New Sea Slugs on Koh Tao and the Hidden Diversity of the Muck

Throughout the past several years, the program has been conducting ever increasing amounts of research on the invertebrate organisms of the island. In 2012, our research expanded into the complex and charismatic world of the nudibranchs, and shortly after, the remaining sea slugs. In December 2015, the program published the findings of the research in the journal Marine Biodiversity.

Sea slugs are an incredibly diverse and fascinating type of organism that are found in marine environments worldwide. You can read more about sea slugs and the roles they play in our learning resources segment here. At the program, sea slugs play important roles in two of our key activities, beyond the incidental observations doing restoration work. The first of these is the inclusion of nudibranchs (and by extension the rest of the sea slugs) in our Ecological Monitoring Program (EMP) surveys which you can read about here. The second is the significance of sea slugs in our muck exploration efforts which have led to a number of interesting findings. In 2012 the research started out mapping the inventory of all known species of sea slugs found on Koh Tao, almost 3 years later the findings of the research came to a number of interesting and regionally significant conclusions.


Sea Slugs on Koh Tao

To identify the baseline of what had been known thus far about the sea slugs on the island, data was collected via informal surveys with a number of island locals and historic inputs on online forums, including social media. Additional inferences were collected from historic EMP data. The collection of these data sources provided a rough idea of what species had been observed on the island and to some degree, how many and where they were to be found.

Following on from this came many many months of surveys both within and outside the coral reef areas to locate sea slug populations and species differences. Many of these surveys, particularly those done outside the reef area were tracked by GPS to provide an accurate idea of the area covered and distances surveyed.


A total of 87 species of sea slug were found during the length of this particular study, including 25 species that had never scientifically been recorded in Thailand and an additional 7 species were new records within the Gulf of Thailand. The number of species found seem to show an uncharacteristically high diversity for an island of its size in the Gulf of Thailand and account for a sizeable portion of sea slugs found in most recent published literature throughout the country. Both water bodies bordering Thailand have observed a relatively low species abundance when compared to nearby regions such as the Philippines.

Abundance estimates suggest that the vast majority of species found on the island are classified as rare according to the abundance scale developed. This was due to multiple factors such as cryptic sizes and colourations making it harder for many species to be seen by the unaided eye. Furthermore the geographic abundance found that most of the species observed on the island had only been found at one or two different locations on the island. The abundance data was largely collected from the informal surveys with local diving naturalists and sea slug enthusiasts. However the dominant factor found linked in with the habitat preference.

Habitat preference data suggested that almost half of all slugs found could be found in the muck habitat, with 80% of these specific to the habitat. Therefore, typical exploration of the reef would yield an incomplete picture of the true diversity and abundance of species that can be found in the vicinity of the island and surrounding sites. Over the next several months and years the program will continue our exploration of the muck and will hopefully uncover more of the secrets of the hidden muck.

A total of 32 new species records were found for the Gulf of Thailand region in this paper, however this whole story is still incomplete. During review and publication of this manuscript, new survey techniques and niches were explored yielding even further new records. These will be published at a later date and thus conclude inventory specific yield of this project. However we here at the team believe that new records will never truly stop and with so many species yet to be discovered, we look forward to pushing ahead with more in depth research on sea slugs of Koh Tao.

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48 Moo 3 Chalok Baan Kao
Koh Tao, Surat Thani
84360 Thailand

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