Scientific Name: Costasiella sp.
Common Name: “Leaf Sheep”

Adult Size: 3 – 7 mm
Depth Range: 2 – 30 m; most common in shallow lagoons and seagrass beds
Biotope / Habitat: Found crawling on algae, especially Avrainvillea and other filamentous green algae, which serve as both food and camouflage
Diet: Feeds on siphonous green algae; uses specialized mouthparts to suck out cell contents. Retains functional chloroplasts (kleptoplasty) in its cerata, allowing it to harness solar energy for additional nutrition
Reproduction: Simultaneous hermaphrodite; lays minute, coiled egg ribbons on algal surfaces. Eggs hatch into free-swimming veliger larvae before settling on algae
Fun Fact: Known worldwide as the “leaf sheep” for its resemblance to a tiny cartoon sheep with a green, leaf-like body. Its ability to photosynthesize via kleptoplasty makes it one of the most extraordinary sea slugs—half animal, half “plant.”

Adorable, tiny, and brilliantly green, Costasiella sp. is a favourite among divers and underwater photographers. Its sheep-like face and algae-inspired body make it a viral star of the sea slug world, captivating anyone lucky enough to see it in person.