Tag: sea slug

Tiny sesame sea slug species discovered in the waters of northern Taiwan

This tiny nudibranch, which measures less than three millimetres in length, was first spotted by lead author Ho-Yeung Chan during a recreational dive in 2019.

Translucent, speckled, and barely the size of a grain of rice, a new species of sea slug has been identified in the coastal waters of Keelung, Taiwan. Because of its minute size and distinctive black and yellow markings, researchers from National Taiwan Ocean University, National Museum of Natural Science and National Taipei University of Education have named the creature Thecacera sesama.

“Taiwanese divers call it ‘sesame’ in Chinese and it is also small like a sesame seed, hence the name,” the research team explained regarding their decision to honour the local nickname in the scientific nomenclature. This tiny nudibranch, which measures less than three millimetres in length, was first spotted by lead author Ho-Yeung Chan during a recreational dive in 2019.

Thecacera sesama sp. nov. Details of appearance and morphological features, hand-drawn on a tablet PC by Chen-Lu Lee. 

The discovery was a stroke of luck that began during Chan’s undergraduate studies:

“During a recreational dive in the summer during the undergraduate study of HY Chan in 2019, he accidentally discovered Thecacera sesama sp. nov. in northern Taiwan waters.”

The Research Team

Despite its unique appearance, the importance of the find was not immediately obvious. In a modern twist on traditional taxonomy, Chan “never realised Thecacera sesama was a new species until he consulted the sea slug expert ‘Hsini Lin teacher’ on Facebook.”

Living specimens of Thecacera sesama sp. nov. Image credit: Ho-Yeung Chan et al.

Documenting the species proved to be a significant logistical feat due to the volatile environment of the Keelung coast. The research team noted that the most challenging part of the study was the unique weather conditions of the region.

Taiwan experiences frequent typhoons in the summer and large waves during the winter monsoon season, with sea temperatures often dropping below 16 degrees Celsius. These factors mean that diving for nudibranch research is only possible for about four months of the year, making sightings of such tiny creatures entirely a matter of chance.

Living specimens of bryozoan with Thecacera species. Image credit: Ho-Yeung Chan et al.

The life of T. sesama is remarkably focused, as the researchers observed that the species exhibits only four primary behaviours: feeding, searching, mating, and laying eggs on bryozoans, which are tiny aquatic invertebrates often called “moss animals”. Interestingly, the specific bryozoan that T. sesama calls home may itself be a species new to science.

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. 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.”

The Research Team

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. The full research on Thecacera sesama was published in the open-access journal ZooKeys on 11 May 2026.

Original source:

Chan H-Y, Lee C-L, Chen W-C, Chang C-H, Shao Y-T, Pang K-L (2026) Thecacera sesama sp. nov. (Nudibranchia, Polyceridae) from Taiwan, evident from morphology and phylogenetic analyses of the 16S rDNA and cytochrome c oxidase I gene. ZooKeys 1279: 269-284. https://doi.org/10.3897/zookeys.1279.184298

Cover image:

Two individuals of Thecacera sesama sp. nov. feeding on a bryozoan. Image credit: Ho-Yeung Chan et al.

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Two new species of wart sea slugs discovered from North Sulawesi, Indonesia

The discovery adds to the rich biodiversity of the Indo-Pacific region.

Five women scientists from Germany, Indonesia, and Wales have discovered two new species of wart sea slugs from North Sulawesi, Indonesia — Phyllidia ovata and Phyllidia fontjei.

Wart sea slugs in the family Phyllidiidae are common inhabitants of the Indo-Pacific Ocean, preying on sponges and stealing toxins from their prey for their own defence. To scientists and nature enthusiasts alike, they are renowned for their warning coloration and their chemical defence mechanisms. Approximately 350 species of sea slugs have been documented in North Sulawesi, and some 100 of these are new to science and still need to be formally identified, but now, two colourful species, much rarer than most of their relatives, have been named and described.

  • A photo of a vibrant black and white nudibranch with orange tips resting on the ocean floor.
    Phyllidia schupporum. Photo by Rafi Amar
  • A photo of a colorful nudibranch with a black and pale pink mottled pattern crawling over a sandy ocean floor.
    Phyllidiopsis sinaiensis. Photo by Nathalie Yonow
  • A photo of a vibrant sea slug with a bumpy, orange and yellow patterned body resting on the ocean floor, surrounded by underwater vegetation.
    Phyllidiopsis cardinalis. Photo by Sven Kahlbrock
  • A colorful sea slug with a pink-orange body and black stripes is photographed on rocky ocean floor, surrounded by algae and coral.
    Paradoris hypocrita. Photo by Sven Kahlbrock
  • Close-up photo of a nudibranch with a pink body and prominent dark stripes, resting on a sandy ocean floor.
    Phyllidiella zeylanica. Photo by Sven Kahlbrock
  • A photo of a vibrant, pale purple and black sea slug resting on sandy ocean floor.
    Phyllidia varicosa. Photo by Hsini Lin

Phyllidia ovata was named in reference to its unique appearance that resembles an egg, both in its shape and its pattern. It has been photographed several times by enthusiastic underwater photographers in Indonesia, Japan, Taiwan, the Philippines, and Australia during the past 23 years, but was only recognised as a new-to-science species now. This medium-sized, up to 5 cm long, wart sea slug has finally been given a name based on an animal collected by a scuba diver in North Sulawesi.

Phyllida Ovata. Photo by Heike Wägele

Phyllidia fontjei was named in honour of the late Dr. Fontje Kaligis, an Indonesian researcher who was instrumental in opening new avenues of international cooperation for the advancement of our knowledge of the — sometimes hidden — biodiversity in North Sulawesi. With a maximum documented size of 16 mm, this small species of wart sea slug is difficult to find. Nevertheless, Phyllidia fontjei has been photographed during the past 15 years – in Indonesia and Malaysia, but is more common in the Andaman Sea in the Indian Ocean. This species has been described based on a single reference animal, which scientists refer to as a holotype. The holotype has been histologically examined, allowing for a very detailed analysis of its anatomy.

Phyllidia fontjei. Photo by Heike Wägele

To a large extent, these discoveries were made possible thanks to people who do not have formal scientific education but are passionate about documenting and protecting Earth’s diversity. Photographs and data posted on citizen science platforms like iNaturalist, social media sites like Facebook, and dedicated sea slug communities such as NudiPixel and the now defunct Sea Slug Forum, provided the researchers with vital information for identifying these nudibranchs as new to science and establishing a more realistic geographical distribution when only few specimens are available for study.

“We all use these platforms in many different fields of taxonomy as they provide useful records when the species are distinctive, and have done so for more than two decades,” says Dr Nathalie Yonow of Swansea University, one of the authors of the study.

The discovery has been published in the open-access journal ZooKeys.

Research article:

Wägele H, Raubold LM, Papu A, Undap N, Yonow N (2025) On two new Phyllidia species (Gastropoda, Nudibranchia, Doridina) and some histology from the Coral Triangle. ZooKeys 1245: 1-18. https://doi.org/10.3897/zookeys.1245.153046