In SCUBA diving we like to keep our hand signals clear and simple. Therefore, anything looking remotely like a sea slug or flatworm gets referred to as “nudis”. This leads to a bit of confusion when I claim that the adorable sea slugs commonly known as Shaun the Sheep are in fact not nudibranchs but sacoglossans.
Now you’re probably wondering why differentiate them – what could possibly be so special about the order Sacoglossa? The biggest answer is kleptoplasty - and this is where things get weird. These tiny sea slugs are some of the only Metazoans that are able to not only sequester but also functionally retain chloroplasts from the algae they feed on – this means they can use foreign chloroplasts to actually photosynthesise themselves. Why they do it is still a mystery in the scientific community, and though there are quite a few theories, none have explained how they’re able to in the first place.
And while there are quite a few species of Sacoglossa around Koh Tao, none are as famous or recognisable as Shaun the Sheep. Located at Sairee Shore, these sea slugs of the genus Costasiella, often C.kuroshimea and C.usagi, tend to be found on the algae genus Avrainvillea, which are green paddle/fan-shaped siphonous algae clustered around in sandy areas.
Research on sacoglossans is still sparse, so to improve our understanding of their distribution, I have decided to combine the mapping project of the DMT-program (Divemaster in Training) together with the eco-program. Instead of just mapping a traditional dive site like Chumphon, my goal consists of mapping the algae while trying to observe how the Leaf Sheep are spread across them. Over the course of six shore dives with multiple buddies and varying success rates, I was able to create a map of a 500m2 area, giving a rough idea on how to find these sea slugs.
Dive 1:
Here’s how the mapping unfolded. For the first dive, I planned to get an idea how to actually find the specific green algae, as well as find a reference point for all my later mapping dives. Unlike experienced Eco-Instructor Matt Knott, I was unable to locate the artificial reef from the surface. Therefore, I headed to the nearest buoy, which connects to a concrete block and a pipe running along the ocean floor. Descending there, I followed the pipe until reaching Roctopus’ own artificial reef and headed to the first visible structure – the octopus. Following Matt’s suggestion, I took a heading of 260° to lay down a transect line, which I learned to use during my eco-DMT. After approximately 28,5m distance from the octopus is when you hit a coral-rock formation that’s covered with sea squirt. Continuing with the heading, you then reach a patch of brown algae at around 40m. Behind this patch, I lie down the next transect line, making sure it is parallel to the first Avrainvillea sp. that hits the 260° heading.
During my first attempt of mapping the algae, I tried laying out quadrants to cover the area. However, after 60mins dive time and six quadrants, I realised that this was a very inefficient method that covered too little area and took too much time. Except for finding out how to find the algae, I was therefore unsuccessful in collecting meaningful data during my first shore dive.
Dive 2:
Reevaluating my strategy, I decided to forget the quadrants and focused on working solely with transects that I borrowed from Matt. The new plan relied on laying out one long transect line following the 260° heading at the same starting point and then laying out 5m transects to either side at every 2m mark (0m, 2m, 4m, …). I would then swim along the shorter transect lines, similar to the surveys we do during the eco-program, and note down any green algae I spot and the sea slugs it’s hosting within a 1m radius. This method allowed me to collect data systematically while covering a much larger surface area in less time.
Therefore, during my second dive, I was able to cover a 10x10m square while documenting 15 Avrainvillea sp. with 11 sea slugs distributed amongst them.
Dives 3-6:
With an efficient strategy, I continued counting algae and sea slugs over the next four dives. Finding a rhythm, I was able to always find the same starting point and covered approximately 100m2 each dive. As these surveys were conducted in a timeframe of three weeks, I started noticing certain trends. The more I trained my eyes to spot the Costasiella sp., the more I spotted them not only on the algae but also in the sandy patches in between. I was able to witness them moving from one algae to another, mating with each other, and laying the egg spirals found on many of the Avrainvillea sp.. And as the weeks went on, I noticed an increase of sea slugs in early-development-stages, indicating that many were hatching starting end of January. It was quite fascinating seeing some specimen as chunky as half of my pinkie nail to almost microscopic sizes. During this time, I also encouraged my dive buddies to go looking for them, which resulted in many confused stares when I told them there were 13 sea slugs on one algae.
This project was only possible through my amazing instructors at Roctopus. Without the workshops and training during my Eco-DMT, I would’ve never been able to spot, let alone count, all of these sacoglossan sea slugs. Through meticulous buoyancy control, patience, and experience with scientific dives, I was able to observe these tiny creatures without disrupting them or the sandy environment around them. Matt Knott, Phil Loader, and Frankie Gard-Storry were all extremely supportive of my unconventional mapping idea and encouraged me to continue kidnapping, sorry, I of course meant encouraging, my fellow DMTs into joining all of my dives.
Like many fellow DMTs, I had procrastinated the whole mapping project to the end of my program, which meant I didn’t have too much time afterwards to revisit the sight as often as I’d like to. However, the last artificial reef clean I was part of was so efficient, everyone who joined got to look for the Shaun the Sheep afterwards. Instead of going over my researched sector, I used this opportunity to explore the surrounding area, which ended up as one of the greatest dives to end my DMT program with at Roctopus. I found up to 23 minuscule Costasiella sp. on multiple algae and there were probably more I missed, but witnessing so many of the sea slugs I had researched during my uni studies in real life truly made it feel like I am doing the right thing by following my passion in marine biology and conservation. Even though I am heading back to Uni Göttingen, Germany, to finish my bachelor’s degree in biodiversity and ecology, I hope to one day return and contribute to the ongoing research on these adorable tiny creatures.
If you’re interested in my mapping project, you can find it here at the bottom of this blog.
Author: My Quynh Bui, EcoDivemaster
Pictures by: Matt Knott and Frankie Gard-Storry
