Hello hello! You might have seen some of these fun fellows around the interwebs, and though they might not look real, they do exist. The slug on the left is a Costasiella kuroshimae, affectionately nicknamed "leaf sheep," and the one on the right is an Elysia chlorotica, which is also known as an emerald elysia. I call them sea leaves or leaf slugs. So, why slugs, why today, and why these two?
Slugs, because sea slugs are beautiful and amazing creatures, today because why not, and these two because they have a few things in common. They're sea slugs, they're green, they're cute, they're tiny...oh, and did I mention they can photosynthesize? Yep. Photosynthesizing animals. Did your jaw drop? Mine did when I stopped to really think about it.
Overview:
Costasiella kuroshimae
- About the slug
- Habitat n' feeding
Elysia chlorortica
- About the slug
- Habitat n' feeding
- Photosynthesis
Alright, let's do this. Just a heads-up: The sea sheep are cute but they're not as interesting as the Elysia, so I'll mostly be focusing on the latter.
Costasiella kuroshimae - About the slug
These leafy slugs aren't actually nudibranchs, or the colourful group that we normally think of sea slugs. They're part of the Sacoglossa clade, also known as the sap-sucking sea slugs. A decent number of them steal chloroplasts from the algae they eat to photosynthesize (this is called kleptoplasty), but for simplicity, let's focus on this famous one.
They can reach up to a whopping 1 cm (about 0.4 inches for any Americans reading this) and are extremely cute. They're also hermaphrodites and spend up to 2 weeks of their lives as plankton after hatching.
Costasiella kuroshimae - Habitat n' feeding
Costasiella kuroshimae are generally found near water's surface to photosynthesize better. They munch on a kind of algae known as Avrainvillae (details in the Elysia chlorotica section) and are found in the Pacific and Indian oceans in warm areas.
(Ocean Info)
Elysia chlorotica - About the slug
These are also Sacoglossan!
They're usually 2-3 cm long, but can be up to 6 cm. Mature slugs are green due to those tasty chloroplasts they steal from algae, but young, pre-algae Elysia chlorotica are brown with red spots. They're also hermaphrodites and live an average of 11 months (Blanchet).
Elysia chlorotica - Habitat n' feeding
This species of slug is amazing in many, many ways. In addition to their capability to photosynthesize, they are incredibly hardy and can survive in "near-freshwater to brackish salt water." (Blanchet). They prefer shallow water, probably because there's more light available.
A slug obtains chloroplasts by sticking a scraping radula into the cell wall of an alga (for this species, that alga is Vaucheria litorea) and sucking out its contents, eventually storing the chloroplasts in small pockets (known as diverticula) along the digestive tract. This allows for the chloroplasts to stay safe while the rest of the cell's contents are digested (Blanchet).
Elysia chlorotica - Photosynthesis
As with most photosynthesizing sea slugs, chloroplasts are stolen from the algae they eat and taken into the slug's own cells for photosynthesis. In a lot of these slugs, chloroplasts die off pretty quickly because the slug doesn't have the various algae things needed to support them. However, this species is a bit different. For one thing, they can support the stolen chloroplasts for much longer than similar species - more than seven months.
How does that work? Well, the slug actually has somehow managed to take algal genes and incorporate them into its own genome (Schwartz et al., 2014). Not only has the genetic material of an alga (singular form of algae, not a typo) somehow transferred into the slug and become incorporated in its very core, but those genes that originated in algae work in an animal. It amazes me every time I think about it. Usually when you shift a gene between different groups of life, you have to do something called codon optimising because different groups will have different building blocks floating around and some are more common in one group than another. Codon optimising edits the genetic code to use the most common building blocks for the group or species the gene is moving to, making sure that the organism(s) is/are able to produce whatever the gene codes for effectively. It's a bit like Lego - maybe one kid has mostly red two-by-ones, but another has only three red two-by-ones and the second wants to make something cool that the first one did. It would be hard for the second to use the same colours to make the structure, but they could make it using the two-by-ones they have and make something that is the same shape. That's kind of the idea behind codon optimizing - the second kid would have a very tricky time making a red structure, so it's much easier to change the instructions to call for another colour of bricks. Same physical shape, just a different colour. The shape is the most important part when dealing with the chemicals that make biology work - the colour (or codon, if you will) is a means to an end.
This is hard enough between very similar species - say, two similar strains of yeast. The genes transferred here move between two entire kingdoms of life!! If this was a show, you'd be cued to clap right around now.
Anyway, moving on! The slug passes this information down from generation to generation and those genes code for the bits and bobs that can support chloroplasts for months on end. In addition to that, it's been found that the presence of chloroplasts in a slug's system increases the amount of bits and bobs required to support them. Something about their presence triggers the genes that were shifted from algae to slug to make more of the things that the slug will need to help its newly acquired chloroplasts photosynthesize (Chan et al., 2018). Makes sense, right? I'm just shocked that any of this works at all, to be honest.
A Concluding Thought
So there you have it! Two incredible species of sea slug and hopefully a bit more appreciation of how they've created a unique way of life. The two images at the top are from Wikimedia Commons (I apologize for the quality of the Costasiella kuroshimae). Thank you so much for reading!
Surprise!! I didn't include this in the overview because ... well, surprise :D
I realized that (at least in my research) I find a lot of basic, surface-level articles by organizations like National Geographic and general news sources that are great for an introduction to a topic, but when I was looking for something more in-depth, it was pretty much just scientific articles. I wanted to bridge the gap and provide an intermediate resource, so now you'll find some more detailed information as well as sources/further reading (in APA format because why not). The scientific articles can be hard to read, but I'll do my best to add in-between resources when I can.
If information wasn't cited, it likely came from Wikipedia, my prior knowledge, or a quick search.
Blanchet, C. (n.d.). Elysia chlorotica. Animal Diversity Web. https://animaldiversity.org/accounts/Elysia_chlorotica/
Chan, C. X. , (2018) Active Host Response to Algal Symbionts in the Sea Slug Elysia chlorotica, Molecular Biology and Evolution, 35(7), 1706–1711, https://doi.org/10.1093/molbev/msy061
Schwartz, J. A., Curtis, N. E., & Pierce, S. K. (2014). FISH Labeling Reveals a Horizontally Transferred Algal (Vaucheria litorea) Nuclear Gene on a Sea Slug (Elysia chlorotica) Chromosome. Biological Bulletin, 227(3), 300–312. http://www.jstor.org/stable/24364084
Sea sheep: An animal that uses photosynthesis. Ocean Info. (2023, May 31). https://oceaninfo.com/animals/sea-sheep/
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