My Glaucus atlanticus Report

May 14, 2009
From: Kenneth Hattersley

Concerning message #22410:

Here is my school report on Glaucus.

 Glaucus atlanticus, a member of the phylum Mollusca, is a magnificent creature. Sometimes the G. atlanticus is referred to as the Blue Slug, Sea Lizard, and sometimes Sea Dragon. I will relinquish my vast knowledge and explain to you the mating habits, eating habits, ways of living, defensive measures, and other random facts based on this wonderful creature. Let's begin our journey through the anatomy and life of the Sea Lizard, starting with its unique food intake.

To start off, this slug is basically anywhere the Porpita porpita (Blue Button), Velella velella (simply known as velella or the By-the-Wind-Sailor), and Physalia physalis (Portuguese Man of War) are found. The reason behind this is that the slug feeds on these specific members of Hydrozoa, but more exclusively the Physalia. These blue slugs use something called rhinophores to seek out their prey. A rhinophore is essentially a sense of "smell." The word comes from two Greek words, Rhino meaning nose, and phore meaning carrier. People use their noses to pick up scent causing chemicals in the air with receptors that send messages to be deciphered by your brain. This is the same basic idea with rhinophores. The rhinophores pick up the chemicals in the water and sensory receptors, once again, send them to be deciphered by the slug's brain giving them an alert as to which way food is. They then attempt to propel themselves in that direction, this propulsion problem I will discuss later. Once the prey is found the daring slug will latch itself to the tentacles of the feared Portuguese Man of War and go to town. Interestingly enough the slug is immune to the powerful sting, and turns it around and uses it for its own benefit and protection, once again to be explained further on. The slugs not only eats these members of the Hydrozoan family, but will in a lot of cases as a result of limited food or close quarters, eat its own kind. In a lot of cases the Sea Dragon will come across one of its own and begin to eat it head first, slug cannibalism. This cannibalism was completely unheard of until somewhat recently. The Sea Lizards accomplish the task of eating the thick, chitinous outer layer of their prey velella and their own kind with the help of their own chitinous, razor like teeth that interlock like a zipper when the slug closes its mouth. There is a single long row of razor sharp, curved, pointed teeth all of which are serrulated like the leaves of a rose or the sharp side of a bread knife which are specialized for tearing through the hard, chitinous, outer rim, called the tentaculiform papillae, of their prey.

This slug also has another crazy feature... its unique defense mechanism. This slug eats the tentacles of poisonous, deadly hydrozoans and, while doing that sorts the nematocysts and picks out the most venomous of them and uses them for their own protection. The slug stores these stolen nematocysts in the cnidosacs in the ends of their cerata, or tentacle like structures at the end of the "arms," for future use. Due to this unique defense, G. atlanticus can deliver a much more deadly, painful sting than that of a Man of War. Australian kids have a game they play with the washed ashore bodies of the Man of Wars, or so called by them Blue Bottles, where they pick them up and throw them at each other. Sounds dangerous to me but people grow accustomed to a lot of things; anyway, as they play this game they sometimes accidentally stumble across a Glaucus, pick it up and hurl it at their buddy, delivering a much more painful sting. This is due to the way these creatures choose the nematocysts. It is said that the blue color is a "tell tale" sign of "Danger! Do not touch!" Therefore the G. atlanticus is very usually left alone. As you can see, accidents sometimes happen. In the water it is not known if the slugs mistake humans for food due to smell and migrate towards people (don't know why they would), but in any case that isn't a threat due to the method of movement.

The G. atlanticus has a very interesting, yet little known about mode of transportation. This slug has an air sac on what seems to be the dorsal side. However, although it looks as though the side of the Sea Lizard you can see in the photograph above is the dorsal side, it isn't. The G. atlanticus spends its life floating along the top of the water, dorsal side down and foot up, just cruising along in search of food putting its rhinophores to use. I'm not quite sure how the Sea Lizard gets to the food its rhinophores help locate. This is the only thing about this creature that somewhat baffles me. Movement is not actually controlled by the slug itself. Although it has been seen to try, the slug is not able to control its own movement. The slug can sometimes be seen attempting to wriggle its way across the sea along the top of the water, but self propulsion for this species of slug is a fail. The Sea Lizard relies on the wind and ocean currents but either way this is a pretty interesting, slow way of movement and altogether life. The Blue Slugs entire life after the larval stage is spent upside down floating on its back buoying itself with its air sac. An interesting touch to this slug's defense is its coloration.

The foot (which faces upwards) is colored a dark blue to grey to camouflage itself from predators that attack from above such as seagull or pelicans. Its underside (dorsal on this specific creature) is a light, silvery grey to keep safe from predators from below suck as sea turtles, that also eat Man of Wars thus, are immune to the slugs powerful sting.

These creatures mating habits are also very abnormal. The Blue Slug is what is called a hermaphrodite, meaning it is both sexes in one. It has a hole on the right side of its body (when looking for dorsal side) between the two larger ceratal clumps, holding both the male and female reproductive organs. It has been said that the creature cannot, or just does not fertilize itself. The slug has an immensely long penis, sometimes longer than its body. Reason for this abnormally larger reproductive organ is to have the ability to reach past the out hanging ceratal clumps of its potential partner. The only other animal that's penis size in relation to body size that can rival that of the Sea Dragon is that of the sessile creature the barnacle. Barnacles have the largest penis in relation to body size in the entire animal kingdom. Back on topic, the female organs produce egg strings that either spiral or string out of the hole that seems to be on the left if the slug is viewed overlooking the foot. Each egg string will hold 12-20 eggs in them. The eggs in this egg string will remain attached to each other until the larvae hatch and are on their way. A lot of times, in stress, the slug will Evert its penis for no apparent reason whatsoever. In one case, a man was watching the mating process and reported seeing a larger of two Glaucus attempting to eat the others penis before mating. Now, there have never been any other reports of before mating one eating the others reproductive organ but it is said that this could be possible due to the often cannibalistic ways of the Blue Lizard. Another question also arose during my there a preference of being either male or female during mating? The answer came up soon after, with a response that stated that during mating both slugs involved will be male and female releasing both sperm and eggs in one mating session. There is also another sea slug very closely related to the G. atlanticus.

This almost identical relative of the Glaucus is known as the Glaucilla marginata. These two slugs are almost exactly alike with few key differences. To start this section off we will begin with a common factor. These two slugs both have what is called bilateral symmetry, in other words if you were to cut them in half directly down the middle it would be exactly the same on both sides. Both sides have the same amounts of cerata that are all equal in length and width. I'm not certain whether the amount of nematocysts is the same on both sides but considering the bilateral symmetry, I'd imagine they would be. The colors and designs along the dorsal and foot are also equal on both sides of the slug. Both slugs have eyes that extend from their heads and can see in all directions. Now, one of these key differences that set these two similar slugs apart is size. The Glaucus is much larger in size ranging from about 2-3 mm as a juvenile to 20-30 mm or more as an adult (*note* these creatures are found mainly in Australia, thus, the metric system is used). The smaller Glaucilla gets only to about 5-10 mm as an adult. Both of these creatures feed on the same things and have the same method of movement, but I assure you size isn't the only difference. The cerata of the G. marginata, unlike those of the G. atlanticus, are multi layered, having several overlapping rows of cerata. The cerata of the G. atlanticus have only one row of longer cerata per "arm." The locality of these creatures varies widely.

Attempts to keep these creatures in captivity with one another has turned out to be an extreme fail, resulting in death of the majority of the captive slugs due to cannibalism and the eventual death of the remaining captives due to one thing or another. The idea behind such quick death in these captive specimens is thought to be because of the fact that the slugs in captivity were found on the beaches and had been there for periods of time, and had probably been roughed up quite a bit by the waves as they came in. No matter how long they last in captivity the will always be happier in the ocean where they belong.

The locality is definitely needed to go into with more detail. As I stated previously, the Sea Lizard basically follows around its food source, the Portuguese Man of War, to all stretches of the earth. In hearing that, you can now assume that the locality of these magnificent, complex creatures is not partial to one specific region. They call any warm, sub-tropic, salt based body of water home. From the waters of Africa and South Eastern Europe, to the coasts of Australia, and even in our own backyard, right here in the Gulf of Mexico. So when you're visiting the beach over summer and those strong winds are blowing in off the coast, they are bringing more than the easily visible Man of Wars. If you spot a Man of War, be wary and take extra caution as these little floating slugs are much harder to see than the hydrozoans, due to their miniscule size.

As you can see if have taken a liking to these beautiful, awesome, complex creatures the Glaucus atlanticus, and have learned basically all there is to know about the natural marvels...from their eating habits, to their form of motion, to the way they make babies. I now have passed this information on to you.

Kenneth Hattersley

Hattersley, K.P., 2009 (May 14) My Glaucus atlanticus Report. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Dear Kenny,

Thanks for this copy of your very comprehensive report on Glaucus atlanticus. I did not realise when you wrote earlier that you were preparing such a project. I hope your classmates - and teacher  - were impressed

Best wishes,
Bill Rudman

Rudman, W.B., 2009 (May 14). Comment on My Glaucus atlanticus Report by Kenneth Hattersley. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from


Glaucus atlanticus

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