Solar powered sea slugs

Meet the sea slugs who double up as plants (well kind of) by creating their own energy via photosynthesis.

eating
The sea slug (Elysia chloritica) amongst its algal food source (Vaucheria littoria)

There are two ways of gaining energy in the natural world, you can either eat other organisms and convert them into energy (heterotrophy) or you can absorb sunlight and convert that into energy (autotrophy). It is perhaps one of the greatest evolutionary adaptations to be both heterotrophic like animals and autotrophic like plants. Organisms that can do this are known as mixotrophic. It is well documented that some plants like the Venus fly trap can also eat insects and small animals, but the reverse is practically unheard of. However that’s exactly what some sea slugs are able to achieve by stealing the ability to photosynthesise from the algae that it eats. It is a very unique process that is exceedingly rare in the animal kingdom and is known as Kleptoplasty.

What is Kleptoplasty?

When plants photosynthesise they convert sunlight into energy which they can then use to survive. The chemical reaction of photosynthesis takes place in a part of the cells called chloroplasts. These are the solar powered factories that give all plants their green colour. When the sea slug (Elysia chloritica) feeds upon the algae (Vaucheria litorea) it keeps the chloroplasts from its food and uses them to create its own energy from sunlight. These stolen chloroplasts are kept in specialised regions of the body close to the skin so they can react with sunlight. As well as being able to increase the amount of energy they can obtain, the chloroplasts also give E.chloritica its brilliant green colour that allows it to camouflage itself from predators within the algae. The word Kleptoplasty is derived from the Greek word Kleptes that translates to thief.

Gaining new genes

Whilst ‘stealing’ the chloroplasts is an impressive accomplishment for E.chloritica it is only the first step to acquired photosynthesis. In order to maintain the alien cell parts the slugs must take something else from the algae, its genes. Specifically the psb0 gene from V.litorea which is obtained by horizontal gene transfer. Although this sounds complicated it is actually relatively quite simple. It just means that genes are transferred between different organisms rather than down generations. Once acquired the psb0 gene allows E.chloritica to produce proteins to maintain the chloroplasts and prevent them from breaking down. This is not a permanent fix but it does allow the slugs to maintain photosynthesis for up to 9 months which is fairly impressive.

Who else can do this?

E.chloritica is not the only species of sea slug that is capable of Klpetoplasty there are several others, although it is one of the best studied examples. However there is also a completely different type of organisms capable of Kleptoplasty. Plankton can also pull off the trick, in particular the planktonic ciliate Myrionecta rubra. This is slightly different to how sea slugs manage it as they steal both the chloroplasts and nucleus from their prey. This allows for complete genetic control over the stolen cell parts. What is even more impressive is that M.rubra can have its new chloroplasts stolen again by the larger dinoflagellate plankton Dinophysis acuminate. This is known as serial Kleptoplasty and could be described as a form of planktonic karma. Mixotrophic plankton like these are now thought to play a much more important role in energy cycling through the ecosystem than previously thought.

Myrionecta-rubra_5-8
The plankton (Myrionectsa rubra) viewed under a microscope. The green colouring is produced by its stolen chloroplasts.

To sum up

Kleptoplasty in sea slugs and plankton is one of the most interesting and useful evolutionary adaptations found anywhere in the natural world. To be able to steal parts of a cell from another organism as well as the genetic information required to control them is very impressive. It is also hugely useful as it allows them to gain energy from both eating organic matter and photosynthesising sunlight. Just imagine if every time you ate a salad you could walk outside and charge yourself up in the sun.

Links

https://www.pnas.org/content/105/46/17867.short

http://www.plantphysiol.org/content/123/1/29.short


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