Psychedelic slugs

All of these are nudibranchs. 1.1 (Row 1 picture 1), 2.1, 2.4, 3.1, 4.2, 4.3, 6.2, 6.3, 6.4 Silke Baron, CC BY 2.0 (1.1, 2.4, 3.2, 4.2, 6.3, 6.4 adjusted, cropped); 2.1, 4.3 cropped; 6.2 adjusted). 1.2, 1.3, 2.3, 3.3,4.1, 5.1 5.2, 5.3, 5.4, 6.1 Orangkucing, CC BY-SA 3.0 US (1.2, 1.3, 2.3, 4.1, 5.1, 5.2, 5.3, 5.4 adjusted; 3.3, 6.1 adjusted, cropped). 1.4 Steve Lonhart, NOAA MBNMS. 2.2 Hidekatsu Kajitani, NPS. 2.5 Claire Fackler, CINMS, NOAA, CC BY 2.0. 3.1 NOAA.

Nudibranchs—pronounced new-duh-branks—are some of the most beautiful creatures in the sea. Just by looking at them, most people wouldn’t expect that they’re sea slugs, which is what they’re commonly called. They come in a wide variety of shapes—some with frills along their edges or short tentacles on their backs or the tips of their limbs if they have them—and can be vividly colorful. Some have a rosette of tentacles on their lower back that look like fern fronds, but are actually external gills. Many others have no gills and just absorb oxygen through their skin. They also use some of their frilly tentacles to smell and taste. This makes them appear delicate.

Their undulating form at times resembles a piece of silk slowly floating on the wind, although a nudibranch is much thicker. Sometimes they just look like a torn piece of seaweed. Others are more mound-like, resembling a snail’s foot or like a shell-less abalone. Some have features that look like soft leafless trees on their backs. When found in tide pools, they tend to look like lumps of jelly, but when in water they unfurl in some amazing shapes. And in the tropics their bright and varied colors rival those of butterflies.

Some are so good at camouflage, even disguising themselves as seaweed, that you can look right at them and not know they’re there. You may have to follow the slime trail they leave behind to find them.

They are mollusks and are related to snails. They crawl like snails, but swim upside-down with their undulating foot upwards. They’re brightly colored as a warning to predators to stay clear or it will get stung.

They are voracious eaters and bad tempered. Most of the time they indiscriminately attack all other nudibranchs, even those of their own species. This sometimes results in fights where they lunge and bite each other. Sometimes one will eat the other. But they prefer eating sea anemones, consuming at each feeding between 50 to 100 percent of their own body weight. Anemones try to avoid this by living high in the intertidal zone. They also have some behavioral defenses, and when damaged they can regrow the tissue and clone themselves.

Some nudibranchs like to eat chunks out of anemones by using their radula—their coiled tongue covered with tiny teeth used for scraping and drilling holes. Researchers aren’t sure why they aren’t stung by the anemones’ tentacles. One hypothesis is that the nudibranch will sneak up and rub itself against the anemone’s trunk, coating itself with the anemone’s mucus so when the anemone’s tentacles touch it, the anemone will sense it as just being a part of itself. Then these sea slugs can eat all they want without being stung. Others think nudibranchs use their own protective mucus. Perhaps different types of nudibranchs use different methods.

Not all sea slugs are nudibranchs. Here, one of the largest sea slugs, which is not a nudibranch, is about to rapidly suck in a hapless nudibranch. Steve Lonhart, NOAA MBNMS.

At least one type appears to hunt in packs. These are white ones with rows of horn-like spurs running down their backs usually found in the Atlantic and Mediterranean. When a nudibranch starts eating a anemone, if the anemone detects something, it leans over in that direction and the attacker is more likely to be stung, but it doesn’t do this when it’s surrounded by three to more than six nudibranchs, so this mobbing behavior might help protect the slugs.

Oddly, nudibranchs also have stinging cells, but aren’t their own—they’re stolen from anemones. Nudibranchs can eat parts of the anemone that contain stinging cells, but the stingers don’t fire. Not only can it swallow the stinging cells without setting them off, while absorbing the nutritional parts, the stinging cells move from its digestive system out to its own tentacles, where it can then use them against anything that touches it by firing off a volley of toxic harpoons.

I find it amazing that they can do this. It’s like if you ate an anemone and then your body moved the venomous cells out to your finger tips where they would sting anyone you touched.

Each species tends to specialize in eating only one type of prey and some obtain their stinging cells by eating corals or jellies. Certain types of nudibranchs obtain toxins from toxic algae—some of which only live in sponges—storing and concentrating these toxins in specialized glands for later use. Others absorb algae that take up residence inside them and contain bacteria that make the toxins they use in a three-way nudibranch/algae/bacteria symbiotic relationship.

Now think about this for a second. They are taking another animal’s weapons and putting them to their own use. It must have taken a long time to evolve that amazing ability.

Some of them can also steal chloroplasts from the algae they eat and put them to use turning sunlight into energy, like in plants, but they’re only able to retain the chloroplasts for up to three months. During that time they can go without eating, but when they have to replace them, they pig out, eating for hours. Still, this essentially turns them into solar-powered sea slugs, or partly solar powered at least.

Blue dragon. Sylke Rohrlach, CC BY-SA 2.0.

One unusual stinging nudibranch that looks like it has fancy wings is called the blue dragon. It lives upside-down floating on the ocean’s surface with its blue side up and its silver side down and contains symbiotic algae that produce food for it. They get their stingers from eating Portuguese man-of-wars, which are among the ocean’s most poisonous creatures.

 

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