Range: All the world’s oceans—from the tropics to Antarctica—at all depths, but most abundant in warm, shallow waters and the intertidal zone
Population: Total pop. unknown. Scientists frequently discover new species
How to spot them: Generally oblong in shape—can be thick or flattened, long or short; often brightly colored but can be colorless; feathery gills and horns; span from 0.25 to 12 inches long
IUCN Red List Status: A few species, such as the blue glaucus, are endangered
The ocean is teeming with all kinds of fascinating creatures, but one of the strangest—and most beautiful—may very well be one you’ve never heard of. Nudibranch (NEW-dih-bronk)—from the Latin for ‘naked gills’—is the umbrella name for more than 3,000 species (and counting) of shell-less mollusks, part of the sea slug family. Splashed with technicolor hues and intricate patterns, these photogenic sea slugs are small but fierce—and often deadly, as science has discovered. Serious research on the ocean’s tiny jewels kicked off fairly recently, during the 1960s. Today, there are only about 40 to 50 nudibranch experts around the globe delving into the slugs’ unique adaptations and pushing research forward.
One of the most incredible discoveries they’ve made to date is the creative way nudibranchs protect their naked, fleshy bodies, once they shed their larval stage shells. Aside from those flashy colors, which serve as a warning to predators looking for a quick snack, nudibranchs are able to render themselves toxic by stealing the defensive poisons from their prey—and then stockpiling it in beautiful feathery gills, called cerata. The sea lemon, for instance, uses toxins from the sea sponges it eats to give itself a sour taste. Other nudibranchs are much deadlier—one species chows down on the hyper-toxic tentacles of a Portuguese man-of-war, then serves up a lethal burst of those chemicals when a predator goes in for the kill.
Learn more facts about these spectacular sea slugs.
On the Prowl
These tiny carnivores use actual teeth or even digestive enzymes to eat coral, sponges, anemones, and other prey. But some species—like the Indo-Pacific’s killer sea slug Gymnodoris rubropapulosa—even cannibalize their own kind. Watch the deadly, slow-speed chase.
While interest in nudibranchs is growing, they’re typically difficult to breed and even harder to keep alive in captivity. Nudibranchs have a very specialized diet in the wild, and unlike other aquarium specimens, they’d rather starve to death than adapt to eating flakes or frozen shrimp.
Two for One
All nudibranchs are hermaphrodites, possessing both male and female sex organs. When mating, both partners are fertilized and both lay eggs. In one unusual case, the species Chromodoris reticulata, sheds its male sex organ after mating—only to regrow within 24 hours from two backups coiled inside its body!
Some nudibranchs, like Pteraeolidia ianthina, are powered by the sun. They ingest algae from plants or coral around them and use the energy created by the algae’s photosynthesis as their own.
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