Sea Swallow

Overview

The blue dragon (Glaucus atlanticus) is a striking, pelagic nudibranch that drifts upside-down on the surface of temperate and tropical oceans. By maintaining a gas-filled sac in its stomach, it remains buoyant at the air-water interface, where it is carried by winds and currents.

Appearance

This small mollusk typically reaches lengths of 3 to 4 cm. It features a slender, flattened body with three pairs of wing-like, finger-tipped appendages known as cerata. Its coloration is a masterclass in countershading: the ventral side (facing the sky) is a vibrant cobalt blue to blend with the water, while the dorsal side (facing the depths) is a silvery-grey to camouflage against the bright surface light.

Biology & behavior

As a member of the pleuston, the blue dragon spends its entire life at the surface. It is a hermaphrodite, meaning each individual possesses both male and female reproductive organs. During mating, they typically meet face-to-face to exchange sperm, after which they produce strings of eggs that drift freely or are attached to prey remains.

Feeding

Despite its small size, the blue dragon is a specialized predator that feeds on venomous siphonophores, such as the Portuguese man o' war, as well as other cnidarians like blue buttons and by-the-wind-sailors. It is immune to their toxins and possesses the remarkable ability to sequester the prey's stinging cells (nematocysts) within its own cerata. These stolen defenses are then used to deter its own predators, making the blue dragon capable of delivering a painful sting to humans if handled.