One of the most commonly touted attributes of science is that its ability to self-correct. New models and theories are constantly being generated based on experimental results but just as frequently, these new theories and ideas are challenged and sometimes, proven to be wrong by other scientists. Encouragingly, this type of academic rigor is not exclusively applied to high-impact research (recently examples include a deceptively simple method to create stem cell and a bacteria that uses arsenic to build its DNA) but also to the seemingly insignificant curiosities of life. The case of the green sea slug falls into this latter category.

Green sea slugs get their vibrant emerald green colour from the algae that they eat, specifically from the chloroplasts contained within the algae. Chloroplasts are specialized compartments in plant cells that house all of the machinery required for photosynthesis. Think of them as little solar panels, converting sunlight into the energy plants need to grow. Green sea slugs are able to extract chloroplasts from algae and store them in special cells along their digestive tract. In some cases, chloroplasts are stored for more than nine months! For a long time, scientists believed that the main role of these chloroplasts was to generate energy for their new slug hosts by photosynthesis. This finding generated a lot of hype and understandably so. Green sea slugs are one of a small handful of animal species that can photosynthesize and quickly became known as “solar-powered slugs” and “leaves that crawl”.

In 2014, the idea that green sea slugs use their chloroplasts exclusively to generate energy through photosynthesis was challenged when a group of researchers found that blocking photosynthesis had no effect on weight loss or survival rate of green sea slugs during starvation. If the main purpose of chloroplasts was to generate energy for the slugs during starvation, then blocking photosynthesis should lead to lower survival and more weight loss. Based on these new findings, the researchers proposed a new theory for why green sea slugs hoard so many chloroplasts. Instead of using them as solar panels to create energy, the sea slugs are breaking down the chloroplasts into its many components and eating those parts as food. Maybe green sea slugs store chloroplasts for the same reason that bears store fat and squirrels store nuts before winter hibernation. This is not to say that green sea slugs don’t photosynthesize. They are undoubtedly capable of photosynthesizing and may use it to some extent to survive periods of starvation but just how significant of a contribution that is remains to be seen.

Another topic of contention in the ongoing saga of the green sea slug revolves around algal genes and their presence in the sea slug genome. A number of earlier studies claimed that green sea slugs had stolen genes from algae and incorporated them into their own DNA. Importantly, the algal genes that were reportedly found in the sea slug genomes encoded proteins that are critical to the photosynthesis reaction, lending support to the previous idea that slugs store chloroplasts mainly to photosynthesize. However, later studies found no evidence of gene transfer between algae and sea slugs. These different results could be due to the different species of green sea slugs and techniques used in each study and the various technical limitations of each technique.

Now, a study has provided new evidence showing that the green sea slug Elysia chlorotica contains an algal gene in its genome. The researchers first collected slug larvae and extracted their chromosomes. Using a technique called fluorescence in situ hybridization (or FISH), they found that the algal gene prk was present on one of E. chlorotica’s chromosomes, providing perhaps the most definitive evidence to date of gene transfer between green sea slugs and their algae food source. FISH works by using brightly labeled pieces of DNA to search for a complementary match. In this case, the researchers used DNA coding for an algal gene (prk) and labeled it with a fluorescent tag. If the same gene is present on a slug chromosome, it will form a specific match and attach to the fluorescently labeled DNA. By looking for brightly glowing dots, researchers could directly see whether or not a specific gene is present.

Will this new finding stand the test of time? Only more experiments will tell. Until then, the continuing story of the green sea slug reminds us that we should not judge a book by its cover or as one scientist puts it, “a slug just by its color.”

References:
Schwartz JA, Curtis NE, & Pierce SK (2014). FISH Labeling Reveals a Horizontally Transferred Algal (Vaucheria litorea) Nuclear Gene on a Sea Slug (Elysia chlorotica) Chromosome. The Biological bulletin, 227 (3), 300-12 PMID: 25572217

Christa G, Zimorski V, Woehle C, Tielens AG, Wägele H, Martin WF, & Gould SB (2014). Plastid-bearing sea slugs fix CO2 in the light but do not require photosynthesis to survive. Proceedings. Biological sciences / The Royal Society, 281 (1774) PMID: 24258718