A new species of coralllivorous nudibranch has just been discovered by our friend and colleague, Dr. Rahul Mehrotra. The new species, which he named Phestilla viei, was described in a recent article in the journal Marine Biodiversity, titled “A new species of coral-feeding nudibranch (Mollusca: Gastropoda) from the Gulf of Thailand.” Not only does this paper provide record of this new species, but it also helps to clarify the systematics of the genera, which currently contains 8 coral-feeding species that have been revised several times over the last few decades.

phestilla viei nudibranch

This study was conducted as part of Rahul’s PhD, which has yielded several other impressive firsts for the world of sea slugs, including the observation that there are corals which feed on sea slugs, as well as three other first records of species found from Koh Tao; including Unidentia aliciae in 2019, as well as Arminia occulta and Armina scotti in 2017.

The new species, Phestilla viei, was first found on the island of Koh Tao, Thailand by Conservation Diver board member, Spencer Arnold, who was a co-author on the paper. He found the first individual on a coral of the species Pavona explanulata, a resilient coral species which is becoming more prevalent on the island as it replaces some of the less robust species in the face of climate change and localized anthropogenic stresses. The novel species was named after local nudibranch enthusiast and citizen scientists, Mr. Vie Panyarachun, well known for his vast contributions in recording and curating sea slug records from Thai waters.

After the first observation, Rahul and his team conducted a series of surveys to locate more individuals and evaluate their ecology, anatomy, and genetics. All of the individuals found were on the same species of coral, implying that they are an obligate parasite of the sceleractinian, Pavona. Because of this obligate relationship, the morphology of the nudibranch is such that it closely resembles the coral, making it very difficult to find. Rahul and his co-authors stated in the paper that the easiest way to find them is to actually look for the feeding scars and egg trails, and then to look for the individuals making them.

Another amazing trait which makes this cryptic species so difficult to observe is that it actually ‘steals’ symbiodinium (the unicellular algae which power the coral) and integrates them into their bodies rather than digesting them, in a process termed “kleptosymbiosis.” Although this species is a corallivore, it does not appear to be a major threat to coral populations, as it was never observed to eat the entire coral, and no overpopulations or outbreaks have been documented.

Although this was the first scientific description, observations of the species have been made in other parts of the Western Pacific, including Indonesia, Madagascar, Papua New Guinea, and the Philippines. Now that more divers know about this species, and where to look for it, surely it will be observed in many other countries over the next few years.

After reading the article, we had some questions for Dr. Mehrotra, so we interrupted his work to interview him about his findings and current projects.

Q1: This is a very interesting finding, when did you first become aware of these species, and how did you first spot it?

I came across reference to a similar species in a couple of references, however with very little corresponding information. After finding a paper documenting this from Hong Kong feeding on Pavona decussata, I considered the possibility that we may have this on Koh Tao. The truth is, I simply needed to share this paper with Spencer who used his remarkable skills in a matter of weeks to find a similar species feeding on a different prey at Koh Tao. 

Q2:As this is the fourth new species you have described for the region, do you think Koh Tao is a particular hotspot for sea slugs, or do you expect other regions in the Gulf to have similar levels of biodiversity?

This is the fourth species of nudibranch to be described from Koh Tao, which has indeed been the home of much of the sea slug research from Thailand over the past decade. However, our data from other locations suggests that, while Koh Tao does seem to offer a diversity of species distinct from what is commonly found elsewhere in the Gulf, the richness of discoveries is largely due to the focused study in the area. Many places in Thailand appear to host species that are awaiting a closer investigation.

Q3: You named this nudibranch after Mr. Vie Panyarachu, how has he influenced your work over the years, or the status of nudibranchs in Thailand in general?

Vie Panyarachun is a keen recreational diver and naturalist who has played a remarkable role in curating records of sea slug diversity in the country. His work as a citizen scientist has helped create a repository of species as well as playing an active role in sharing the world of sea slugs with the SCUBA diving community in Thailand.

Q4: why do you think it is important that populations of nudibranchs in the region are recorded and monitored?

Nudibranchs and other sea slugs have been shown to play a variety of roles economically and ecologically, however few of these have been investigated thoroughly. Monitoring sea slug diversity has already led to remarkable documentation on the hidden impacts of climate change and species invasions. The importance of monitoring sea slugs in the Western-Pacific, home to the most biodiverse marine environments in the world, allows us to dig deeper into the complex ecology of the region. In a time where hundreds of invertebrate species are known to have been regionally displaced or gone extinct only to be discovered/rediscovered after the fact, detailed monitoring of such understudied groups may allow us to catch large scale threats before they take hold. Many sea slugs have been associated with very specific prey organisms, and thus the fate of any given species is also intricately tied to the resilience of its prey. The species we describe here for example has been found exclusively on a single species of coral, with similar species of coral either hosting their own distinct species of nudibranch, or none at all. Many reef building corals such as the prey of P. viei are susceptible to threats such as coral bleaching. We have little evidence at present to suggest that populations of P. viei would could show enough dietary plasticity to shift hosts to a more resilient prey should these threats escalate. 

Q5: What are you currently working on, or towards, regarding sea slugs in Thailand?

I am in the process of completing my work from Koh Tao, documenting further ecologically relevant observations. My work on sea slugs will be transitioning to other fields soon too, so as to provide a more solid framework for further research on sea slug biology in Thailand. 

We want to pass our sincerest congratulations to Rahul for this valuable and important work, and wish him all the best in his future ventures, which we follow very closely and support in any way we can. We also want to congratulate Spencer Arnold on his find, and his invaluable assistance to this study, he has an eye for finding tiny sea slugs like no one else. We also want to congratulate Mr. Vei on the species being named for him, and for all his contributions to the field over the years.

phestilla nudibranch
A close-up of Phestilla viei on it’s host coral