N is for Nudibranch

Did you know? Nudibranchs' bold colors come from their diet!

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If sea slugs sound like the stuff of nightmares, think again. Adorned in vibrant purples, ruby red swirls, yellow spots, neon green streaks, bright blue spikes and glowing orange flecks, nudibranchs are more like the stars of a psychedelic daydream.

Illustration for N is for Nudibranch with a Nudibranch Perched Atop a Rock

The name nudibranch means "naked gills," describing the shell-less sea slugs' exposed respiratory organs. There are more than 3,000 known nudibranch species and potentially thousands still yet to be discovered. They thrive throughout the world's oceans but are most prevalent in tropical, shallow waters.

Sea slugs come in all sorts of shapes and range in size from .25 inches to a foot. They can be short, round, long or flat with soft spines, horns and bumps. Some have branching tendrils that extend from their backs or wavy, wing-like frills that outline their forms. A few (arguably less exciting) nudibranchs are actually colorless.

The soft-bodied mollusks carnivorously patrol the seafloor chomping on algae, anemones, corals and sponges. Their flamboyant colors come from their diet, as do the poisons that some of them secrete. Other nudibranchs pilfer stinging cells from their prey to use as another line of defense against predators.


Sea slugs are hermaphrodites, which means they have both male and female reproductive organs and can mate interchangeably. One Australian species, the siphopteron, has even been observed stabbing its mate in the head, injecting it with a prostate gland fluid.

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