Costasiella kuroshimae, known commonly as Leaf Sheep, is an ocean-dwelling grazer that “steals” photosynthesizing chloroplasts from the food it eats in order to generate energy. The Leaf Sheep is a species of sea slug that munches on algae instead of grass, like the sheep you find on dry land. Reaching a length of 5 mm, the tiny aquatic slug has the distinction of being one of the few animals that are able to photosynthesize.
Unlike the Leaf Sheep’s landlubbing relatives, it thrives in the salty seas near Japan, Indonesia and the Philippines. Like many sea creatures, the Leaf Sheep consumes algae to survive. In doing so, the Leaf Sheep pulls the photosynthesizing chloroplasts from the algae and incorporates them into their own body. This process is known as kleptoplasty, in which the predator leaves the algae’s plastids intact, allowing it to briefly benefit from energy generated through photosynthesis.
Though unique in the animal kingdom, the Leaf Sheep is not the only photosynthesising fauna. Several species of sea slug possess this ability. The Eastern Emerald Elysia is a photosynthesizing sea slug that lives only off the eastern coast of North America. At up to 30 mm in length, the Elysia is slightly larger than the Leaf Sheep. Size is not the only area in which the Elysia outdoes the Leaf Sheep. Whereas the Leaf Sheep only temporarily engages with photosynthesis, the Elysia actually incorporates the algae’s genes into its own DNA!
The Elysia uses its sharp front tooth to suck nutrients, including photosynthesizing chloroplasts, from algae. The gene for repairing these chloroplasts is then encoded onto the Elysia’s genome. “While the next generation must take up chloroplasts anew from algae, the genes to maintain the chloroplasts are already present in the slug genome,” says Professor Sidney Pierce, who has studied the phenomenon. To receive maximum sunlight, the Eastern Emerald Elysia is usually found in the shallow waters of marshes, tide pools, and creeks.