SERIES: Invertebrate of the Week

Invertebrate of the Week #14 – Elysia chlorotica: a (possibly) solar-powered sea slug

Elysia chlorotica. Photo: Patrick J. Krug
Elysia chlorotica. Photo: Patrick J. Krug. Source: Evolution Happens.

Meet Elysia chlorotica, often hailed as a “solar-powered sea slug,” and affectionately named the “Eastern Emerald Elysia.”

Elysia chlorotica can be found in waters along the Atlantic Coast of North America (from Nova Scotia to Florida) and along the Gulf Coast of the United States.

It inhabits saline marshes, intertidal pools, and brackish canals at a depth between 0 – 0.5m. That penchant for a shallow depth range within the reach of sunlight provides a clue to what gives this beautiful sacoglossan its a stunning emerald green color: chlorophyll.

Elysia chlorotica. Photo: New Scientist.
Elysia chlorotica. Photo: New Scientist (http://goo.gl/HGSLlG)
Elysia chlorotica. Photo: Nature
Elysia chlorotica. Photo: Nature (http://goo.gl/xzA9TD)
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Research by scientists like Sydney ‘Skip’ Pierce suggests that the slug may be capable of sequestering the chloroplasts within its body in a manner that allows them to continue photosynthesizing. Those chloroplasts then continue to function and providing the slug with a steady supply of sugars.

After emerging from the larval stage, young Elysia colorata consume large quantities of Vaucheria litorea algae. The slugs pierce the algal filaments and suck out a soup of algal cytoplasm. That liquid diet passes into the digestive tract where the chloroplasts are isolated and sequestered into vacuoles along branches of the digestive tract.

According to Prof. Pierce’s hypothesis, eventually, enough chloroplasts are ingested and sequestered to allow the slug to produce enough products of photosynthesis to provide a steady supply of food. But for that method to work, the story can’t stop there.

You might recall from your basic biology courses that chloroplasts have their own DNA, separate from the organism they are found within. When inside their native V. litorea algae, the chloroplasts rely on a combination of proteins encoded in their own genome and on proteins encoded by the genes of the algae itself. Without the two genomes working in concert, the chloroplasts are unable to function.

One would think that once the algal chloroplasts were translocated into their new Elysia colorata host, they would lose access to the vital proteins produced by their former algal home and would subsequently become dysfunctional.

Here’s where things get interesting. Since the chloroplasts observed in Elysia colorata appear to be fully operational, they must be getting those vital proteins from somewhere. The mechanism by which that is occurring, however, place remains a mystery.

Two possibilities have been put forth. The first possibility is that this species of slug incorporated algal DNA into its own genome at some point, either through horizontal gene transfer or through a viral vector.  Both possibilities, however, have their pitfalls and neither has been sufficiently supported by the studies thus far.

Elysia timida. Photo: Sven Gould/Jan de Vries
Elysia timida. Photo: Sven Gould/Jan de Vries
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Other researchers, like Sven Gould and graduate student Christa Gregor, have offered a different explanation. Based on observations from two similar species, Elysia timida and Plakobranchus ocellatus, they believe that the chlorophyll may just be stored as a fat and protein-rich food reserve to be used in times of need. This might explain why the slugs are able to endure prolonged periods of starvation and become progressively paler when they’re unable to graze on algae.

Perhaps the answer is in some combination of the two possibilities, with the slugs using photosynthesis in concert with ‘chlorophyll as food-storage reserves’.  I can’t help but wonder if the proteins necessary for photosynthesis could be obtained through ingesting a steady supply of protein-rich V. litorea cytoplasm. Any marine algal specialists, biochemists, slug geneticists, etc. reading this want to weigh in?

Further Reading

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