The Blue dragon is nudibranch, a species of shell-less mollusk, also known as a sea slug. Source.

Drifting slowly at the surface of the ocean the Blue dragon (Glaucus atlanticus) appears deceivingly innocent, moving gently with the current, searching for its prey. Instead of picking on something its own size, this brilliantly blue nudibranch actively hunts a collection of dangerous hydrozoans, known as the Blue-fleet, including the well known Portuguese man-of-war. Despite their physical prowess, these colonial cnidaria are unable to defend themselves against the blue dragon and prove no match for the shell-less mollusk with an appetite like no other.

Life upside down

The beautiful blue dragon, Glaucus atlanticus, growing up to 5cm. Source.

Life at the ocean surface does not come without its dangers, as terrestrial predators swoop in from above whilst hunters circle from the depths below. The Blue dragon’s primary form of defense comes in its distinctive physical appearance, a mechanism known as counter-shading. The nudibranchs’ shimmering blue ventral surface blends beautifully with the ocean when viewed overhead, whilst its’ dorsal grey-white side matches the illusion of the sun above, fooling any prospective pursuer.

At a mere 5cm long, the blue dragon is identifiable from its flattened, elongated body which is wide with 6 limbs that fan out to form branched feet known as cerata. In order to prevent any vertical movement through the water column, G. atlanticus possess a large gas float located in the main body cavity allowing it to stay close to the ocean surface by swallowing and expelling air as it chooses. However, the gas float is positioned ventrally, resulting in the nudibranch living life upside down, with its foot facing the sky above.

David vs Goliath

The Portuguese man-of-war, Physalia physalia, the prey of choice. Source.

The Portuguese man-of-war, Physalia physalis, famed for its role in the movie Stormbreaker and feared for its’ fatal sting, is the preferred prey of the Blue dragon. The Portuguese man-of-war is a species of free-floating colonial hydrozoan, also known as a siphonophore, made up of three different organisms or polyps; dactylozooid, gonozooid and gastrozooid. Despite being separate, the polyps act collectively as one, each with an individual role within the colony. The different polyps cluster and are grouped together underneath the characteristic blue gas float, the pneumatophore, which gives the Portuguese man-of-war its name. Extending up to 50 meters (more commonly 10 meters) from main body, venomous tentacles dangle temptingly, continuously searching out prey foolish enough to fall into their path. Surely a tentacle too many for the humble sea slug?

However, the Blue dragon does more than hold its own against the Portuguese man-of-war. Armed with a strong, hinged chitinous jaw, G. atlanticus anchors itself to the main disk or float of the hydrozoan and begins to tear the giant to pieces. Instead of teeth being arranged on jaws, inside the head of the nudibranch is the radula, as shown below, where 24 sharply hooked, teeth-like appendages are precisely organised and used in a tongue-like way to scrape and grasp their prey.

Scanning electric micrograph view of a Blue dragon radula. Source

Once in place, the blue dragon is hard to shake, using digestive enzymes secreted from the salivary gland to break down it prey, having been known to devour an entire P. physalia. However the battle is not over. G. atlanticus must somehow navigate the thousands of stinging cells that cover the cnidarians tentacles. Each tendril (tenetacle) is equipped with nematocysts, cells that contain capsules of venom that once injected into its prey, slowly paralyse from the respiratory system through to the muscular network. Whilst there are species of fish immune to the poison, the Blue Dragon has another mechanism for coping with the toxicity. When digesting its victim, the capsules move through the digestive tract of the nudibranch without being broken down and absorbed, traveling to the cerata where they are stored in specialised pouches known as cnidosacs. These cluster in the very tips of the organisms feet, concentrated here so that no poison diffuses into the rest of body and causes paralysis in the Blue dragon. Not only does the G. atlanticus make an easy meal of the widely feared Portuguese man-of-war, but it also steals the siphonophores’ venom and harnesses it as a weapon of its’ own. The innocent looking nudibranch is transformed into a predator with a sting more powerful than most others.

Reproduction on tenderhooks

The nudibranch hooked penis, second only to the intertidal barnacle in penis to body size ratio. Source.

Possessing both female and male reproductive organs, the Blue dragon is a true hermaphrodite, with a dangerous mating game. When your prospective partner is armed with 84 finger-tips, each loaded with a powerful shot of venom, a mechanism for minimal contact must be called upon. G. atlanticus has developed a large hooked penis, sometimes longer than the individuals body, that allows safe penetration without risk of being attacked by the stinging cerata. After mating, both sea slugs will produce a string of 20-40 eggs, attaching them to some form of floating substrate, often an old carcass Portuguese man-of-war. Each egg is initially encased in a capsule, however they are lost as the egg hatches and the larvae develop. Remaining on the floating raft where they were originally placed, the larvae safely mature here, developing their gas sacs until they can independently float at the ocean surface.

The best defence is a good offence

When discovered in 1777 during Captain Cook’s second voyage across the Pacific Ocean, no one would have predicted the carnivorous appetite of the beautiful Blue Dragon. Living a pelagic life, drifting in tropical and temperate oceans across the world, your chances of encountering this enchanting sea slug are somewhat small, and with much further research still needing to be done, the secret to G. atlanticus’ venomous taste is just beginning to unravel. The mighty siphonophore should live in fear of this little nudibranch, as it floats unsuspectingly, hunting the hunter.


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