Glaucus atlanticus

Glaucus atlanticus Forster, 1777

Glaucus atlanticus @ Boynton Beach, Florida, USA 28-01-2018 by Ariane Dimitris










































Glaucus atlanticus  Forster, 1777

 Classification according to Bouchet et al. (2017)
Taxonomic source: World Register of Marine Species (AphiaID: 140022).
Taxonomic note: The genus Glaucus was established in 1777 by the British naturalist Johann Reinhold Forster. He described the genus based on a number of specimens of Glaucus atlanticus collected during the second voyage of James Cook aboard the HMS Resolution. In 1848, the German naturalist Johannes Gistel proposed the name Dadone to Glaucus, but it later deemed unnecessary and today it is considered invalid. Traditionally it was considered that the genus Glaucus had only two species Glaucus atlanticus and Glaucus marginatus, but in the work of Churchill et al. (2014) an anatomical and molecular study of different species of the genus Glaucus -unexpectedly- revealed the presence of a species complex. Glaucus Atlanticus was the only species found in the Atlantic Ocean, although it is also present throughout the Indo-Pacific, but under the name of Glaucus marginatus four separate species (referred to informally as the clade ‘marginatus’) were discovered. There are three species (Glaucus marginatus, Glaucus thompsoni and Glaucus mcfarlanei) cited in the North Pacific Gyre while there is a single species (Glaucus marginatus) cited in the South Pacific Gyre and the Indian Ocean. In addition, in the South Pacific there is another species Glaucus bennettae. Specimens of Glaucus atlanticus show some genetic differences in their different areas of distribution, but they are small, so it is considered to be a single species.


  • Doris radiata Gmelin, 1791
  • Glaucus distichoicus d’Orbigny, 1837
  • Glaucus flagellum Blumenblach, 1803
  • Glaucus hexapterigius Cuvier, 1805
  • Glaucus lineatus Reinhardt & Bergh, 1864
  • Glaucus longicirrhus Reinhardt & Bergh, 1864

Glaucus atlanticus can reach up to 30 mm in length. The body elongated, with the widest area right behind the head, and finishing in a pointed tail. The overall color is silver, with the rhinophores, oral tentacles and the lower edge of the cerata colored electric blue. The dorsum varies from dark brown to blue, while the foot is silvery in its central area, flanked by bands of light blue color that join in the head and near the tip of the tail. The head is short and has two simple, smooth and very short tentacles, widely separated. The smooth rhinophores are located very low on the sides of the head. Cerata are laid in six groups, the first two, near the head, are pedunculated (growing of a short common base), while the four rear are sessile (growing directly from the body side). The cerata are long and have a conical shape, are inserted into each group in a single series, and the most dorsal cerata is the longest. Each cerata ends in a cnidosac storing active stinging cells from their preys as a defense mechanism, and is easily autotomizable. The foot is wide and well developed, has a rounded anterior end and extends to the tip of the tail on the back. The anus is located on the right side, between the second and third group of cerata, while the renal pore is located at the same level but slightly ahead of the second group of cerata.

It feeds on other pelagic creatures to which it can swim with slow swimming movements performed with the cerata. Among their preys are Velella velella, Porpita porpita and Janthina janthina, also the stinging siphonophore Physalia physalis (according to Miller, 1974), also known as “Portuguese man o’war”. It has the ability to sequester and store the active cnidarian nematocysts in its own tissues, as a protection against predators. The manipulation of these animals by humans often results in painful and potentially dangerous stings, as they concentrate the poisonous nematocysts in the cnidosacs, so their sting is more powerful than the cnidarian’s. It has been documented that in captivity it may attack other specimens of the same species if there is no other food. Like almost all heterobranchs, it is hermaphroditic, each individual having both male and female reproductive organs, but unlike most sea slugs which mate by joining their right side, this species mates by joining their ventral sides (Ross & Quetin, 1990). After mating both individuals produce egg masses formed by straight threads of white eggs up to 17.5 mm long that float freely in the water (Ross and Quetin, 1990). These authors also reported that freshly collected individuals produced 4-6 egg strings per hour with 36-96 eggs per string and a total of 3.300-8.900 eggs per day. Being a pelagic animal (that lives on the water surface), it floats upside down, with the foot towards the surface, thanks to an air bubble in its stomach that gulps periodically and the water surface tension on its many cerata (Miller, 1974). It is carried away by winds and ocean currents. Although they are animals that live in open ocean water, sometimes they strand on the beaches by the thousands, because of the action of wind, currents and waves.


  • Glaucus, from ancient Greek “Γλαῦκος”, was a Greek prophetic sea-god, born mortal, became immortal after eating a magical herb. It was believed that he came to the rescue of sailors and fishermen in storms, having once been one himself. Also a Greek name meaning “greyish blue” or “bluish green” and “glimmering”.
  • Atlanticus, Latin form of the Greek “Atlantikos”, meaning “of Atlas”, in reference to Mount Atlas in Mauritania. Atlantic is the name for the “sea off the west coast of Africa”, applied to the whole ocean since c. 1600.

It lives in temperate and tropical seas of all the world’s oceans. It has been found in the eastern and southern coast of Africa, in European waters, the east coast of Australia, New Zealand, Hawai’i and Mozambique. It has also been cited in the Gulf of Aden (Thompson & McFarlane, 2008), in Andhra Pradesh, in the Bay of Bengal and on the coast of Tamil Nadu (India) (Srinivasulu et al. 2012), also in Bermuda (Johnston Barnes, O. 01/25/2016). There are recent reports of its presence in the Humboldt Current, off the coast of Peru (Uribe et al., 2013). Also in the Canary islands [D’Orbigny (1839), Pérez et al. (1990), Pérez Sánchez, Bacallado and Ortea (1991), Moro et al. (1995, 2003), Ortea et al. (2001)], Madeira [Bergh (1899)] and Azores [Simroth (1888), Bergh (1899), Wirtz (1998), Malaquias (2001), Wirtz and Debelius (2003)] in the NE Atlantic, and in the Balearic islands [Hidalgo (1916), Bofill and Aguilar-Amat (1924)] in the Mediterranean Sea.

Known georeferenced records of the species: Glaucus atlanticus
: GROC 2010-2011
: Enric Madrenas
: João Pedro Silva
: Bernard Picton
: Manuel Ballesteros.
: M@re Nostrum
: Altres fonts
: Marine Regions


    Western Mediterranean: ★☆☆☆☆
    Eastern Mediterranean: ☆☆☆☆☆
    Atlantic Ocean: ★★☆☆☆

This chart displays the monthly observation probability for Glaucus atlanticus based on our own records.



More pictures


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Further reading

Cite this article as:

Pontes, Miquel, Manuel Ballesteros, Enric Madrenas (2023) "Glaucus atlanticus" in OPK-Opistobranquis. Published: 25/09/2016. Accessed: 14/04/2024. Available at (

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