Leaf Sheep Sea Slug Wins The Guardian's 2026 Invertebrate of the Year

The leaf sheep (Costasiella kuroshimae), a marine sea slug roughly the size of a grain of rice, has been crowned the winner of The Guardian's 2026 Invertebrate of the Year competition. The annual public vote, now in its third year, celebrates the diversity of invertebrates and attracted a global audience. The leaf sheep triumphed over nine other species, with the Patagonian moscardón bumblebee placing second and the horseshoe crab third. The species was first described in 1993 near Kuroshima, an island in Japan's Ryukyu Islands, and also inhabits tropical waters around the Philippines and Indonesia. Its distinctive appearance includes branching structures called cerata on its back, which aid in breathing and contain remnants of consumed algae. The leaf sheep's most remarkable adaptation is kleptoplasty, a rare process in which it retains functional chloroplasts from the algae it eats, allowing it to harness solar energy. This ability gives the slug its vivid green color and enables it to survive for periods using photosynthesis. The competition was organized by The Guardian, which also published related articles about the sea slug's appeal and its solar-powered capabilities. The victory highlights the extraordinary biological adaptations found in even the smallest marine creatures.