The Photosynthetic Sea-Slug

Victoria Navas Via-Dufresne

Staff Writer

Plants are thought to be the only beings to do photosynthesis since they possess chloroplasts, but the sea-slug Elysia Chlorotica has been found to also have the ability to perform photosynthesis.

Elysia chlorotica, or Eastern Emerald Elysia, is a sea slug that averages between 20 and 60 mm. The Elysia Chlorotica lives from the south shore of Florida up to the shores of Nova Scotia, and it is found in depths down to 0.5 m beneath the surface. Depending on their feeding and intake of chloroplasts, these sea slugs range from a red or greyish colour to a bright green colour.

When in the juvenile stage, they mainly eat algae named Vaucheria litorea, in order to accumulate enough chloroplasts and then move towards a photosynthetic diet with some algae ingestion. Chloroplasts are present in plant cells exclusively. By using a process named kleptoplasty, the chloroplasts of the algae are taken in by the Elysia and used in order to perform photosynthesis.

This kleptoplasty works by the partial digestion of the algae leaving the chloroplasts intact for the Elysia to temporarily use. This sea slug has been classified as the being able to use kleptoplasty the longest, with the use of the chloroplasts lasting up to 10 months. The Elysia, when feeding, uses the radula, a tongue-like structure with minuscule teeth, to attack the Vaucheria litorea, penetrating its cell wall. It then sucks in the cells’ contents and digests them in its gut, leaving only the chloroplasts intact and moving them inside its own cells.

Due to their green color in their adulthood, these sea slugs are able to camouflage in their environment from predators. Adult slugs, after consuming enough chloroplasts, are able to survive months without algae when food is scarce. It turns out that Elysia Chlorotica possess a gene in their genome called psbO that is vital in algae in order to perform photosynthesis.

Scientists concluded that the gene was probably transferred to their genome by horizontal gene transfer, consisting of an exchange of genetic material between organisms. It is because of that transfer of information that Elysia is able to adequately use the chloroplasts for photosynthesis. If the sea slug weren’t able to integrate these chloroplasts into its cells, it would have to feed on the algae more often.

Originally Published on Vol.49 Issue 13 on April 15th, 2020