Costasiella kuroshimae is a species of shell-less sea slug in the family Costasiellidae. The creature is often nicknamed the leaf sheep due to the combination of its head, which resembles a sheep, and its dorsal organs, which look like a cluster of leaves.
The physical appearance is packed into a tiny body. Upon reaching adulthood, the body size ranges from 5 to 10 millimeters.
The leaf sheep was first identified in 1993 in the waters off Kuroshima Island, Japan. Beyond the initial location of discovery, Costasiella kuroshimae is also distributed across tropical waters in the Indo-Pacific region, including the Philippines and Indonesia.
The anatomical arrangement of the head and back defines the physical appearance of Costasiella kuroshimae. On the head, the slug possesses two dark eye spots and two sensory structures called rhinophores that extend upward like sheep ears or insect antennae.
Meanwhile, the back is covered with rows of green, protruding organs called cerata. The structure of the cerata connects directly to the digestive gland of the slug, while also playing a role in oxygen exchange and defense against surrounding threats.
Mechanisms of Algae-Derived Chloroplast Retention
Costasiella kuroshimae maintains unique feeding habits in its natural habitat. The slug acts as a selective feeder that relies entirely on green algae from the genus Avrainvillea for food.
When consuming the algae, the digestive tract of Costasiella kuroshimae sequesters chloroplast cells intact without damaging them. The isolated chloroplasts are then distributed and stored inside the cells of the cerata.
The process of absorbing algal chloroplasts into the cells of the cerata is known scientifically as kleptoplasty. The chloroplasts stored within the cerata remain functionally active, carrying out photosynthesis to meet energy needs.
Nevertheless, the photosynthetic capability of Costasiella kuroshimae does not last indefinitely, categorizing it as short-term retention.
Based on a study published in Frontiers in Zoology in 2009, measurements of photosynthetic capacity using Pulse Amplitude Modulated (PAM) fluorometry revealed that chloroplasts stored within the cells of the slug remain functional for only about 1 to 2 weeks.
As a result of the limited timeframe, the bright green color of the cerata gradually fades as chloroplast function declines. The slug must regularly feed on Avrainvillea algae to replenish the chloroplast supply within the body.
In addition to driving photosynthesis, the chloroplast cells stored in the cerata serve as a food reserve (larder). When algae in the environment become scarce, the slug digests the stored chloroplasts to sustain survival.
Seafloor Survival Strategies
Beyond supplying nutrients, the stored chloroplasts inside the cerata function as camouflage. The stored green pigment allows the body of the sea slug to match the shade of the Avrainvillea algal fronds.
The color alignment with the host algae serves as the primary defense mechanism for Costasiella kuroshimae to evade predators around coral reefs.
Avrainvillea algae also play an essential role in the reproductive cycle of the slug. As a simultaneous hermaphrodite, every individual Costasiella kuroshimae possesses fully functioning male and female reproductive organs at the same time.
Following fertilization, the slug deposits eggs onto the surface of the Avrainvillea algae. The eggs are laid in tiny spiral ribbon structures until hatching into larvae.
The reliance on Avrainvillea algae renders the presence of Costasiella kuroshimae particularly vulnerable to environmental disruptions. Damage to the host algal population directly impacts the food supply, and breeding sites of the sea slug.
Reference:
Händeler, K., Grzymbowski, Y. P., Krug, P. J., & Wägele, H. (2009). Functional chloroplasts in metazoan cells - a unique evolutionary strategy in animal life. Frontiers in Zoology, 6, 28. https://doi.org/10.1186/1742-9994-6-28
