It might look like something you'd find in an alien salad, but this bizarre sea slug can actually steal body parts from other organisms and use their powers for itself.
The lettuce sea slug (Elysia crispata) gets its energy by eating algae. But that's not just through the usual method of digesting food – the slug's gut separates out the algae's photosynthesizing organelles, called chloroplasts, and uses them on its back almost like little solar panels, sapping energy from sunlight.
"This is an organism that can steal parts of other organisms, put them in their own cells, and use them," says Corey Allard, cellular biologist at Harvard. "And I thought that was some of the craziest biology I'd ever heard of."
Scientists have long known about the slug's ability to commandeer the powers of what it swallows like some kind of aquatic Kirby, but the specific biology behind its thieving remained murky. In a new study, biologists at Harvard investigated closer.
The team found that rather than digesting the algae's chloroplasts, the slug diverts them into small sacs in its intestines.
Here, they're encased in membranes that the researchers have named 'kleptosomes', which keep the chloroplasts alive and functioning. Finally, they're shuttled to structures on the slug's back, where they can access sunlight to help the slug go longer without food.
When the researchers conducted chemical analyses on the chloroplasts, they found that they not only continued to generate algal proteins – indicating the stolen organelles remained functional – but the chloroplasts now contained slug proteins as well. That shows the hosts are actively working to keep them alive.
Lettuce sea slugs are known to come in a variety of colors, and the Harvard team found that this might partly be based on their health. Well-fed slugs are generally green, but they often turn orange if they go without food.
It seems that the slugs resort to digesting the chloroplasts if there's no other food available. Or there may be a limit to how long they can sustain these alien chloroplasts.
"The actual function of these things could be far more complicated than simple solar panels," says Allard. "They could be food reserves, camouflage, or making [the slugs] taste bad to predators. It's probably all of those things."
Understanding this kind of symbiosis could help scientists unravel longer-term cases of organisms gaining abilities from things they absorb. After all, it's hypothesized that this is how we got mitochondria, the little organelles that give our cells energy.
The research was published in the journal Cell.