Thanks to NordPass for sponsoring today's episode.

Secure your business effortlessly with a 3-month NordPass trial by going to nordpass.com/microcosmos and using the activation code "microcosmos". If you were asked to describe what a sea slug is, you might be tempted to go with the straightforward response: it’s a slug that lives in the sea.

And you know, you wouldn’t be wrong. You might even conjure up an image of a squirming, squishy thing gliding through the water. But as you can see right now, sea slugs can be spectacular, bringing together strange colors and structures that make them seem like an adorable cartoon brought to life.

At least, that’s the case for this little guy. The true array of sea slugs is even more amazing to behold… and also, incredibly confounding to decipher. “Sea slugs” is a very broad term, sometimes defined as marine gastropods that don’t have a shell on the outside of their bodies. That’s because “slug” in general is a loose term, used to describe a type of body that's undergoing the long evolutionary process of losing a shell.

Some sea slugs have lost their shells entirely, while others are a bit more in the middle. For example, nudibranchs are a group of carnivorous sea slugs that can have shells as larvae, but their lack of shells as an adult is a unifying feature. This particular species that we’re looking at belongs to the clade Sacoglossa, which does include some shelled species.

But not these guys. Sacoglossans are sometimes called sap-sucking slugs because they eat algae by sucking whatever the algal equivalent of meat is from them. And some species are famous for what they do next.

They steal from their food, taking the chloroplasts from the plant cells and storing them in their body through a process known as kleptoplasty. We’ve seen some versions of organisms taking on chloroplasts from others in the microcosmos, but they’ve always been single-celled organisms. And for a time, sacoglossans were the only animal we knew of that could take chloroplasts like this.

It was just a few years ago that scientists identified a marine flatworm with the same talent. And what’s most exciting of all is that sea slugs can actually use those chloroplasts, becoming photosynthetic animals in the process. However, the chloroplasts don’t last forever.

Some sacoglossans can only maintain the photosynthetic activity for a few days. But one species was able to hold on to their chloroplasts for 14 months. James, our master of microscopes, came across these sacoglossans in some samples gathered from Alicante, Spain.

He’d stored the samples in a clear plastic container and shone a strong light to help the algae in the sample. And on the second day, while he was looking through a magnifying glass, he found one of these little creatures crawling on the surface of the container. So he took it out with a pipette and put it on a slide.

And underneath the microscope, he found an absolutely adorable creature, with black eyes and yellow leaf-like structures called cerata that reminded him of a stegosaurus. At the top of their heads were a pair of rhinophores, the antenna-like structures that help sea slugs essentially smell chemicals dissolved in the water around them. When he checked his samples again later, he found more sea slugs just like the first one he’d found, climbing towards the surface where there was more oxygen.

And the whole time that James spent staring at those sea slugs, he was so excited. He’d never observed these creatures up close before, and he kept saying to himself that this was his first nudibranch. Nudibranch?

Yes, nudibranch. That other group of sea slugs that we mentioned earlier. You know, the group that's distinct from the Sacoglossans.

We’ll cut to the chase: James was wrong. He hadn’t found nudibranchs. And there’s nothing wrong with being wrong, especially when you’re venturing out of your well-developed comfort zone.

James knows how to distinguish between countless ciliate species based on the most minute details captured in 19th century illustrations. But these are an entirely different type of being. But what we’ve learned in the process of working on this episode is that sea slug people are wonderful, helpful people.

When James shared his footage of these little friends, some experts suggested that he’d found some species of Sacoglossa. And later, someone suggested that he contact Dr. Manuel Caballer Gutierrez, a scientist at The American University of Paris whose many research interests include marine mollusk taxonomy.

Dr. Gutierrez confirmed that these were indeed sacoglossans, and that they likely belonged to the genus Stiliger or Ercolania. But the slugs we’re watching are quite young, which made them hard to identify.

Sadly, the samples didn’t last long enough for us to be able to continue watching them or send them to him for further identification. But after spending some more time looking through the videos James sent, Dr. Gutierrez did narrow in on the species he thought James had found: Ercolania viridis.

We’ve found a few features that might help with this identification, like that young Ercolania viridis tend to have green lines close to their eyes, and that extend towards the back. The species is also known to have a white band going down the back, though it’s not always visible. Their rhinophores are elongated and smooth, with white spots appearing around the eyes.

And there are so many other qualities to try to draw on, from the eggs, to the shape of the cerata to their color, to their teeth. With sea slugs coming in so many different forms, it shouldn’t be that hard to identify a species, especially when they take on such brilliant colors and patterns and shapes, right? But there are a number of challenges.

For one, adult Ercolania viridis can be quite different in color, not just from our younger buddies here, but also from each other. Some are almost transparent, others are green, and some are even black. That’s led to this species also being given the name Ercolania funerea, which of course adds to the confusion.

And to make things even more confusing, there are other sea slugs with similar uses of “viridis” in their name. But sea slug researchers are a self-aware bunch, it seems. We were looking through an old magazine called Shells and Sea Life.

On one page was an advertisement for a Nudibranchs calendar for the year 1985. And a few pages later was an article titled “A Nomenclatural problem in the Ascoglossa— or: Why One Should Never Name a Green Sea-Slug Viridis.” The article was written by the scientist Dr. Kathe R.

Jensen, who helpfully lays out the confounding multiple uses of “viridis” to name sea slug species. And any clarity at all about what we’re looking at right now is thanks to scientists like Dr. Gutierrez and Dr.

Jensen, who take the time to clarify these details not just for other scientists, but for those who might just be curious about sea slugs. In fact, the common theme in our research was just how many sea slug scientists have dedicated their time to crafting online resources to help people in their journey to understand sea slugs. Their websites are formatted in ways that are deeply familiar to anyone who spent the 2000s creating websites dedicated to the things they loved.

Perhaps the most emblematic of this is the website seaslugforum.net. Though it’s been defunct now for more than a decade, the forum still remains. In it, you can find helpful articles explaining sea slugs, messages from curious sea slug finders wanting help in their identification, and responses from people with the knowledge to aid them in their journey.

The internet, for all its threat of permanence, is also an easy place to lose information. So the preservation of the Sea Slug Forum, even in this static state, feels like a comforting corner to recall what is good about being able to access so many people at once, and also what's good about retaining those conversations. On this channel, we often refer to papers that are centuries old, a form of conversation that took place between those who were curious about a new world that had opened with a microscope.

This forum is a continuation of that tradition, but also taken to a new level. It was probably read by so many more people at the time of publication compared to the papers of Antonie van Leeuwenhoek because that is what our modern age has allowed us— a broader conversation, spread among so many more people. And so today our journey through the microcosmos may have taken us in a new direction.

But like traveling around the world, these strange sights come with parallel experiences that challenge and comfort and reveal all at once. Thank you for coming on this journey with us as we explore the unseen world that surrounds us. And thank you again to NordPass for sponsoring today’s episode.

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Some of their names are on the screen right now, and these are the people that make this channel, and videos like this possible and we are so so grateful. If you’d like to become one of them, you can go to patreon.com/journeytomicro. If you’d like to see more from our Master of Microscopes, James Weiss, you can check out Jam & Germs on Instagram, and if you’d like to see more from us, there’s probably a subscribe button somewhere nearby.