. sesama is just the tip of the iceberg for Taiwanese marine biology. Because many species are so small, many more are likely awaiting discovery and formal study.
