It's A Slug's Game Solar-Powered Sea Slug

Sea slugs sport some of nature’s most unreal adaptations, including solarpowered skin and disposable penises. Welcome to their weird world

- Dr Helen Scales

It looks like a tiny, cartoon sheep, but this is in fact one of more than 4,700 known species of sea slug that creep and occasionally swim through the ocean. Known as the leaf sheep (Costasiella kuroshimae), this sea slug lives around coral reefs, where it grazes on clumps of algae. The green colouration comes directly from its food – a leaf sheep retains the algae’s chloroplasts (tiny cellular structures that harness the Sun’s energy to make sugars) and incorporates them into its skin, where they continue to photosynthesise. This trick of stealing chloroplasts, called ‘kleptoplasty’, stops the leaf sheep from starving when there’s no food around. It’s one of a range of weird and unique survival techniques that sea slugs have evolved.

The term ‘sea slug’ is applied to an assorted and flamboyant bunch of molluscs, all close relatives of snails that evolved a shell-free adult life. They are found almost anywhere in the sea, from rock pools to the deep sea, in tropical and temperate waters, and even in the Arctic and Antarctica.

MARVELLOUS MOLLUSCS

Sea slugs come in an amazing variety of colours and shapes. “The geometric beauty of them is startling,” says Heather Buttivant, author of Rock Pool: Extraordinary Encounters Between The Tides, and an expert in finding sea slugs around the British coast. Many, including this tropical sea slug (Cyercenigricans), are covered in projections called ‘cerata’. When this species gets disturbed, perhaps by a hungry fish or crab, it jettisons some of its cerata, like a lizard dropping its tail, to distract the intruder, while it makes its escape.