The Leaf Sheep Sea Slug, scientifically known as Costasiella kuroshimae, has gained widespread attention due to its endearing, cartoon-like appearance that seems to blend the features of a mollusk and a plant. Its tiny size and vibrant green hue have made it an internet sensation, sparking curiosity about its biology. The most compelling question surrounding this creature is whether this animal truly possesses the ability to photosynthesize like a leaf. This rare phenomenon in the animal kingdom makes this small sea slug one of the most fascinating organisms in the ocean.
What is the Leaf Sheep Sea Slug?
Costasiella kuroshimae is a diminutive marine organism, typically measuring only 5 to 10 millimeters in length. It belongs to the Sacoglossa order of sea slugs, a group characterized by their specialized diet of algae. The slug’s translucent body is adorned with numerous leaf-like appendages called cerata, which are responsible for its common name, the Leaf Sheep. These invertebrates are found in the tropical and subtropical Indo-West Pacific, inhabiting shallow coastal areas near Japan, the Philippines, and Indonesia.
The Direct Answer: Functional Kleptoplasty
The Leaf Sheep Sea Slug can photosynthesize indirectly through a process called functional kleptoplasty. Kleptoplasty is a biological phenomenon where an organism sequesters and retains functional chloroplasts—the organelles responsible for photosynthesis—from the food it ingests. This mechanism allows the slug to temporarily utilize the energy-producing machinery of a plant cell.
The slug does not possess the genetic blueprint to manufacture its own chloroplasts, so it cannot be classified as a truly photosynthetic organism. Instead, it “steals” the developed organelles from the algae it consumes, which is why these sacoglossan sea slugs are often referred to as “solar-powered.” The stolen chloroplasts remain active within the slug’s cells for a period of time, fixing carbon and producing photosynthates. Once incorporated, they continue to convert sunlight, water, and carbon dioxide into sugars, providing the slug with supplemental energy, especially during periods of starvation.
How Chloroplasts Are Acquired and Maintained
The kleptoplastic process begins with the slug’s highly selective diet, which centers on green algae from the genus Avrainvillea. The sea slug uses its single radular tooth to puncture the cell wall of the algal filament and then sucks out the internal contents, a process known as sap-sucking. The slug is adept at digesting the bulk of the algal cytoplasm while leaving the chloroplasts intact.
These unharmed chloroplasts are transported through the digestive tract and incorporated into the cells lining its digestive diverticula, which fill the cerata. The numerous cerata provide a large surface area for light absorption, maximizing the efficiency of the stolen chloroplasts. Because the slug lacks the necessary algal genes to repair or replace degraded photosynthetic proteins, the chloroplasts eventually lose their function. This temporary nature means the Leaf Sheep Sea Slug must periodically feed on Avrainvillea to replenish its supply and sustain its solar-powered ability.