The Blue Dragon, or Glaucus atlanticus, is a small marine animal belonging to the group of nudibranchs, or sea slugs. This creature is known for its intricate coloration and distinct appearance, earning it common names like sea swallow and blue angel. It is a pelagic species, meaning it spends its entire life floating on the ocean’s surface, a lifestyle that has shaped its unique form.
Detailed Physical Characteristics
The Blue Dragon typically reaches a maximum length of about 3 centimeters (1.2 inches). Its body is elongated and tapers to a point, giving it a streamlined, almost lizard-like shape. Its vibrant coloration is a form of camouflage called countershading.
The sea slug exhibits a combination of dark and pale blue on its ventral side, and a silvery-gray on its true dorsal side. A dark blue stripe is often visible running down the length of its foot. Extending laterally from its body are six appendages, called cerata, which branch out into finger-like extensions. These cerata are arranged in three pairs and serve multiple functions, including respiration and defense.
Habitat and Unique Floating Behavior
The Blue Dragon is found globally in the open waters of tropical and temperate oceans, including the Atlantic, Pacific, and Indian Oceans. It is a member of the neuston, a group of organisms that live at the interface between the water and the air. Since it is not a strong swimmer, the species is entirely dependent on winds and ocean currents to travel.
To maintain its position on the surface, the Blue Dragon swallows an air bubble into its stomach, which provides the necessary buoyancy. Due to the location of this internal gas sac, the sea slug floats upside down.
This inverted position is directly linked to its camouflage strategy. The blue side faces upward, blending with the water surface, while the silver-gray side faces downward. This countershading makes it difficult for predators below to distinguish the slug against the bright surface light, protecting it from both aerial and aquatic threats.
Why This Sea Slug Is Dangerous
Despite its small size, the Blue Dragon poses a danger to humans due to its unique feeding habits. This sea slug preys upon venomous siphonophores, most notably the Portuguese Man O’ War (Physalia physalis). The Blue Dragon is immune to the venom of its prey and consumes the organism.
The slug does not digest the stinging cells, known as nematocysts, but stores them in specialized sacs at the tips of its cerata. This process, called kleptocnidae, allows the Blue Dragon to concentrate the venomous cells from multiple prey items. The resulting sting is often more potent than that of the Portuguese Man O’ War itself. Contact can cause severe pain, skin inflammation, and symptoms including nausea and vomiting. The stinging cells remain active even after the sea slug has died, so specimens washed ashore should never be touched.