Divers drool at the sight of them and underwater photographers can spend entire dives trying to capture them in perfect pose. They’re found throughout the world’s oceans, but are most abundant in shallow, tropical waters and flaunt some of the most intriguing shapes, lavish hues and intricate patterns of any animal. They can be thick or flattened, long or short, and can be as small as six millimetres or as large as 30 centimetres long.
Nudibranchs, it turns out, are no ordinary molluscs. Unlike other shell-less sea slugs, most wear feathery gills on their backs, a feature that formed their scientific name, Nudibranchia, meaning naked gill.
Blind to their own beauty, their tiny eyes discerning little more than light and dark, they rely on smell, taste, touch (using their head-mounted sensory tentacles called rhinophores) and chemical signals to track food and navigate.
The 3 000-plus known nudibranch species are well equipped to defend themselves. Not only can they be tough-skinned, but they’ve also traded the family shell for less burdensome weaponry: toxic secretions and stinging cells. A few make their own poisons, but most pilfer from the foods they eat and deploy the stolen artillery in acts of self-defence. Species that dine on toxic sponges, for example, store the irritating compounds in their bodies and secrete them when disturbed. Other nudibranchs hoard capsules of coiled stingers, called nematocysts, ingested from fire corals, anemones and jellyfish.
Nudibranchs derive their colouring from what they eat, which can either help camouflage or warn potential predators that they’re distasteful or poisonous.
To further ensure their species’ survival, nudibranchs are hermaphrodites (they have both male and female organs) and can fertilise one another, an ability that speeds the search for mates and doubles reproductive success. Depending on the species, pairs may lay up to two million eggs in coils, ribbons or tangled clumps.
Still, nudibranchs have hardly given up their secrets. Scientists estimate that they’ve identified only half of all nudibranch species and even the known ones are elusive. Most live no longer than a year, then disappear without a trace, their boneless, shellless bodies leaving no record of their brief, brilliant lives.
