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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 called Elysia chlorotica is a small 5 cm-long marine gastropod. It lives in shallow waters along the east coast of North America. This strange slug looks like a leaf. It’s green! When the sun shines, it spreads out, as if to enjoy the sun. How is that possible? Elysia chlorotica feeds on filamentous algae such as Vaucheria littorea. During digestion, the photosynthetic cells from the algae are only partially destroyed: their chloroplasts remain intact and allow Elysia to use the products of photosynthesis to feed itself. This is an example of chloroplastic symbiosis or kleptoplasty [1], in other words, chloroplast robbery. These chloroplasts contain chlorophyll, a pigment that captures light during photosynthesis and gives the sea slug its green colour. They are present in the cells of his highly branched digestive tract. That’s why Elysia chlorotica looks like a green leaf, presenting leaf vein-like structures. This property seems to be specific of this family, since several related species have the same behaviour.

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 cells absorbed from their prey: green, red or brown algae. They then integrate them into their digestive system and use them at their own service. This is generally for a limited period of time, but predation allows the stock to be renewed. The most stable relationship is observed in symbiosis involving photosynthetic organisms. This is the case with corals, which are organized colonies of polyps (see Corals: Ocean engineers are under threat). Their tissues contain symbiotic zooxanthellae, which are photosynthetic microalgae belonging to the genus Symbiodinium.

But for the sea slug Elysia chlorotica, things are very different. The mollusc has acquired chloroplasts during its development, i.e. during the transition from larval to adult form. Chloroplasts then remain functional throughout the life of the sea slug. Thus, Elysia chlorotica seems to feed on algae only at the beginning of its existence, then it is drawing its energy exclusively from photosynthesis. Experiments have shown that in the presence of light and CO₂, Elysia chlorotica is able to incorporate CO₂ into its organic matter through photosynthesis. However, the fact that the presence of chloroplasts is essential to the life of sea slugs remains controversial.

In plants, the chloroplast needs the permanent import of proteins synthsized in the cytoplasm. The fact that the chloroplasts sequestered in the sea slug’s digestive system are able to carry out photosynthesis for months is therefore somewhat problematic. Sequencing of the host genomes – the slug – and the symbiont – the algae – showed that genes essential for photosynthesis had been acquired by the animal by horizontal gene transfer from the nucleus of the algae. The proteins thus encoded are then redirected to the plastid [2]. Thus, in the case of this sea slug, predation – i.e. the consumption of a green algae – has been accompanied by a horizontal gene transfer between the symbiont nucleus and that of the host. This system is at the origin of a metabolic novelty, “green animals”, that are able of performing photosynthesis for several months, thanks to chloroplasts remaining functional in the host tissues [1].

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. PNAS USA 105, 17867-17871

Elysia chlorotica, the slug that behave like a leaf

To go further: Elysia chlorotica in action

References and notes

Further readings