Kleptoplasty

How sea slugs give themselves superpowers

Their slimy shenanigans might have applications for humans, too

Jul 03, 2025 02:10 PM

Slug shot

SOME SEA slugs are kleptomaniacs. Elysia crispata, a species of these marine molluscs found in the western Atlantic and the Caribbean, is among the most notorious. When the slugs eat algae, their bodies pinch bits of the algae’s cells, known as chloroplasts, that enable photosynthesis. These are put to good use, giving the slugs their verdant hue which, along with their frilly back, earned them the moniker “lettuce slug” (see picture). They also continue to function inside the slug for about a year, providing them with photosynthetic energy. Scientists have known about this process, termed kleptoplasty, for decades. But how the heist was pulled off remained a mystery.

A recent paper published in the journal Cell sheds light. The study, led by biologists at Harvard University, found that the slugs build sac-like structures known as kleptosomes out of their own cells in which the chloroplasts are stashed. The kleptosomes stop the slugs from immediately digesting the stolen chloroplasts, but allow them to draw on this store of food in times of scarcity. It is like “a living larder growing on their back”, says Corey Allard, the study’s lead author.

To reach this conclusion, the scientists fed slugs that had recently consumed algae a chemical tag designed to identify any new proteins created within their bodies. Following a six-hour incubation period, the researchers extracted the stolen chloroplasts and analysed the tagged proteins within them. Among them were signs of Rab7a, a protein that is usually found in processes where cells engulf foreign bodies. This led the scientists to suspect that the chloroplasts were held inside structures made by the slug. Observation under a powerful microscope confirmed that each stolen chloroplast was indeed wrapped in just such a membrane.

The researchers then investigated what the sea slugs do with the kleptosomes. They did this by comparing the starvation resistance of E. crispata with that of Aplysia californica, a sea slug that feasts on algae but stows no chloroplasts. While A. californica died after about four weeks, E. crispata survived for up to four months. They found that when subject to a period of starvation the slugs switched from storing the chloroplasts to digesting them, turning a tell-tale orange as the chlorophyll depleted, much like autumnal leaves.

Although kleptoplasty has been observed in a handful of other creatures (single-celled protists and marine flatworms), these are much less studied and the processes involved may be entirely different. What’s more, chloroplasts are not the only goods that sea slugs steal. Some in the Berghia genus have innards that snatch the stinging cells off ingested sea anemones, storing them within appendages on their backs. When a predator threatens they hurl these stolen barbs, which Dr Allard describes as bombs that fire harpoons when they explode. Another (as-yet unnamed) species glows after feasting on fluorescent corals. Dr Allard is studying the whole gang and hopes to work out their respective modi operandi.

Such slimy shenanigans have exciting implications for humans’ own evolutionary history. Kleptoplasty is thought to be a precursor to endosymbiosis, a process in which one single-celled organism lives inside another cell. Endosymbiosis was fundamental in the evolution of eukaryotic cells, the cells that make up all complex life. Cell components such as chloroplasts and mitochondria were once free-floating bacteria until they were engulfed by a host cell. That process took millions of years: sea slugs offer an analogue that takes place within a single lifetime, says Nicholas Bellono, a biologist at Harvard University and co-author of the latest study.

There could be practical applications, too. Understanding how the cells of one organism incorporate the components of another to acquire novel functions could inform medicines that grant human cells new disease-fighting abilities. Dr Allard is particularly excited about the potential for treatments for neurodegenerative and metabolic diseases. In the natural world, it appears, theft pays handsomely. ■