One of the many reasons the snorkeling community is so great is for their unmatched enthusiasm and intrigue for sighting wildlife of any kind. The ooooos and aaaas are just as loud for a 15 meter whale shark as they are for a 5cm slug, and honestly sometimes louder. Nudibranchs are the perfect example of this. See one of these colourful fellas out on the reef, and suddenly it’s the highlight of the snorkel. Once you’ve spotted one, you can’t wait to tell your fellow snorkelers so that they can swim over to admire your prized find. So what is so special about these multicoloured sea slugs?
Nudibranchs are a group of soft bodied mollusks, meaning they are a part of the extremely diverse taxonomical group that includes cuttlefish, nautilus shells, giant clams and limpets, to name a few. But unlike many of their mollusk cousins, nudibranchs lack any kind of armour or shell, they are, as their name suggests, naked. Their name derives from their anatomy as ‘Nudi’ comes from the Latin word ‘nudus’ meaning naked and ‘branch’ come from the Greek word for lungs. Their lungs, or gills, are the colourful frills decorating their backs. So if you see a tiny colourful worm with frills on its back moving rather slowly over the reef, you’re likely looking at one of 3000 nudibranch species.
Why So Colourful?
Each nudibranch has its own unique flare, they combine colour, pattern and form in every imaginable way, so that flicking through a nudibranch identification guide can feel rather psychedelic. They are truly beautiful. But being tiny, slow and rainbow coloured isn’t exactly the best tactic to staying alive on the reef, or is it? Nudibranchs use colour to their advantage in a number of ways.
Some nudibranchs use colour to blend in. These species specialise in living on specific hosts such as brightly coloured sponges. The nudi will eat the sponge, uptake its colour pigments and incorporate them into its tissues so that it blends in perfectly with its home.
Other species do the opposite, they will purposefully not blend in, making themselves extremely visible to passers by. This tactic is a warning for potential predators that I might be tiny but I pack a punch (aka I’m very poisonous). These colourful slugs will eat poisonous animals, uptake their toxins and become poisonous themselves. Like the Glaucus Atlanticus, commonly known as the blue dragon nudibranch, which preys on the extremely poisonous Portuguese Man ‘O War tentacles. Uptaking its stinging cells, the nudi becomes even more toxic than the Man’O War itself. These guys can be found floating on the surface of the water, using the wind to travel and can often wash up on beaches in strong weather. So if you encounter a bright blue dragon-like slug on the beach, please don’t touch!
The Original Solar Panels
Nudibranchs are all about green energy, they literally carry little solar panels on their backs. This group of nudis called the Philodesmium family, have yet another ingenious way of taking advantage of their surroundings. As they feed on sea anemones they harvest symbiotic algae from the anemone’s cells and instead of digesting them, store them in the tissue of the ‘cerata’ on their backs. These cerata are like little hairs on the nudibranch’s top side which face the sun. The algae then absorbs the sunlight, photosynthesises and shares its energy with the slug. Talk about working with what you’ve got! Amongst this group of clever nudibranchs is the famous, and arguably cutest nudi; Shaun the sheep.
Even Their Eggs Are Beautiful
Knowing what we now know of nudis it is to be expected that even their methods of reproduction are flamboyantly unusual. Nudibranchs follow each other’s slime trails, to find one another in the vast expanse of the ocean. Once, they do find the love they’ve been looking for, the mating commences.
Most nudibranchs are hermaphrodites, meaning they have both male and female reproductive organs. Now, I’ll try to not get too graphic here. The nudis spend a couple of minutes to a couple of hours together, doing a little dance, whilst they exchange sperm. Both nudibranchs can then store that sperm for up to a few weeks at a time whilst they develop their ova, and then internal fertilisation can take place.
Once fertilised, it is time to lay their eggs. Nudibranch eggs are just as beautiful as the nudibranchs themselves. They lay them in pink, spiral shaped ribbons that resemble a rose attached to the reef.
Where to find them?
Nudibranchs can be found in every part of the world, from the icy waters of Antarctica to the warm waters of Indonesia so if you’re dying to meet one you’re in luck! Nudibranch whereabouts are currently of particular interest to environmental researchers. Although found all over the world, each species is highly specific about the conditions it can live within. This coupled with their very short life spans (often less than a year), means that their populations quickly reflect environmental changes, providing rapid feedback on ecosystem health.
In fact, you as a snorkeler can help this important research. Using the sea slug census, a global citizen science initiative, you can log your sightings of any nudibranch, anywhere in the word and contribute towards vital data that helps us further understand environmental change.
Final thoughts
Through writing this blog, I myself fell right back in love with these charismatic critters. They are yet another example of animals manipulating their environment to survive in the most unlikely and imaginative ways possible (and they do it in style).
Blue dragon picture by Sylke Ronrlack, shared under the Creative Commons license
