This tiny blue sea slug floats upside down on the ocean surface using a gas-filled stomach. It eats Portuguese man-of-war and concentrates their venom in its tissues, becoming more venomous than its prey. It's simultaneously beautiful and deadly.
Taro Taylor from Sydney, Australia, CC BY 2.0, via Wikimedia Commons
An upside-down life at sea
The Glaucus atlanticus, or blue dragon, is a species of sea slug that spends its entire life in the open ocean. Typically growing to only three centimeters long, it drifts on the surface of temperate and tropical waters worldwide. Its buoyancy is maintained by a gas-filled sac in its stomach, which causes it to float upside down. The animal uses the water's surface tension to stay afloat, carried by winds and ocean currents.
This inverted orientation is important for its survival. The slug’s foot, which faces the sky, is a blue that blends in with the ocean's surface, camouflaging it from airborne predators. Its true back, which faces down into the water, is a silvery-grey that matches the bright, reflective surface of the sea when viewed from below. This defensive coloration, known as countershading, provides protection in the exposed pelagic environment. The blue pigments are also thought to help reflect harmful ultraviolet radiation.
A stolen arsenal of venom
The blue dragon preys on other floating organisms, most of all the venomous Portuguese man-of-war (Physalia physalis). This sea slug is immune to the man-of-war's stinging cells, called nematocysts. Using a hard, serrated, tooth-like ribbon called a radula, it scrapes tissue from its much larger prey. A protective mucus layer in its gut prevents the nematocysts from firing as they are consumed.
Instead of digesting the stinging cells, the blue dragon transports them through its digestive system to be stored in specialized sacs, called cnidosacs, at the tips of its finger-like appendages, or cerata. This process is called kleptocnidy. The slug appears to select and store the most potent nematocysts, concentrating them in its own tissues. This makes the blue dragon's sting significantly more powerful and dangerous than that of the man-of-war it consumed. The venom remains active even after the slug has died. A single sting can cause nausea, vomiting, and acute allergic contact dermatitis in humans.
💡Fun Facts
*Glaucus atlanticus* is a hermaphrodite, meaning each individual has both male and female reproductive organs, though they still require a partner to reproduce.
During mating, two blue dragons lock together ventrally, face-to-face, which is unusual for nudibranchs.
After mating, both individuals can lay strings of 10 to 20 eggs on floating objects, including the carcasses of their prey.
Large groups of blue dragons are sometimes blown ashore by strong winds and are collectively known as a "blue fleet".
These are wild animals in the open ocean. Sightings on beaches in the Western Cape are rare and typically occur when strong onshore winds blow them ashore.
Admission
Free
Accessibility
Sightings may occur on any sandy beach accessible to the public. The terrain is natural and variable.
