The Glaucus atlanticus, commonly known as the Blue Dragon, is a striking mollusk that lives in the open ocean. This sea slug is a type of nudibranch, meaning it is a shell-less gastropod, closely related to snails and terrestrial slugs. Characterized by its brilliant blue and silver coloration and unusual, feather-like appendages, the Blue Dragon is a mesmerizing creature of the sea. It leads a pelagic lifestyle, drifting on the ocean’s surface as a carnivorous predator.
Identifying the Blue Dragon and Its Habitat
The Blue Dragon is a small animal, typically reaching a maximum length of about 3 centimeters (1.2 inches) in maturity. Its body is flattened and tapered, featuring six distinctive appendages that branch out into numerous finger-like projections called cerata. This morphology gives the nudibranch its common “dragon” appearance.
The sea slug employs countershading to avoid predators. Its underside, which floats upward, is a vibrant blue or blue-white, allowing it to blend with the surrounding water. Conversely, its dorsal surface, which faces downward, is a silvery-gray color, helping it disappear against the bright surface of the ocean when viewed from below.
The Blue Dragon spends its life floating on the surface of the open ocean. It maintains buoyancy by swallowing air, which is stored in a gas-filled sac in its stomach. This mechanism causes it to float upside down, carried along by the wind and ocean currents. The species is found globally across the temperate and tropical waters of the Atlantic, Pacific, and Indian Oceans.
The Specialized Diet of the Blue Dragon
The Blue Dragon is a specialist predator with a highly specific and dangerous diet consisting almost entirely of other venomous, surface-dwelling organisms. This sea slug hunts and consumes cnidarians, a phylum of aquatic animals that includes jellyfish and their relatives. It is uniquely immune to the potent stinging cells of these prey animals, a necessary adaptation for its survival.
The primary prey of the Glaucus atlanticus is the Portuguese Man-o-War (Physalia physalis), a siphonophore known for its powerful venom. The Blue Dragon uses a jaw and a specialized structure called a radula, which has serrated teeth, to grasp and consume large chunks of the prey. It also feeds on other surface-dwelling hydrozoans, such as the By-the-wind Sailor (Velella velella) and the Blue Button (Porpita porpita).
The Blue Dragon’s ability to consume these highly venomous creatures is driven by its need for defense, not just sustenance. The mollusk is a voracious eater, and in the absence of its preferred prey, it has been observed to engage in cannibalism, attacking and eating other Blue Dragons.
Weaponizing Its Food Source
The Blue Dragon’s diet is not merely about survival; it is a biological strategy known as kleptocnidae. This process involves the sea slug stealing the stinging cells, called nematocysts, from its cnidarian prey. The Blue Dragon does not digest these specialized cells; instead, it redirects the undischarged nematocysts through its digestive system.
These stolen stinging cells are then stored in specialized sacs, known as cnidosacs, located at the tips of its cerata. The slug is selective, storing only the most potent and undischarged nematocysts. When threatened, the Blue Dragon can deploy this accumulated arsenal for its own defense.
Because the Blue Dragon actively concentrates the most powerful stinging cells from multiple prey items, its own sting can be significantly more potent than that of the Portuguese Man-o-War it consumed. Handling a Blue Dragon, despite its small size, can result in a very painful and potentially dangerous sting to humans. The concentrated venom can cause symptoms such as intense pain, nausea, and allergic reactions.