Overview
The Bunny Leaf Slug (Costasiella usagi) is a microscopic, captivating sacoglossan sea slug celebrated for its uncanny resemblance to a cartoon rabbit. Reaching mere millimeters in length, this miniature marine gastropod roams shallow coral reef flats across the Indo-West Pacific, grazing on velvety mats of green macroalgae. Beyond its charismatic appearance, Costasiella usagi possesses the extraordinary biological capability of kleptoplasty, sequestering viable chloroplasts from its algal food to conduct solar-powered photosynthesis within its own living tissues.
Biology and Physical Characteristics
Costasiella usagi is characterized by a translucent, pearlescent white head adorned with two erect, sensory rhinophores that look strikingly like rabbit ears, tipped in dark pigment or delicate mauve. Between the rhinophores sit two tiny, beady black eyes positioned close together on the forehead, completing its facial ‘bunny’ profile. Its back is densely covered in leaf-shaped fleshy cerata filled with branching digestive diverticula. These cerata glow with vibrant emerald green due to millions of intact chloroplasts extracted intact from digested algal cells, often tipped with luminous blue, magenta, or white dots.
Behavior and Ecology
A specialized herbivore, Costasiella usagi feeds almost exclusively on colonial green algae of the genus Avrainvillea. Using a specialized single row of radular teeth shaped like tiny sabers, the slug pierces individual algal cell walls and suctions out the intracellular contents. Rather than digesting the algae’s photosynthetic chloroplasts, the slug shunts them into the epithelial cells of its cerata, where they remain functionally active for weeks, generating sugars and organic nutrients directly from ambient sunlight. When threatened by micro-predators, the slug can autonomously drop individual cerata (autotomy) to distract attackers.
Conservation and Human Impact
Although not currently evaluated with an individualized threat category by the IUCN, Costasiella usagi is widely distributed across healthy coral reef systems in the Coral Triangle. Like all sensitive reef invertebrates, its primary threats stem from coral reef degradation, agricultural nutrient runoff that smothers delicate macroalgae beds in silt, and escalating ocean acidification that disrupts marine gastropod reproductive physiology.
Interesting Factoid
The species name ‘usagi’ is the Japanese word for ‘rabbit,’ honoring the slug’s distinctive rabbit-like ear rhinophores and facial markings!
Last updated: October 2026