The blue dragon, Glaucus atlanticus, is a type of mollusk, or sea slug, known as a nudibranch. These are shell-less gastropods, like your common garden slug, but with a little more going on. Its distinctive and stunning blue hues have garnered it quite a lot of attention in recent years. Blue dragons have a silvery grey-blue dorsal slide, a dark and pale blue belly, and dark blue stripes on the head. Because they float belly up just beneath the bright sea surface, their countershading coloration gives them a valuable form of camouflage. Their vivid blue disguises them against the backdrop of the ocean, while their pearlized silver/grey belly blends in with the bright sea surface, hiding it from predators below. It’s also thought that their blue color offers added protection from harmful UV rays. These colors have also inspired a series of nicknames, such as sea swallow, blue ocean slug, dragon slug, and blue angel. Their striking appearance makes them easy to recognize, though nearly impossible to find in their natural habitat. Beyond its color, the blue dragon has a flat, tapered body with six ‘feathery’ appendages that branch out into finger-like cerata. These slender appendages are where their stinging cells sit, which they use for both defense and hunting. They have radular teeth resembling a knife’s serrated edge. This interesting creature has been mistaken for many different animals due to its color patterns and six arms – from a sea insect to the larval stage of an angel shark. Most nudibranch species live lower down in the water column or on the sea floor, but blue dragons spend their life floating upside down, partially kept afloat by an air-filled sac in their stomach to aid buoyancy. This allows them to basically crawl on the undersurface of the water. While they are usually content to drift with the ocean currents, they also have the ability to move on their own. They can propel their bodies through the water with their appendages using a combination of the millions of tiny hairs on their flesh along with small muscular contractions. Despite its impressive arsenal of defense tactics, the blue dragon rarely reaches more than 1.2 inches long at maturity, weighing anywhere from just 3- to 100-grams, belying its dangerous reputation.
This pelagic species occurs in temperate and tropical waters of the Pacific, Atlantic, and Indian oceans. Though they occasionally sink down to the ocean floor and rest on the sand, blue dragons typically move about the ocean's surface, floating upside down and allowing the ocean waves and winds to carry them from place to place. Sometimes the winds carry them to undesired locations, namely shores where they can become stranded and die. Recently, it seems their habitat is expanding, as blue dragons have been turning up in areas not traditionally associated with the species, including Australia's east and south coasts, European waters near Mozambique, the coasts of South Africa, Taiwan, and Texas. This is likely because of the impacts of climate change on ocean temperatures, increased storm activity, and the movement of one of their main prey, Portuguese man o’ war. In Texas, they can be found washing ashore in the spring.
Blue dragons are a foraging predator with an unusual appetite: venomous siphonophores, including the by-the-wind-sailor (Velella velella), the blue button (Porpita porpita), and the infamous Portuguese man o' war (Physalia physalis). They might be 300 times bigger, but they’re no match for a blue dragon’s attack. When it inevitably floats by a man o’ war or other cnidarian, the blue dragon locks onto the larger creature’s tentacles and in a shocking turn of events, steals their venom. Hard disks and protective layers of mucus inside their skin make blue dragons immune to the nematocysts. Using its serrated radula, the slug rips off chunks of the tentacles and moves the stinging cells through its digestive tract out into its finger-like cerata without being stung. While the blue dragon isn’t venomous by itself, because what they collect is a concentrated concoction of venoms from several different creatures, blue dragons can actually have more powerful stings than the much larger creatures they prey on. So, if you float by a blue dragon sometime soon: look, but don’t touch. Picking one up may result in a painful sting and symptoms similar to those of its prey. If they are unable to find their preferred foods, these slugs won’t hesitate to eat other blue dragons. In fact, blue dragons are the prevailing predators of their own species. They have very few others, though loggerhead sea turtles may be one. A study on this species found that 42% of blue dragons had remnants of their own species in their stomachs.
Like other nudibranchs, blue dragons are hermaphrodites, meaning that all individuals contain both male and female reproductive organs and produce both egg and sperm. Despite this fact, they still have to mate with another slug to produce viable eggs. Their mating habits resemble those of their hunting habits; they simply float along until they find another dragon. Reproduction is an act they must engage in with caution to avoid getting stung by their partner. The male reproductive organ has evolved to be especially large and hooked so that it can safely reach the other dragon without being affected by their venomous cerata. Once mating takes place, both individuals lay a long spiral-shaped string containing about 20 eggs, and around 55 egg strings per hour. They often deposit the strings on the carcasses of their prey, if available; otherwise, they’ll leave them on any other floating mass they encounter that provide the newborn slugs with time to develop their own air sacs before setting off into the water. Life expectancy ranges from one month to one year.
A group of blue dragons floating together is called a “blue fleet.” Because these fleets have an increased amount of exposed surface area, they are often washed ashore by oceanic winds. Blue dragons curl into balls to protect themselves when they're caught in waves and being pushed toward the beach. When they become stranded on the sand, their venom remains active even after they die – something to keep in mind if you find them on the beach. In recent years, blue dragons have become very popular on the internet. Many people want to add them to their aquariums, leading to an increase in their appearance in the exotic pet trade. However, keeping them as a pet is a bit impractical due to their diet. Climate change is an additional threat, specifically ocean acidification, which is caused by the increase of C02 in the atmosphere. Ocean acidification reduces the amount of food available for Portuguese man o’ wars, which in turn results in fewer man o’ wars and less prey for the dragons. As is the case with many marine invertebrates, there is little information available regarding the conservation status of Glaucus atlanticus. Their pelagic lifestyle makes it difficult to assess their risk levels. Additionally, researchers can have a hard time telling species in the Glaucus genus apart. In fact, in the 1800s there were at least five other species that would ultimately be reassigned to Glaucus atlanticus.
Where I learned about blue dragons, and you can too!
World Register of Marine Species
Natural History Museum
Smithsonian National Museum of Natural History
Extraordinary animals: an encyclopedia of curious and unusual animals
By Ross Piper
Sea Slug Forum