Glaucus atlanticus

Figure 1: Glaucus atlanticus (Left) Natural History Museum, 2014

I first gained interest in researching about Glaucus atlanticus when I was researching about the Portuguese Man O’ War (Physalia physalis) and I found that one of their main predators was this small, yet extraordinary, sea slug, and I was curious to how such a small animal, would be able to do this whilst avoiding the Portuguese Man O’ War’s well known venom which can be highly deadly.

Glaucus atlanticus also known more commonly as the Blue sea slug is a small type of sea slug, and is a type of marine gastropod mollusk (A member of the Glaucidae family) known for spending it’s life within the open oceans. This Sea Slug was first studied in 1777 century by Forster, the animal was named ‘Greek god of the sea, Glaucus, who was forced to dwell in the sea forever’ this was due to spending it’s lifetime within the open ocean. In 1992 they were considered a type of insect which lived within the sea, due to it’s larval stage. However they were finally correctly classed as a marine gastropod.

It’s taxonomy group goes as follows:
Phylum: Mollusca
Class: Gastropoda
Order: Nudibranchia
Suborder: Aeolidinia
Family: Glaucidae

It’s main body area is elongated and also very flat, with a small head. On the side of the body are four or three pairs of clusters of papillae which are the cerata, which a possibility of 84 in total. The foot of the slug is very flat and slender, whereas on the ventral side the foot is much longer.
The animal itself can be up to around 3cm long and holds air bubbles within it’s stomach, and this factor enables Glacucus atlanticus to float on the surface of the oceans (Pacific, atlantic and Indian oceans) and will be found floating upside down after taking in the air which is then kept stored within their stomachs, aiding them in keeping afloat, and will let the wind and/or current be their form of travel around the oceans.
They are often mistook for being harmless creatures, whereas they can actually be quite a threat to living things. They have been known to turn to cannibalism with members of their species if they cannot find food, they also feed on a variety of hydrozoans (Animals related to jellyfish and corals) such as the By the wind sailer (Velella velella), the Blue button (Porpita porpita) and the Portuguese man o’ war (Physalia physalis), which is one of the main things which their diet consists of, which to a human can be fatal, however Glaucus atlatincus holds barriers within their body which acts as a barrier providing them with a special mucus, they will then go and eat both the fired and unfired stinging cells.
It will then store the poison it has gathered from the Portuguese Man o’ War, and stores it within it’s body, through sucking it up with a special structure known as ‘Cerata’ and then takes it and keeps in within canidosacs, this consists of 84 sacs which are found on the upper surface of the animals body. The reason for eating the stings is not only to collect the poison to use to defend itself, but to also get nourishment from them.

Figure 2: Glaucus atlanticus feeding on a Portuguese Man O’ War, Glaucus atlanticus, 2007
Figure 3: Glaucus atlanticus about to feed on a Blue button jellyfish (Porpita porpita) 2011

The way they feed on their prey such as the Portuguese man O’ war is through capturing the edge of the hydrozoean and tearing of a large chunk of it, this is through the use of the rows of pointed denticles they hold within their chitinous jaw, enabling them to hold onto their prey without coming off due to things such as a more rough surrouding.
In recent research of Glaucus atlanticus they have found that many of their denticles seem to be damaged in some way, this gives us the possible idea that they put in a considerable amount of effort when they attach themselves to their prey, this statement was made by Irina Roginskaya.

One of the main things people will notice about the Glaucus atlanticus is their bold, vibrant, blue colour they bear on their bodies, this actually comes to good use as this enables they to use things such as countershading which they use for camouflage to aid in protecting them from predators, this occurs as they have a silver grey colour from it’s dorsal side, which floats at the sea surface due to the air bubble effecting the way they float, and then a dark pale blue ventrally, this helps Glaucus atlanticus stay hidden away for predators within the air aswell as within the sea.
It is also a hermaphrodite, meaning it has both sets of reproductive organs. These means that after mating has occured, the slugs will both produce eggs.
The males penis is large and hooked, this is believed to aid them around the cerata, which could be highly dangeous as this is where they store the venom which they collect from their prey.
They will often lay their eggs on driftwood and on the corpses of their prey, however as there is a lack of hard substrate in the open ocean to lay their eggs on, they often will have to leave the eggs to float freely in the water and after hatching, will reside wherever they were laid, until their airbags have fully developed, as they would not able to travel when these have not developed correctly.
Glaucus atlanticus will produce strings of eggs, coming to about 17.5 mm, and these are encased in a thin mucus membrane, the eggs are around 60-75 µm wide and 75-97 µm long. After around 48-60 hours a trochophore (planktonic marine larva) will begin to form and then after 3 days a veliger (planktonic larva) with a larvel shell will leave the egg string.

The only possible threat this animal has to humans, is if there were ever a case they were to be blown on-shore, then they will be carrying a high amount of venom in their cerata, which could prove fatal to humans.


Glaucus atlanticus (Blue dragon sea slug) Mrs MacLellan, Amelia v. 2014. no. 22/10/2014. 2014.

Marine invertebrates of Bermuda Blue Ocean Slug (Glaucus atlanticus) Scocchi, Carla; Woods, James v. 2014. no. 10/26. 2011,

Glaucus atlanticus (Taxonomy) Mrs MacLellan, Amelia v. 2014. no. 22/10/2014. 2014

Glaucus atlanticus (Feeding & Distribution) Mrs MacLellan, Amelia v. 2014. no. 24/10/2014. 2014

Glaucus atlanticus Newman, LJ; Estep, KW v. 2014. no. 10/26. 2012.|MainLayout::init

Figure 1: Taxonomy,

Figure 2: Glaucus atlanticus Forster 1777,

Figure 3: Life of a blue button Jellyfish,


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