Solar-powered Sea Slugs

Plants are known for using the sun’s energy to produce chemical energy in a process called photosynthesis. This is something we all learned in grade school science, but what often isn’t addressed is the slugs that also use photosynthesis! Yes, a couple species of sea slugs in the genus Elysia have found a way to incorporate chloroplasts into their cells, using the sun to produce sugars.

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One such species of slug is E. chlorotica, or the eastern emerald Elysia. Chlorotica’s diet consists of an algae (Vaucheria litorea). Algae cells are cut open by the slug using it’s radula (pictured as letters ‘r’ in diagram below) and the contents are sucked out. While digesting most of the plant matter and passing the rest as waste, the creature preserves the chloroplasts. This is when the astonishing transformation occurs.

 

e = esophagus | m = mouth | mx = maxilla | o = odontophore | op = odontophore protractur muscle | r = radula | rp = radula protractor muscle | rr = radula retractor muscle
e = esophagus | m = mouth | mx = maxilla | o = odontophore | op = odontophore protractor muscle | r = radula | rp = radula protractor muscle | rr = radula retractor muscle

Young chlorotica begin as brown colored slugs with bright red spots, but upon feeding on algae the slug begins to turn its beautiful emerald tone. This process is not permanent initially, but with continuous consuming of algae the chloroplasts find a stable spot in the cells. After this point, the slug will remain green.

Lifecycle of the eastern emerald Elysia
Lifecycle of the eastern emerald Elysia

As mentioned, chloroplasts allow for energy from light. Photosynthesis occurs within the slug’s cells, creating sugars to supplement their diet. Initially, scientists wondered if the slugs could survive on this alone. During an experiment, eastern emerald Elysia were placed in containers with sunlight, but no algae to feed on. They survived for several months! However, further study showed these slugs could survive just as well in the dark. How much the photosynthesis actually supports the slugs remains unknown.

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Integration of chloroplasts into their cells is a definite sign of photosynthesis-supporting genes. But how did these slugs gain the genes in the first place? A gene for stable chloroplasts (psbO) was found in the eggs and sex cells of Elysia chlorotica, leading to the understanding that it came from something called horizontal gene transfer. HGT is the movement of genetic material through means beyond sexual reproduction. It’s not known how or when the transfer of genes occurred, but now that the gene is in their cell nuclei these slugs are a rare hybrid between plant and animal.

Elysia viridis, another species of "solar-powered" slug
Elysia viridis, another species of “solar-powered” slug

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