Diversity In a Tight Space: A Study of Sea Slug’s Abundance at Sempu Strait

Essay Hydrophis Archive

Sempu Strait, a narrow water body that separates the Mainland Java with Sempu Island. This narrow three kilometers strait extends for about 300 meters at it nearest point, making it nearly possible for people to get to the otherside just by swimming!. But seriously though, don’t even try it, especially when it’s the peak of fishing season where about three hundred fishing boats would come and go from the nearby Pondokdadap Fishingport that located right at the heart of this strait.

The Sempu Island itself, is a 877 hectares uninhabited small island that have been declared as Nature Reserve (Id: Cagar Alam) area by the Dutch since March, 15th 1928. Well, it’s been 92 years since its inception by now, but despite this island boasts its status as Nature Reserve, the overall biodiversity inside this island is so poorly documented. Only at 2018, when Indonesian Institue of Science (Id: Lembaga Ilmu Pengetahuan Indonesia; LIPI) published its comprehensive study around the overall terrestrial plant’s diversity, hence a piece of info around biodiverity inside this island is eventually revealed. Then at 2019, the BKSDA (Indonesian Nature Conservation Agency) launched its book around the bird’s diversity in this Island. This recent publication also adds a valuable resource despite it needs more than 90 years for the Indonesian Government to finally fulfill the most basic data around biodiversity on its one of the most protected area on this country.

School of barracudas on the Tiga Warna Waters. Originally posted on my instagram @anthonikovii

Albeit the information around terrestrial life inside this island is slowly getting built by those publications, there’s still one thing that everyone might not giving any attention. How about the life under the water? Until now, there is no way I could know how much species of marine lifeforms inside this strait’s water as there are no basic data that available. Whereas, the fishing industry has been lurking in this tight space for more than two decades, and now we are on a race with overexploitation as a study revealed that there are no adult groupers or snappers could be found on numerous spot on this strait during the daylight.

As the informations about the fishes are scarce, so does the information on the lower taxa of the ocean’s denizens. As an ecosystem, coral seas are very dependent on every small aspects of its environment. They are all connected through a chain of relations that bring life to its own little world. At 2019, one tiny creature caught my attention that made me publish an article about it existence on this strait. As you guess, that tiny creature is sea slug, a soft bodied, shell-less gastropod with vivid coloration and striking appearance. This creature is so tiny that some of its species are only as big as your fingertip, and finding them inside the vastness of the underwater realm is not an easy job for everyone.

Bursatella leachii, one of the sea slug species I’ve encountered during this study. Originally posted on my instagram @anthonikovii

Seeing the unseen, revealing the unknown

Sea slugs, are feeding exclusively on certain organisms, like poriferans, cindarians, ascidians, and algae that also associated within the coral reef ecosystem. So that studying this little organism’s biodiversity could bring us to a bigger frame for understanding the overall biodiversity around the underwater organisms on this area.

is tiny creature. At 2019, I dived on eleven dive sites with my buddy, covering areas from 0 to 25 meters depth with around 50 minutes spent underwater on each occasions (might be shorter if we dived more than once a day). On every occasion, we scanned the substrate, desperately hoping for a striking and unusual coloration to be magically appeared among the gloomy colored background. We dived into the rocky ledges, the murky silt-bottomed water, the colorful reef patch that teeming with fish, and diving among the debris just to get a glimpse of this tiny creature.   

After hundreds minutes spent underwater and thousands of photos taken, we finally revealed something. Sempu Strait, as we know, holds around forty five species of sea slug on its water, and if we dissect its species number depends on its respective orders, we have 35 species that belong  to  the  Nudibranchia  (robust nudibranchs),  6  species that belong  to  the  Aplysiida /Anaspidea (sea hares),  2 species that belong  to  the  Cephalaspidea  (headshield slugs and relatives),  and  2  species  belong  to  the  Sacoglossa (sap-sucking slug). Click here if you need an explanation around sea slugs, so you won’t get lost in translation.

The most dominant families of sea slug found during the study are Chromodorididae, Phyllidiidae, Facelinidae, and Aplysiidae, where the most common species being the pimply phylid nudibranch Phyllidia pustulosa. Overall, we found most of the sea slug on rubble-bottomed or sandy water. It is also revealed that the western part of this strait holds more species of sea slug rather than its eastern counterparts.

Overall, this preliminary study reveals that for a small space, this strait holds a hefty amount of sea slug species. We couldn’t bring this place in comparison with other place in Indonesia for now because the data is very limited. But on a long term, hopefully we will slowly reveal its biodiversity and foresee its potential to attract more scientists to further investigate the diversity not only on this taxa, but also another group of sea creatures that lived inside this area.


Many thanks to you that have been reading until this point.

This essay is a short explanation on my published work on revealing and describing marine diversity around Sempu Strait Waters. Which I’ve done for more that four years by now. If you’re interested about this topic, feel free to see my paper here as I forementioned before.

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