Plants are not the only organisms that photosynthesize using chloroplasts. Elysia, a genus of sea slugs, are notorious for stealing chloroplasts from green algae and using photosynthesis to feed themselves. These “solar-powered sea slugs” have been objects of intrigue since the late 1800s. 

Scientists have worked out a lot about how and why these sea slugs steal chloroplasts (rightfully called kleptoplasts in this context). Kleptoplasts — from the Greek kleptes, meaning thief — help slugs tide over difficult times when algae might not be readily available, letting them generate their own energy from light. One question that has been keeping scientists awake at night, though, is how the slugs manage to keep these kleptoplasts functional in their own bodies for up to months on end.

Algal chloroplasts are not designed to work efficiently in slug bodies. Within algae, chloroplasts are protected from light-induced damage by several mechanisms, but these processes seem to be absent in slugs. So what are the slugs doing to protect their stolen goods from damage? A new study published in the journal eLife has some answers.

By comparing how algal chloroplasts and slug kleptoplasts function under different light and oxygen conditions, the researchers found that the slugs have devised their own strategies to keep the kleptoplasts safe. The slugs essentially tweak the chemistry of certain kleptoplast components to reduce the formation of harmful molecules called reactive oxygen species. When exposed to light, the slugs also convert any extra energy to heat, again protecting the kleptoplasts from light-induced damage. 

Although the scientists are still working to find the molecules that help Elysia use their kleptoplasts more efficiently, this study helps us understand how these animals evolved their selective thievery. In the long run, the researchers hope to figure out how chloroplasts came to be in the first place, as they are thought to have originated from endosymbiotic cyanobacteria.