By Jemma Powell | Science Editor
Nudibranchs (noo-dee-branks) are small, carnivorous invertebrates that live on the sea bed in shallow tropical waters. Generally oblong in shape and often only 2cm long, these creatures sound completely uninspiring. That’s where you’d be mistaken. These tiny little molluscs are arguably some of most bizarre creatures on planet earth you’ve never heard of, and deserve more of a reputation.
If you’ve ever seen a nudibranch, the first thing you’ll notice is the extraordinary colours and designs on their skin. There are more than 2000 known species of nudibranch and each of these has a unique combination of some of the most vibrant colours seen in mother nature. From the pure white with fluffy black ‘ears’ (commonly known as sea bunnies), to the Chromodoris magnifica with its neon blue and purple stripes, bright orange and white ‘wings’, and deep red ‘horns’. There seems to be no colour and appendage combination too bizarre.
Nudibranchs owe their vivid colours to the food they eat. Feeding on a wide variety of organisms such as coral, anemones, and even jellyfish, the sea slugs harvest their victim’s pigment and put it to their own use. Across the genus, colour is used either as a deterrent, a pseudodeterrent, and even just as camouflage for the colourful world they live in.
With no natural sharp claws or tough shells of their own, some species of nudibranchs protect themselves by stealing other organisms’ weapons.
The main form of defence for aeolid nudribranchs is to approach a cnidaria a group of organisms whose characteristic feature are venom-filled harpoons such as jellyfish. They then eat the cnidaria, neutralise the venomous spikes and repurpose them for their own use. Some species of aeolids simply store the venom itself as a defensive mechanism if ingested, whereas others put the cnidarias stingers in finger-like structures protruding from their backs, essentially creating their own mini harpoons.
There are multiple examples in the natural world of animals being able to survive without certain parts of their body. Some even do this on purpose as an escape mechanism, such as lizards who can shed their tails under attack (caudal autotomy). The lizard can escape the grasp of a predator and regrow its tail to a similar standard.
Nudibranchs take this concept a step further. If under threat and the venomous spikes, stored poison, and camouflage isn’t working, nudibranchs can jettison their entire body, leaving behind its major organs and even its beating heart.
Some studies have found nudibranchs can and do abandon up to 85% of their entire body mass in self decapitation.
As a genus, they have surprisingly few natural predators (mainly due to their stolen venom and lack of nutritional benefits), so performing autotomy in the face of imminent danger is uncommon. Incredibly, this process was hypothesised to be a defence against parasites and viruses. They can leave their own infected organs behind if
they start to fail. Even more remarkably, not only do they survive this but they have regrown all that body left behind in just two weeks.
The potential for applying this process to human medicine is very exciting, and currently being researched in multiple institutions worldwide. One particular genus of nudibranch (Phyllodesmium) is also capable of photosynthesis. This genus feeds primarily on coral, and can eat the photosynthetic algae on them without damaging their capacity to convert sunlight to energy.
In a similar way to the repurposing of venomous spikes, the nudibranchs can then move these algae to certain parts of their body surface (cerata). Once there, the algae continue to photosynthesis providing up to a quarter of the slug’s energy requirements.
Sadly, as with many creatures around the world, their survival is under threat from pollution, habitat degradation and climate change. But, this species is not gone yet,
and while they’re still here they can be saved.