Elysia viridis: this sea slug is one of the few known animals that engage in photosynthesis, which allows it to survive without food for months at a time
Elysia viridis is a sacoglossan sea slug with an unusual ability -- it can "steal" the photosynthesizing cells from algae and then incorporate those cells into its own body, where they continue to convert sunlight into energy, producing nutrients that supplement the sea slug's diet.
This process is known as kleptoplasty, which means "the theft of cells." It allows the sea slug to survive with limited food (or no food at all) for months at a time.
As this article explains:
In sacoglossan sea slugs, kleptoplasts are harboured in cells of the digestive diverticula, enabling their animal host to survive photo-autotrophically for periods ranging from days to up to one year after being "stolen" from algal cells.
Due to these remarkable features, sacoglossan sea slugs have frequently been termed "leaves that crawl" or "solar-powered sea slugs."
Elysia viridis can have a green or reddish-brown appearance, depending on the amount of light that it receives and the color of the algae that it consumes. Its body is also covered in glistening flecks of blue and green.
Above: the photo at the top shows Elysia viridis with its parapodial lobes folded over its body, while the photo at the bottom shows the same species with the lobes completely unfurled
The sea slug's body is flanked by two flat, wing-like lobes that can be folded inward or unfurled, allowing the animal to regulate its light exposure; when these flaps are stretched open, the sea slug looks remarkably similar to a leaf. Its body has a flat, wavy appearance, and its digestive glands are clearly visible, resembling the veins of a leaf.
Above: Elysia viridis as a baby
Several other sea slugs are also known to engage in photosynthesis, but this phenomenon is still remarkably rare among animals at large. Most of the animals that do photosynthesize are marine invertebrates, including sacoglossan sea slugs and certain nudibranchs.
Above: Elysia chlorotica, another photosynthizing sea slug from the same genus
Journal of Experimental Botany: Crawling Leaves: Photosynthesis in Sacoglossan Sea Slugs
Senckenberg Society for Nature: Identification and Biology of Elysia viridis
Nature New Biology: Persistence of Functional Chloroplasts in Elysia viridis
PLOS Biology: Kleptoplasty: Getting Away with Stolen Chloroplasts
University of Southern Denmark: Functional Chloroplasts Inside Animal Cells (PDF)
Journal of Experimental Biology: The Making of a Photosynthetic Animal
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