25 Most Colorful Sea Slugs on Earth

Michele Collet
Michele Collet
Scribol Staff
Environment, January 26, 2012
  • The word ‘slug’ tends to conjure images of gross cabbage-eating creatures that leave slime trails in their wake; ugly drab-colored garden pests that wouldn’t be entered into even the lowliest of beauty contests. In the ocean, however, slugs can be quite the opposite…

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  • Regaled in a breathtaking mix of bold and vibrant colors, and appearing in a wide variety of shapes and sizes, the sea slugs known as nudibranchs at once fascinate and delight. We’ve previously featured the amazing glaucus atlanticus here on Environmental Graffiti; now it’s time for some other amazing sea slugs to show us what we’ve got.

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  • All of the images collected here show nudibranchs, particularly colorful types of sea slugs. There are other species that fall under the sea slug umbrella but we’ll bet few others display such an extraordinary array of different colors as nudibranchs.

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  • Nudibranchs can be as small as half an inch in length or as large as over 20 inches long (that’s the daddy of nudibranchs, the sea lemon!). Oddly, perhaps, for such brilliantly colored creatures, nudibranchs have more or less black and white vision and so cannot see their own vivid tones.

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  • “Nudibranch” means “naked gills” in Ancient Greek; an appropriate enough moniker given the exposed organs found on many species’ backs through which the little critters breathe.

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  • Molluscs with no shells, at least in their adult stage, nudibranchs might seem somewhat vulnerable as they crawl about underwater in search of food, but they’re not as defenseless as they might at first appear. For one, they’re able to withdraw their feathery gills into their bodies at the first sign of danger, and as we shall see, their flashy colors don’t necessarily say: “Eat me…”

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  • Nudibranchs also have two tentacles, called rhinophores, on the backs of their heads that they use to smell, feel and taste their surroundings and generally get about the place. Most nudibranchs can also pull these sensors inside their bodies when they are threatened. Incidentally, the name “rhinophore” comes from the Greek “rhino”, meaning “nose”, and “phoros”, meaning “bearing”.

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  • As members of the order of gastropods, nudibranchs are characterized by the their large sticky “foot”, a large flat muscle beneath their bodies that allows them to move from place to place, albeit slowly. Like their terrestrial cousins, they even leave slime trails – though unlike your average garden slug, nudibranchs can also swim short distances.

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  • These slime trails left behind by nudibranchs serve a couple of purposes. For one, they send a signal to potential mates as to the roaming nudibranch’s whereabouts; second, they warn other nudibranchs of danger through chemicals released into the slime by the critter that created it.

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  • One might think that, without the protection of a shell, nudibranchs would be as scarce as hen’s teeth – but no: there are over 3,000 species distributed throughout the Earth’s oceans, everywhere from murky depths to sunlit shallow waters. Furthermore, what these incredible creatures lack in protective covering is more than made up for by some unique and fascinating forms of defense.

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  • The skin of nudibranchs is itself tough and bumpy, but that isn’t enough to deter most predators, such as hungry fish and ravenous sea turtles! In place of heavy armor – the shells they have cast off during the course of evolution – nudibranchs have more subtle weapons with which to see off enemies: toxic secretions and stinging cells are much more of a deterrent!

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  • What’s more, as we’ll soon see, it’s not just the fact that nudibranchs have these chemical weapons in their armory and how they deploy them that’s interesting; the way in which they obtain them is fascinating too. See, while some species manufacture their own poisons, most steal the toxins of the foods they eat.

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  • For example, some nudibranchs consume toxic sponges, processing and storing the irritants before secreting them from their skin cells and glands when they are on the receiving end of unwanted attention from aggressors. The chemical defense clouds the water around them, putting off would-be predators.

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  • Certain nudibranchs are immune to the stingers (nematocysts) of fire corals, anemones, and even siphonophores like the deadly Portuguese man o’ war. Rather than digesting the stinging cells, they move them through their bodies and keep them in the skin of their innocent looking and beautiful flowing cerata – the tentacles on top of their bodies – ready to discharge them against enemies when necessary.

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  • Yet before it even gets to the point of needing to use toxins and stingers, the nudibranchs have other defensive measures at their disposal. Their bright colors are not for the purpose of flaunting fashion-model good looks but instead serve as a warning to others that danger lurks in their squidgy bodies. Some rely on highly contrasting patterns and colors – brilliant blues and reds against a green background, for example.

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  • Interestingly, some nudibranchs have no toxins to deploy but instead mimic the gaudy colors of cousins that do. Predators will stay away from both species if they have had a previous distasteful experience with one.

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  • Camouflage is also made use of by nudibranchs. Many species have coloring that conceals and protects them by mimicking the appearance of surrounding plants. Incredibly, nudibranchs can also acquire the color of the foods they eat. A nudibranch that eats an orange sponge, for example, may take on the reddish pigment of its lunch and then be able to hide in its midst, invisible to preying eyes.

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  • As carnivores, nudibranchs themselves prey upon a range of other species – from sponges and coral to anemones, barnacles and fish eggs. Prone to the odd spot of cannibalism, they will also eat other nudibranchs and sea slugs. Different species are, however, quite choosy about what they eat, often sticking to just one type of prey.

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  • One of the most amazing facts about these sea slugs is that they can farm food inside their bodies! In fact, it’s not a stretch to say that at least one species is solar-powered – which is about as cool as it gets in our book!

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  • Take the example of the blue dragon (Pteraeolidia ianthina). This nudibranch eats organisms filled with algae – which, as plants, make their food using the sun through the process of photosynthesis.

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  • Rather than digest the algae, the clever blue dragon stores it in its cerata (tentacles) where it continues to use sunlight to make sugars that the nudibranch subsists on. The blue dragon can go for days on end without eating if it catches enough rays in shallow water. What a useful trick!

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  • One other cool fact about these sea slugs is that they are hermaphrodites, meaning each one has the reproductive organs of both sexes. Now this makes sense. It isn’t as if they can head out to the latest club looking for romance – and potential partners might be few and far between for these solitary and slow-moving creatures. In short, whatever flirty nudibranch comes along next is fair game – and the nudibranch can adapt to either lay or fertilize the eggs.

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  • The eggs themselves are rather fascinating, too. They are laid in long spirals, ribbons or clumps – as many as 2 million eggs in one sitting! Some nudibranchs are born as small but fully formed juveniles that begin crawling right away (and certain species even have shells that they later lose). Most, however, emerge as larvae that will gradually make their way to the ocean bed until they are ready for their transformation into adulthood.

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  • Humans have begun to find nudibranchs of use in the search for new medicines, with the chemicals used by the sea slugs liable to help scientists in finding new ways to healing human organs. And people in places as far and wide as Alaska, Chile and Russia are known to eat sea slugs – cooked or even raw!

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  • One thing’s for sure: there’s plenty more to learn about these incredibly colored gastropods.

    Sources: 1, 2, 3, 4, 5, 6, 7

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