Blue Glaucus | Oceana
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Corals and Other Invertebrates

Blue Glaucus

Glaucus atlanticus


Atlantic, Pacific and Indian Oceans


Open ocean

Feeding Habits

Foraging predator

Conservation Status



Order Nudibranchia, Family Glaucidae


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Despite its impressive arsenal of defense tactics, the blue glaucus rarely reaches more than 3 centimeters long. And, unlike most benthic nudibranchs, this species lives throughout the entire water column. An air bubble stored in its stomach keeps the nudibranch afloat. The creature often floats on its backside, showing its brightly colored underbelly to airborne predators. The bright blue color acts as camouflage against the backdrop of ocean waves while the animal’s grayish backside blends with the bright sea surface, concealing it from predators below. This is an example of a phenomenon known as countershading, helping the creature to avoid both flying and swimming predators while floating in open water. 

Like other sea slug species, the blue gaucus isn't venomous by itself. When feeding on its preferred prey, Portuguese man o' wars, the blue gaucus stores the stinging nematocysts created by the prey’s notoriously long, venomous tentacles — these tentacles may average up to 30 feet long! The stinging cells are stored and concentrated for the future, so when the blue dragon is threatened or touched, it can release these stinging cells to deliver a far more potent sting than the Portuguese man o' war can alone. The blue gaucus, like all nudibranchs, is hermaphroditic — each individual produces both eggs and sperm. An individual cannot fertilize its own eggs, however, and pairs still must mate. Long, spiral-shaped eggs are produced by both male and female, and often float freely in the water or stick to nearby surfaces.


Additional Resources:


the Full Creature Index