The blue dragon sea slug, Glaucus atlanticus, is a marine gastropod known for its striking appearance. Often called the blue sea dragon or sea swallow, this creature features a flat, tapering body with bright blue and silver coloration. It possesses feathery cerata, which are finger-like appendages branching out from its body. Measuring around 3 centimeters (1.2 inches) in length at maturity, this small mollusk inhabits the ocean.
Global Distribution
Blue dragons inhabit warm temperate and tropical waters across the world’s oceans. Their distribution spans the Atlantic, Pacific, and Indian Oceans. Records indicate their presence in diverse locations, including the coasts of South Africa, European waters, Australia, Bermuda, Indian coastlines, the Gulf of Mexico, and off Phuket, Thailand. While their range is extensive, their appearance in specific regions can be sporadic, due to ocean currents and environmental conditions.
Pelagic Habitat
These sea slugs are pelagic, meaning they live in the open ocean rather than near coastal areas or the seabed. They spend their lives floating inverted at the water’s surface, forming part of the neuston community. This surface-dwelling habit is maintained by swallowing an air bubble, stored in a gas-filled sac in their stomach. The location of this air sac causes them to float upside down, with their underside facing upwards. This adaptation allows them to remain buoyant and drift along the ocean’s surface.
Adaptations for Open Ocean Life
The blue dragon exhibits adaptations for its life at the ocean’s surface. Its coloration provides effective camouflage through countershading: the bright blue side faces upwards, blending with the ocean’s blue surface from above, while its silver or grey side faces downwards, making it difficult for predators below to distinguish it against the reflective water. This dual coloration helps protect it from both airborne and underwater threats.
The diet of Glaucus atlanticus consists of venomous siphonophores like the Portuguese Man O’ War, By-the-wind Sailors, and blue buttons. These sea slugs are immune to the venom of their prey. They absorb and concentrate the stinging cells, known as nematocysts, from their prey into sacs at the tips of their cerata. This allows them to deliver a more potent sting for their own defense, making them potentially dangerous to touch.
Movement and Dispersal
Blue dragons are not strong swimmers and are dependent on environmental forces for movement. They are carried across vast oceanic distances by prevailing winds and ocean currents. This passive mode of transport explains their wide global distribution and why they are occasionally found washed ashore on beaches. Such beaching events occur after storms or periods of strong onshore winds, as groups of blue dragons, sometimes called “blue fleets,” are pushed towards the coast. Even when stranded or deceased, their stored stinging cells remain active, posing a hazard to beachcombers.