The Blue Dragon (Glaucus atlanticus) is a sea slug known for its unusual appearance and vibrant coloration. This small marine gastropod is often called the sea swallow or blue glaucus. Its striking blue and silver body hints at a specialized existence far removed from the seafloor.
Identifying the Blue Dragon
The Blue Dragon has an elongated, tapering body that typically reaches a length of up to 3 centimeters (just over one inch). Its most distinguishing features are six appendages that branch out into rayed, finger-like extensions called cerata. These cerata are arranged in three pairs along its sides, giving the creature a feathery appearance.
The vivid coloration of the Blue Dragon is a form of camouflage known as countershading. The side facing upward (its foot) is marked with deep blue stripes, blending it with the ocean surface when viewed from above. Conversely, the side facing downward (its dorsal surface) is silvery-gray color, helping it disappear against the sunlit water when viewed by predators from below.
Global Distribution and Habitat
The Blue Dragon is a pelagic species, meaning it inhabits the open ocean rather than staying near the coast or seabed. It is distributed globally across tropical and temperate waters in the Atlantic, Pacific, and Indian Oceans. This extensive range means the slug follows the currents and winds that dictate its movement.
The creature’s entire life is spent floating at the water’s surface, a behavior made possible by a specialized buoyancy mechanism. The Blue Dragon maintains an air bubble in a gas-filled sac within its stomach, which allows it to float. Due to the placement of this gas sac, the slug floats upside down, with its blue side facing the sky.
Since it relies entirely on water movement, the Blue Dragon is considered a passive drifter, traveling with other organisms that float on the surface, collectively known as neuston. This dependence on ocean currents and winds is why the slugs are sometimes carried into coastal areas and wash up on beaches, particularly following strong storms. When stranded, they are highly noticeable due to their bright blue color against the sand.
Survival and Defense Mechanisms
The Blue Dragon’s diet is composed almost entirely of other floating, pelagic organisms, most notably the highly venomous Portuguese Man o’ War (Physalia physalis). Despite its small size, the sea slug is an effective predator, using its serrated teeth and strong jaw to consume the entire body of its prey. It also feeds on other stinging siphonophores like the By-the-Wind Sailor and blue buttons.
The slug has a remarkable adaptation that allows it to ingest the stinging cells, or nematocysts, of its venomous prey without triggering them. Instead of digesting the nematocysts, the Blue Dragon stores them within specialized sacs at the tips of its cerata. This process, called kleptocnidy, is a defense mechanism that turns its prey’s weapon into its own.
By storing and concentrating the ingested stinging cells, the Blue Dragon creates a defense that is more potent than the initial sting of the prey it consumed. This concentrated venom is used to deter potential predators. For humans who encounter the creature on a beach and touch it, the sting can be extremely painful, producing symptoms similar to, or even worse than, a Portuguese Man o’ War sting.