Photosynthetic Sea Slug *Costasiella kuroshimae* Exhibits Kleptoplasty

The marine ecosystem continues to present biological mechanisms that challenge conventional classification, exemplified by the diminutive sea slug, *Costasiella kuroshimae*. Often nicknamed the 'leaf sheep' due to its morphology, this organism possesses the specialized capability of kleptoplasty, allowing it to functionally mimic plant-like energy production, a trait typically reserved for flora and certain microorganisms. This unique survival strategy places the slug among a small cohort of creatures, including other sacoglossan sea slugs and some marine flatworms, that blur the established boundaries between animal and plant kingdoms.

The process begins when the slug grazes upon specific marine algae, notably species from the genus *Avrainvillea*. Instead of fully digesting the ingested material, *C. kuroshimae* selectively sequesters the chloroplasts—the organelles essential for photosynthesis—from the algal cells. These sequestered chloroplasts are then integrated into the slug's own tissues, primarily within the leaf-like protrusions known as cerata, enabling the mollusk to generate energy directly from sunlight. This photosynthetic supplement is vital, supplying the slug with necessary carbohydrates and lipids, particularly when primary food sources become scarce.

*Costasiella kuroshimae* was initially identified in 1993 near Kuroshima Island, Japan, which serves as its type locality within the Ryukyu Islands. These shell-less marine opisthobranch gastropod mollusks inhabit shallow, tropical and subtropical coastal waters extending from Japan through the Philippines and Indonesia. They are small, generally measuring between 5 to 10 millimeters, or up to 0.3 inches, in length. Their physical structure includes two dark eyes and two rhinophores projecting from the head, sensory organs used to detect chemical signals in the water to locate sustenance.

The retention of functional chloroplasts, or kleptoplasts, for extended periods presents a significant biological question, given the substantial differences between an animal cell's internal environment and that of an alga. While *C. kuroshimae* demonstrates short-term retention, relatives like *Elysia chlorotica* have shown evidence of incorporating algal genes into their own chromosomes to sustain chloroplast function for up to nine months. For *C. kuroshimae*, the sequestered chloroplasts function as a nutrient reservoir, or 'larder,' supporting survival during starvation, although the precise genetic mechanism for long-term maintenance is less understood than in related species. Furthermore, the cerata housing these organelles also serve dual functions in respiration and defense, while the chloroplasts contribute to the creature's vibrant green coloration, providing effective camouflage among the algae it consumes.

The study of these organisms provides valuable insights into cellular biology and the potential for functional gene transfer, a concept with relevance to broader biological research. These gastropods, sometimes nicknamed 'Shaun the Sheep' for their resemblance to the animated character, also play an ecological role in moderating local algae populations near coral reefs. However, the species remains susceptible to environmental pressures, including climate change and habitat loss, emphasizing the importance of continued exploration and conservation efforts for this unique component of marine biodiversity.