You are what you eat: new evidence shows that sea slugs carry algal gene

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”.

The green sea slug Elysia chlorotica (Source)
The green sea slug Elysia chlorotica (Source)

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. Continue reading