The blue glaucus (Glaucus atlanticus), also known as the “blue dragon” or “sea swallow,” is a unique marine nudibranch (a shell-less sea slug). It exhibits a striking appearance with its vibrant blue and silver coloration. Measuring typically around 3 centimeters (1.2 inches) in length, it is distinguished by its unusual form.
Global Distribution
The blue glaucus has a widespread distribution, inhabiting the world’s temperate and tropical oceans. It is commonly observed across the Atlantic, Pacific, and Indian Oceans. This pelagic species lives in the open ocean, far from coastal areas or the seabed. Its presence extends from the east and south coasts of South Africa to European waters, the east coast of Australia, and Mozambique. Its distribution is influenced by warm ocean currents and prevailing winds, which transport these organisms across vast distances.
Open Ocean Habitat
This sea slug primarily inhabits the surface waters of the open ocean, specifically the pelagic zone. Strong currents and winds dictate much of its existence, as the blue glaucus passively drifts with these forces. It maintains its position at the water’s surface by using surface tension and a gas-filled sac within its stomach. This buoyancy mechanism causes it to float upside down, exposing its underside to the sky. The blue glaucus is part of the pleuston, a community of organisms living at the air-water interface.
Adaptations for Surface Living
The blue glaucus has specialized adaptations for its open-ocean, surface habitat. Its distinctive coloration, known as countershading, provides effective camouflage against predators from both above and below. The slug’s blue side faces upwards, blending with the ocean’s surface and sky when viewed by aerial predators. Conversely, its silver or grayish underside faces downward, helping it disappear against the bright surface light when seen by underwater predators.
A gas bubble in its stomach keeps the blue glaucus buoyant and inverted, allowing it to remain at the water’s surface. Its cerata, or finger-like appendages, also contribute to buoyancy and can be used for slow swimming movements.
Prey and Predators
The blue glaucus is a specialized predator, primarily feeding on other pelagic organisms, particularly venomous siphonophores. Its main prey includes the Portuguese man o’ war (Physalia physalis), the by-the-wind sailor (Velella velella), and the blue button (Porpita porpita). It ingests and stores stinging nematocysts from its prey, concentrating them in its cerata for defense. This potent defense means the blue glaucus has few natural predators. Loggerhead sea turtles consume blue dragons, and cannibalism among larger individuals occurs, especially when food resources are scarce.