That's where the newly discovered gene comes in. The type of algal gene found in the slug is central to sustaining photosynthesis -- it makes a key enzyme that repairs damaged chloroplasts and keeps them working.
It's a useful bit of theft that gets passed on to slugs down the line. "The gene is incorporated into the slug chromosome and transmitted to the next generation of slugs," said study co-author Sidney K. Pierce, an emeritus professor at University of South Florida and at University of Maryland, College Park, in a press release.
The next-gen slugs still have to pilfer chloroplasts from V. litorea, Pierce noted, but the genes to maintain them are already on the animal's genome.
"There is no way on Earth that genes from an alga should work inside an animal cell," said Pierce. "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."
The team notes that its finding represents one of the only known examples of gene transfer between multicellular species, and that it could be relevant to human gene therapy research that targets genetic diseases.