A Green Sea Slug Steals Power From Algae

The discovery makes this a true plant-animal hybrid

Illustration of the Elysia chlorotica by Mary Peart from "Report on the Invertebrata of Massachusetts" by Augustus A. Gould, W. G. Binney Licensed under Public Domain via Wikimedia Commons

The sea slug Elysia chlorotica is as brilliant green as a new leaf. And in a very leaf-like and un-slug-like way, it can absorb carbon dioxide. It also stays alive for months without food, as long as the laboratory is well-lit.

Exactly how the slug, commonly called the emerald green elysia, gains its solar power has been a research question for decades, and scientists are gradually pulling together the whole story. The latest evidence supports the idea that the slug steals genes from the algae it eats, according to a statement from the Marine Biological Laboratory in Woods Hole, Massachusetts.

The slug’s thievery has been known for years, but researchers didn’t realize how far it went. For Scientific American, Ferris Jabr writes

About the length of a postage stamp or two, these slugs feed on algae by sucking all the delicious gelatinous cytoplasm and crunchy protein nuggets out of the underwater plants. In the process, they slurp up algal chloroplasts, also known as plastids—green jelly bean-shaped organelles that perform photosynthesis, capturing the sun’s energy and combining it with carbon dioxide and water to make food. Most sap-vacuuming slugs digest the chloroplasts right away, but some species store the plastids for weeks to months in large transparent digestive glands, turning the animals brilliant shades of green.

The elysia in question is one of those hoarders, but an experiment by researchers at the University of Dusseldorf made the picture murky. After administering a drug that shuts down photosynthesis, the slugs still survived after 55 days. They were paler and smaller — so it’s probable that they simply digested the non-functioning chloroplasts they had stored. But the stolen organelles remained active for several months. Jabr explains why this is suprising:

In order to photosynthesize, the chloroplasts inside an alga depend on many genes in the alga’s own nucleus and the proteins for which they code. Tearing chloroplasts out of algal cells and asking them to make food inside a slug’s gut is like expecting the bottom half of a blender to puree some carrots sans the blade and glass jar.

The slug could have stolen DNA as well as chloroplasts from the algae, but several studies failed to find the genes keeping the photosynthesizing organelles ticking. The new study, however, found them.

The researchers used flourescent DNA markers to light up algal genes in the genetic material of larval and adult slugs, they report in the Biological Bulletin. The gene is one that repairs damage to chloroplasts and is transmitted to the next generation of slugs.

“There is no way on earth that genes from an alga should work inside an animal cell,” study co-author Sidney K. Pierce says in the statement. “And yet here, they do. They allow the animal to rely on sunshine for its nutrition. So if something happens to their food source, they have a way of not starving to death until they find more algae to eat.”

If the study’s findings hold up, this is the first case of gene transfer from one multicellular organism to another. (Bacteria do it all the time, no big deal.) That makes this sea slug a true plant-animal hybrid.

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