It takes a diver with very keen eyesight to spot one of the ocean’s most adorable molluscs. This species of tiny sea slug, Costasiella kuroshimae – also known as the leaf sheep – grows, at most, to a centimetre in length (about a fingernail’s width). And they’re superbly camouflaged.
Their green bodies perfectly match the colour of the seaweeds they live on, because this is also what they eat. And when they do eat it, something amazing happens: they turn into solar-powered sea slugs.
Just like land plants, seaweeds have tiny structures inside their cells called chloroplasts, which carry out the process of photosynthesis. They harness energy from sunlight to convert carbon dioxide into sugars.
When the leaf sheep eats seaweed, like sheep grazing on grass, they can digest those sugars. Alternatively, they can keep hold of intact chloroplasts without chewing them into a sugary mush, and push them inside their bodies to save for later use.
The structures along the leaf sheep’s back, which look like little leaves, are known as cerata. Each structure contains an extension of the sea slug’s digestive system and is chock full of chloroplasts, giving the cerata their grainy texture.
Amazingly, these swallowed chloroplasts keep on photosynthesising and producing more sugars. So, as long as the sea slug stays in shallow, tropical waters with plenty of access to sunlight, they have a source of food that they can carry around with them.
Scientists first spotted this species in the early 1990s on the Japanese island of Kuroshima, hence the species name kuroshimae.
Since then, divers have looked carefully at the right kinds of seaweed – leaf sheep are very choosy with their food and only live on Avrainvillea green algae – and found these tiny sea slugs throughout Indonesia and the Philippines.
Like many species of seaweed-munching sea slugs, leaf sheep lay eggs in neat spirals, which hatch into larvae and drift through the water. For a short time, the young sea slugs have tiny shells before abandoning them and living shell-free.
Stealing chloroplasts from seaweed is a process scientists refer to as kleptoplasty and it occurs in various other species of sea slugs. Along British and other European shores, green elysia sea slugs (Elysia viridis) can be found living among the velvety digits of Codium seaweed (aka dead man’s fingers).
They don’t have leafy projections on their back like leaf sheep, but instead have two wing-like flaps that they unfurl to catch as much sunlight as possible to power their home-grown food factory.
When spread out like this, these sea slugs take on the resemblance of floating leaves. Another species, Elysia marginata, not only steals chloroplasts, but can perform an even more astonishing trick. Similar to a gecko detaching from its tail, these sea slugs can separate their heads from their bodies.
The process takes several hours and the decapitated body can survive for a few days, but it doesn’t regrow a new head. The original head, however, wanders around on its own for a while, before sprouting a new body.
This head-chopping behaviour may have evolved as an effective, although rather extreme, way of getting rid of a body infested with parasites.
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