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Elysia chlorotica, the slug that behave like a leaf

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Elysia chlorotica, a sea slug able of drawing its energy from photosynthesis!

The sea slug Elysia chlorotica is a small marine gastropod, 5 cm long. It lives in shallow waters along the east coast of North America. This strange sea slug looks like a leaf. It’s green! When the sun shines, it spreads out, as if to make the most of the sunshine. How is this possible? Elysia chlorotica feeds on filamentous algae such as Vaucheria littorea. During digestion, the algae’s photosynthetic cells are only partially destroyed: their chloroplasts remain intact and enable Elysia to use the products of photosynthesis for nourishment. This is an example of chloroplast symbiosis or kleptoplasty [1], in other words, ‘chloroplast theft’. These chloroplasts contain chlorophyll, a pigment that captures light during photosynthesis, which gives the sea slug its green colour. They are present in the cells of its highly branched digestive system. This is why Elysia chlorotica resembles a green leaf. This characteristic appears to be specific to this family, as several closely related species exhibit the same behaviour. However, many protists behave in the same way.

Encyclopédie environnement - Elysia chlorotica — Figure 1. [Source. © Patrick J. Krug, Creative Commons CC BY-NC 3.0 license, via Wikimedia Commons]

In general, many marine organisms retain chlorophyll-containing cells absorbed from their prey: green, red or brown algae. They then incorporate these into their digestive system and use them for their own benefit. Their lifespan is generally limited, but predation allows the supply to be replenished. The most stable relationship is observed in symbiosis involving photosynthetic organisms (See Corals: Ocean engineers are under threat). This is the case with corals, which are organised colonies of individuals known as polyps. Their tissues contain numerous zooxanthellae, photosynthetic microalgae belonging to the genus Symbiodinium.

But for the slug Elysia chlorotica, things are very different. The mollusc acquires chloroplasts during its development, as it transitions from the larval to the adult stage, and keeps them functional for several months, or even nearly a year under certain experimental conditions. During this period, Elysia chlorotica appears to be able to manage without feeding on algae again, deriving a e proportion of its energy from photosynthesis. Experiments have shown that, in the presence of light and CO₂, Elysia chlorotica is indeed capable of incorporating CO₂ into its organic matter through photosynthesis. The exact role of this photosynthesis in the slug’s survival and reproduction remains, however, a matter of debate.

In plants, chloroplasts constantly require the import of proteins from the cytoplasm. The fact that chloroplasts sequestered within the sea slug’s digestive tract are capable of carrying out photosynthesis for several months, without the support of the algal nucleus, is therefore particularly intriguing. An initial targeted study had suggested that a gene essential for photosynthesis, psbO, had been transferred from the algal nucleus to that of the sea slug via horizontal gene transfer, with the corresponding protein subsequently being redirected to the chloroplast [2]. However, this result—obtained through a targeted approach focusing on a few candidate genes—has not been confirmed by complete genome sequencing of Elysia chlorotica, including its germ cells: no trace of a transferred algal gene was found there [3]. Similar observations in other kleptoplastic sea slugs reinforce the idea that this gene transfer is probably not the key to the puzzle. Other avenues are now being explored, such as a form of autonomy specific to the stolen chloroplast or mechanisms limiting its oxidative stress, which would allow it to remain active for longer without genetic support from the alga [4].

How and why are functional chloroplasts maintained within sea slugs, without the support of the algal nucleus? This question, long attributed to a gene transfer that is now being called into question, remains largely unresolved: in a recent review [5], which examines the alternative explanations currently under investigation, Cruz and Cartaxana themselves describe it as an ‘unresolved mystery’.

To go further: Elysia chlorotica in action

A lecture on Elysia by Dr. Sidney Pierce at TEDx Tampa Bay (Florida, USA)

References and notes

[1] Rumpho M.E., Dastoor F.P., Manhart J.R. & Lee J. (2006) The Kleptoplast. In: Advances in Photosynthesis and Respiration – The Structure and Function of Plastids. R.R. Wise & J.K. Hoober, eds, Springer Pub., Vol. 23, pp 451–473

[2] Rumpho M.E., Worful J.M., Lee J., Kannan K., Tyler M.S., Bhattacharya D., Moustafa A. & Manhart J.R. (2008) Horizontal gene transfer of the algal nuclear gene psbO to the photosynthetic sea slug Elysia chlorotica. Proceed. Natl. Acad. Sci. USA 105, 17867–17871

[3] Bhattacharya D., Pelletreau K.N., Price D.C., Sarver K.E. & Rumpho M.E. (2013) Genome analysis of Elysia chlorotica egg DNA provides no evidence for horizontal gene transfer into the germ line of this kleptoplastic mollusc. Molecular Biology and Evolution 30, 1843–1852.

[4] Havurinne V., Handrich M., Antinluoma M., Khorobrykh S., Gould S.B. & Tyystjärvi E. (2021) Genetic autonomy and low singlet oxygen yield support kleptoplast functionality in photosynthetic sea slugs. Journal of Experimental Botany 72, 5553–5568.

[5] Cruz S, Cartaxana P (2022) Kleptoplasty: Getting away with stolen chloroplasts. PLoS Biol 20(11): e3001857. https://doi.org/10.1371/journal.pbio.3001857

Elysia chlorotica, the slug that behave like a leaf

To go further: Elysia chlorotica in action

References and notes

Further readings