Locating one of the ocean’s most charming mollusks requires a diver with exceptionally keen vision. This tiny sea slug, Costa Sierra Crosimae—commonly referred to as a leaf sheep—reaches only a few centimeters in length, approximately the size of a fingernail. Their exquisite camouflage makes them hard to spot.
Their vibrant green bodies blend seamlessly with the seaweed they inhabit, which also happens to be their primary food source. An incredible transformation occurs when they consume it.
Similar to terrestrial plants, seaweed contains small structures called chloroplasts within its cells, which facilitate the process of photosynthesis. These chloroplasts harness sunlight energy to convert carbon dioxide into sugars.
When the leaf sheep feed on seaweed, akin to sheep grazing in a meadow, they can digest the sugars they consume. Alternatively, they can retain the entire chloroplasts without damaging them and incorporate them into their bodies for later use.
The features along the back of the leaf sheep resemble small leaves and are known as cerata. Each ceratum houses an extension of the sea slug’s digestive system, filled with chloroplasts, giving it a textured appearance.
Remarkably, these engulfed chloroplasts continue to photosynthesize, generating additional sugars. Therefore, as long as these sea slugs dwell in shallow tropical waters with abundant sunlight, they have a sustainable food source.
The scientist who first discovered this species in the early 1990s on Japan’s Kuroshima Island named it Black Himae.
Since then, divers have been diligently searching for the specific type of seaweed that these leaf sheep prefer, which is exclusively Avrainvillea green algae. These delightful sea slugs have been located in Indonesia and the Philippines.
Like other sea slugs that maintain various seaweed species, leaf sheep lay their eggs in a meticulous helix, allowing them to hatch into larvae that drift through the water. Initially, the young sea slugs possess small shells before eventually discarding them to live shell-free.
The process of adopting chloroplasts from seaweed is known as keratoplasia, which can be observed in many other types of ocean slugs. For example, the green Elysian sea slugs (found along the British and other European coasts, Elysia viridis) utilize Codium seaweed (also known as the dead man’s fingers).
While these slugs lack the leaf-like projections seen in leaf sheep, they possess two wing-like extensions that unfold to maximize sunlight absorption for their self-sustaining food factories.
In this position, these marine slugs resemble drifting leaves. Another species, Elysia marginata, not only captures chloroplasts but also performs astonishing feats. Similar to geckos that shed their tails, these sea slugs can separate their heads from their bodies.
This process takes several hours, and while the detached body can survive for days, it does not regenerate a new head. Meanwhile, the original head roams for a while before growing a new body.
This behavior of severing the head may have evolved as a drastic but effective method for eliminating parasite-infected bodies.
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