Photosynthetic sea slugs: plastid survival in animal cells

The only animal cells known that can maintain photosynthetically active plastids in their cytosol occur in the digestive gland epithelia of some marine slugs. The animals steal their plastids (kleptoplasts) from siphonaceous algae upon which they feed.

Research on the slugs and their lasting kleptoplasts stretches back many decades and the issues of how slugs survive starvation and what mediates kleptoplast longevity have sometimes been conflated. It was once thought that lateral gene transfer from the alga to the slug could explain kleptoplast longevity, but most recent data refutes this concept. Even the role of photosynthesis itself, with regard to how some species can survive starvation periods lasting many months, is in question.

Current data suggests that kleptoplast longevity is an intrinsic property the stolen organelles bring along and that some species die in the presence of photosynthetically active kleptoplasts, and concomitant with the accumulation of reactive oxygen  species (ROS) in the digestive tract. Long term animal-plastid interaction, while requiring robust plastids, is determined by an increased starvation tolerance of the animals. Starvation tolerance itself is spearheaded by the ability to suppress ROS stress and not the maintenance of stolen plastids.