Glaucus atlanticus: the mini ninja of the ocean

By Vanessa Uschenko

Glaucus atlanticus, commonly known as the blue sea slug, is a marine gastropod of the nudibranchia clade and is one of only two species found in the Glaucus genus (Rudman 1998).  Found in the temperate and tropical waters of the Pacific, Indian and Atlantic oceans, the blue sea slug can grow up to 3 cm in length (Kumar et al 2012).

Despite its size, the blue sea slug has a reputation for being a voracious predator, consuming some of the most dangerous ocean dwellers. This small invertebrate preys upon many hydrozoa species, preferring the Portuguese Man-o-War (Physalia physalis) when available but has also been documented to prey upon by-the-wind-sailor (Velella velella),and the blue button jelly (Porpita porpita) (Bieri 1966). During conditions with limited food, the blue sea slug will also resort to cannibalism (Scocchi and Wood 2011). Contrary to its size, the blue sea slug is always full of surprises. Its mouth is comprised of a strong jaw made of chitin, holding together many sharp teeth-like structures that the invertebrate uses to latch on and tear off chunks of prey (Savilov 1956). The blue sea slug is not only unaffected by the poisonous sting from its prey, but has developed a unique way of utilizing the venom for its own benefit. Upon swallowing the nematocysts (stinging cells), the blue sea slug is then able to store the poison in their 84 tentacle-like structures called cerata (Natural History Museum 2011). This stored venom is then used as a defense mechanism against potential predators including birds and predatory fish.

A blue sea slug (also called sea swallow, blue glaucus, blue dragon, blue ocean slug, lizard nudibranch), Glaucus atlanticus. Photo by Sylke Rohrlach. Used under the Creative Commons Attribution-ShareAlike 2.0 Generic license.

Another mechanism the blue sea slug uses to avoid predation is camouflage. The blue sea slug uses its bright blue underside  to avoid detection from below, and its grey topside to avoid detection from above (Scocchi and Wood 2011). The blue sea slug has another unique adaptation to life in the ocean. This small nudibranch lives a pelagic upside down life, both floating and swimming (seldom) at the surface using a small air bubble, which they swallow and store inside their stomachs, allowing them to float along the surface, driven by wind and ocean currents, or by using their cerata that slowly allow them to swim closer to both prey and potential mates (Kumar et al 2012). Due to their wandering lifestyle, it is not uncommon for this species to wash up on shore, often in large numbers commonly referred to as “blue fleets”, posing danger to individuals utilizing the beaches (Rudman 1998). As these “blue fleets” wash up many blue sea slugs, the danger of people being stung increases, either by swimming in the waters, or interactions with the species on land.

This species is anything but ordinary. Little is known regarding its reproduction, but it has been documented sexual interactions between this species lasts between 43-59 minutes, and that males have small spines along the penis, a feature common in many animalian species (Alejandrino  et al 2013). It has also been documented that individuals of this species are hermaphroditic and lay large amounts of eggs on either the carcasses of prey they have previously killed or in free floating masses when carcasses are not readily available (Savilov 1968). The unique adaptations of this small pelagic nudibranch are a surprise to many: the ability to harness their prey’s defenses, superb camouflage and their voracious appetite make the blue sea slug a mini ninja of the ocean.


Alejandrino, A., Churchill C.K.C., Foighil, D.O., Valdes, A. 2013. Paralled changes in genital morphology delineate cryptic diversification of planktonic nudibranchs. Proceedings of the Royal Society. 280(1765):1-5

Bieri, R. 1966. Feeding Preferences and Rates of the Snail, Ianthina prolongata, The Barnacle, Lepas anserifera, The Nudibranchs, Glaucus atlanticus and Fiona pinnata, and the food web in the marine Neuston. Publications of the Seto Marine Biological Laboratory. 14(2): 161-170.

Kumar, G.C., Srinivasulu, B., Srinivasulu, C. 2012. First record of the blue sea slug (Glaucus atlanticus) from Andhra Pradesh-India. Taprobanica. 04(01):52-53 Accessed March 26, 2015

Natural History Museum. 2011. Taxonomy. London. Retrieved from: Accessed: March 23, 2015

Rudman, W.B .1998. Glaucus atlanticus Forster, 1777. Sea Slug Forum. Australian Museum, Sydney. Retrieved from: Accessed: March 23, 2015