Glaucus atlanticus (blue sea slug)

These little guys are pretty awesome. They live in temperate and tropical waters, staying afloat by swallowing air and keeping it in their stomachs. They eat the venomous Portuguese Man o’ War jellyfish and keep its stings for their own defence. And they look like cycloptic alien dragons. What’s not to love?

(Photos and info from Wikipedia, the Natural History Museum and Animals Talking in All Caps.)

Edited to add: Here’s what they look like on the inside!


Glaucus atlanticus, the blue dragon (also called sea swallow and blue angel), is a species of small-sized blue sea slug. This tiny animal spends its life floating upside-down on the surface of the Pacific, Atlantic, or Indian Ocean thanks to an air bubble which it swallows and keeps inside its belly, going wherever the currents and the wind take it.

This cute-looking creature is actually an aggressive predator that feeds on organisms much larger than itself, including the most venomous ones. It also can give you a poison sting when picking one up barehanded.

Blue Sea Slug (Glaucus atlanticus)

… also called the sea swallow, blue glaucus, blue sea slug and blue ocean slug) is a species of medium-sized blue sea slug, an aeolid nudibranch, a marine gastropod mollusk in the family Glaucidae… This nudibranch is pelagic, and is distributed throughout the world’s oceans, in temperate and tropical waters. his species floats upside down on the surface tension of the ocean…

(read more: Wikipedia)


Glaucus Atlanticus.

Also known as sea swallow, blue sea slug, blue angel or blue dragon, it is a small sized sea slug that lives in the Atlantic, Indian and Pacific ocean.

They feed mainly on other sea creatures like the Portuguese Man-o-War, a cnidarian often mistaken for a jellyfish. The Glaucus is immune to their venom, and can actually store it in its cerata ( their dorsal and lateral outgrowths on the upper surfaces of the body). As a result they can sting potential predators. The sting is quite painful for humans, so they should be handled carefully.
At times, and given the occasion, they can be cannibalistic.

With the aid of a gas-filled sac in its stomach, the Glaucus floats at the surface. Due to the location of the gas sac, the sea swallow floats upside down. 

Like almost all heterobranchs, Glaucus is a hermaphrodite, having both male and female reproductive organs. After mating, both animals produce egg strings.

The curiosities of the Blue dragon nudibranch

Commonly referred to as the Blue dragon nudibranch, Pteraeolidia ianthina (Nudibranchia - Facelinidae), is a remarkable species of sea slug native to the Indo-Pacific region.

This is an extremely elongate species up to 5cm long, with large, curved arches of cerata (the projections on the upper surfaces of the body) along the length of the body. The cephalic tentacles have two distinctive dark purple (or blue) bands.

Although the body color of this nudibranch is translucent tan, the cerata, which are mostly blue or dark purple, lavender or golden brown, give the nudibranch most of its apparent color.

The Blue dragon nudibranch has many amazing survival strategies. When touched, the nudibranch will “flare” its cerata and the nematocysts will discharge on contact (it is one of the few nudibranchs with a sting strong enough to be felt by humans though usually not in areas with thicker skin such as the palm of the hand).

It is also able to autotomize (lose or detach) the posterior part of its body in order to distract, or free itself from, a potential predator. Later, the missing portion can be regenerated.

Another curiosity of this species is that the cerata contain zooxanthellae of the genus Symbiodinium that exhibit the capacity for photosynthesis, and they grow while reside in the sea slug. This symbiotic relationship with the algae helps the adult nudibranch to overcome a period of food shortage by getting photosynthetic products.

References: [1] - [2] - [3] - [4]

Photo credit: ©Sylke Rohrlach

Locality: New South Wales, Australia

Blue Sea Slug (Glaucus atlanticus)

Kingdom: Animalia
Phylum: Mollusca
Class: Gastropoda
Superfamily: Aeolidioidea
Family: Glaucidae
Species: G. antlanticus
Size: 3cm
Diet: Larger pelagic organisms like Portuguese Man o’ War, by-the-wind sailor, blue button, and violet snail
Distribution: Temperate and tropical waters like the East and South Coast of South Africa, European waters, east coast of Australia and Mozambique
Facts: The sea slug preys on poisonous organisms to them save their poison to use for its own protection.  It also floats upside down on the surface tension of the ocean water.


This is a Blue Sea Slug.

This is a Sea Swallow.

This is a Blue Ocean Slug.

This is a Blue Dragon.

This. Is. Beautiful.

If ever you’re confused as to the name, just use the scientific one, Glaucus atlanticus. When in doubt, Latin.

Anyway, the Blue Dragon is a type of nudibranch, which means it’s a mollusk without a shell (easy for me to remember because, well… nudey-branch… no shell… I know it’s silly, but it’s easy to remember). It is venomous, too – which means if you’re a kid in Australia and a bunch of these wash up on shore one day, they’re… good fun to… throw at your friends?! That’s right. Kids in Australia throw these little beauties at their friends, they’re called “Bluebottle” fights. Don’t have a “Bluebottle” fight. These are animals, leave them alone.

What I really wanted to talk about with these was the finger-like projections… the whispies… They’re called cerata, and those are the organs where the Blue Dragon stores the stinging cells that it steals from the jellyfish that. it. eats.

Pretty cool, right? Yeah, yeah it is. But this precious little 4 cm slug doesn’t just eat any old jellyfish, this is a dragon we’re talking about. The Glaucus atlanticus eats the Man O’ War jellyfish – and just so you know, the Blue Dragon concentrates the venom that it collects and can produce a more deadly sting than the Man O’ War jellyfish. Can’t find a Man O’ War jellyfish, just a bunch of other Blue Dragons? That’s okay as well, sometimes they become cannibalistic.

But hey, they’re pretty, right?

The Blue Dragon aka Blue Sea Slug (Glaucus atlanticus)

The blue glaucus likes to snack on hydrozoans (relatives of jellyfish). It’s especially fond of the tentacles of the highly poisonous Portuguese man-of-war. The blue glaucus is adapted to deal with the Portuguese’s stinging cells without getting hurt. Instead, it deters its own potential predators by storing the swallowed poison inside the finger-like structures sticking out of its body. Pretty nifty! 

(read more: Encyclopedia of Life)

(photo by Tien–Cheng Wang via World Register of Marine Species)

This tiny creature has gotten a fair bit of attention lately because of one simple reason: It’s absolutely crazy-looking. At first glance, it resembles a Pokémon or character from Final Fantasy more closely than a real biological animal. But the Glaucus atlanticus sea slug—commonly known as the blue sea slug or blue dragon—is indeed a genuine species. And if you swim in the right places off of South Africa, Mozambique or Australia, you just might find one floating upside down, riding the surface tension of the water’s surface.

The species has a number of specialized adaptations that allow it to engage in a surprisingly aggressive behavior: preying on creatures much bigger than itself. The blue dragon, typically just an inch long, frequently feeds on Portuguese man o’ wars, which have tentacles that average 30 feet. A gas-filled sac in the stomach allows the small slug to float, and a muscular foot structure is used to cling to the surface. Then, if it floats by a man o’ war or other cnidarian, the blue dragon locks onto the larger creature’s tentacles and consumes the toxic nematocyst cells that the man o’ war uses to immobilize fish.

The slug is immune to the toxins and collects them in special sacs within the cerata—the finger-like branches at the end of its appendages—to deploy later on. Because the man o’ war’s venom is concentrated in the tiny fingers, blue dragons can actually have more powerful stings than the much larger creatures from which they took the poisons. So, if you float by a blue dragon sometime soon: look, but don’t touch.