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.