Overview
The Leaf Sheep (Costasiella kuroshimae) is an enchanting, microscopic marine sacoglossan sea slug celebrated worldwide for its whimsical cartoonish appearance and remarkable photosynthetic capabilities. First described in 1993 from Kuroshima Island in the Japanese Ryukyu Archipelago, this tiny sea slug resembles a miniature grazing sheep crowned with succulent green leaves. Beyond its charismatic appearance, it belongs to an elite group of animals capable of kleptoplasty—stealing living chloroplasts from eaten algae to produce energy directly from sunlight.
Biology and Physical Characteristics
Costasiella kuroshimae is a minuscule gastropod measuring mere millimeters in length. Its translucent white head bears two tiny black bead-like eyes set closely together, flanked by a pair of upright white sensory rhinophores tipped in velvet black that resemble fuzzy ovine ears. The dorsal surface is densely packed with flattened, leaf-shaped appendages called cerata. These cerata are packed with branches of the slug’s digestive diverticula containing active chloroplasts, imparting a luminous bright green hue capped with delicate pink or orange glandular tips.
Behavior and Ecology
The life history of the leaf sheep revolves entirely around its host alga, Avrainvillea. Using a specialized radula equipped with needle-like piercing teeth, it punctures individual algal cell walls and sucks out the cytoplasmic fluid. Rather than digesting the chloroplasts, the slug incorporates these intact photosynthetic organelles into the cells lining its cerata through kleptoplasty. These retained chloroplasts remain functional for months, generating sugars and amino acids through photosynthesis that nourish the slug during periods of food scarcity, rendering it functionally ‘solar-powered.’
Conservation and Human Impact
Evaluated as Least Concern, Costasiella kuroshimae remains prevalent across tropical Indo-Pacific reefs wherever suitable Avrainvillea algae colonies thrive. However, because it relies on delicate, shallow coral reef flats, it is indirectly vulnerable to coral bleaching, ocean acidification, coastal siltation, and rising sea temperatures driven by climate change. In recent years, it has become a cherished icon among macro underwater photographers and marine science educators, sparking global public appreciation for marine microfauna.
Interesting Factoid
The Leaf Sheep is a real-life solar-powered animal: it grazes on green algae like a tiny sheep, steals the plant’s solar panels (chloroplasts), and can survive on pure sunshine alone for up to several months without eating a single bite!
Last updated: October 2026