Scientific Name: Costasiella sp.

Adult Size: Usually between 2 – 8 mm.
Depth Range: Commonly seen from 1 – 20 m, often in shallow, sheltered reef zones or seagrass beds.
Biotope / Habitat: Typically found grazing on algae, particularly species of green algae such as Avrainvillea or Bryopsis. It prefers areas where these algae thrive, including sandy patches, seagrass meadows, and lagoon flats.
Diet: Feeds on siphonous green algae. Like other sacoglossans, Costasiella species practice kleptoplasty—retaining chloroplasts from the algae in their tissues, allowing them to derive some energy from photosynthesis.
Reproduction: A simultaneous hermaphrodite. After mating, it lays tiny, coiled egg ribbons on algae. The planktonic larvae eventually settle onto suitable algae to complete their development.
Fun Fact: Often nicknamed “leaf sheep” or “Shaun the Sheep slug” due to its adorable appearance, Costasiella is a favorite subject for macro photographers. Its small size and algae-like camouflage make it a rewarding challenge to spot.

Costasiella species are tiny sacoglossan sea slugs with a charming, leaf-like appearance. Their bodies are typically translucent green, speckled with darker spots or streaks that help them blend into their algal habitat. The head bears a pair of dark eyes and rolled rhinophores resembling tiny sheep ears. The cerata (dorsal appendages) are arranged in orderly rows, each packed with stolen chloroplasts that give them a vibrant green hue. This remarkable adaptation not only provides camouflage but may also contribute to their energy needs through photosynthesis.