Marine Invertebrates of Bermuda

Blue Ocean Slug (Glaucus atlanticus)

By Carla Scocchi
James B. Wood (Ed)

Abstract  Introduction  Taxonomy  Habitat  Ecology  Recent Research  Commercial Importance  Bermuda Laws  Personal Interest  References  Links 

Blue Ocean Slug, Glaucus atlanticus

Phylum: Mollusca
  Class: Gastropoda
    Order: Nudibranchia
      Suborder: Aeolidinia
        Family: Glaucidae


      Glaucus atlanticus, better known as the blue ocean slug, is a unique marine gastropod that spends it life in the open ocean. Whether this creature is planktonic or pelagic is controversial due to the debate on the locomotive use of its foot. G. atlanticus is commonly associated with and is known for predating upon other open water invertebrates known as “the blue fleet”. The “blue fleet” consists of typically dangerous hydrozoans such as the Portuguese-man-o-war (Physalia physalis), by-the-wind-sailor (Velella velella), and blue button (Porpita porpita). The blue ocean slug has the ability to ingest the nematocysts of their prey and incorporate them for their own protection. Another form of defense is the counter-shading this species exhibits. The ventral side of the organism, which actually floats on the sea surface due to the gas float in its stomach, is a brilliantly blue color matching the color of the ocean. The dorsal side, which faces downward into the ocean, is a silvery grey color to blend in with the surface. G. atlanticus is hermaphroditic and oviparous, laying eggs masses in a mucus membrane. The eggs are often deposited on the dead carcasses of its prey. These animals are of no commercial importance, however they do present problems for beach goers when the “blue fleet” is blown ashore.


      Glaucus atlanticus, better known as the Blue ocean slug, was first described by Forster in 1777. Spending its life in the open ocean, this animal was appropriately named after the Greek god of the sea, Glaucus, who was forced to dwell in the sea forever (Sterrer 1992). G. atlanticus was first thought to be a marine insect followed by the larval stage of an angel shark (Sterrer 1992). This shell-less mollusc was finally classified as a marine gastropod.


      Blue ocean slugs have been found in the open ocean in temperate and tropical waters all around the world (Rudman 1998. Nov 6). There is controversy over whether this creature is a planktonic or pelagic invertebrate, due to contradicting observations on its voluntary movement (Savilov 1956, Thompson and McFarlane 1967). Glaucus atlanticus lives in close association with its close relative Glaucilla marginata (Thompson and Bennett 1970) as well as other open ocean invertebrates such as the Portuguese-man-o-war (Physalia physalis), the by-the-wind-sailor (Velella velella) and the blue button (Porpita porpita) (Sterrer 1992).


      Glaucus atlanticus is a voracious predator, feeding on some of the most dangerous hydrozoans known to humans. G. atlanticus is unharmed by the stinging cells (nematocysts) used for defense in its prey (Sterrer 1992, Rudman 1998. Nov 6). The blue ocean slugs are not only immune to them, but they actually incorporate them into their own bodies as a defensive measure themselves (Thompson and Bennett 1969) . The blue ocean slug’s preferential food is Physalia physalis, or the portuguese-man-o-war, but will often resort to cannibalism when food supply is scarce (Bieri 1966). When food is available and the slug is feeding on either P. physalis, Porpita porpita or Velella velella, it captures the edge of the disc of the hydrozoan and can tear off big chunks of whole colonies. The small slug can engulf and attach to big pieces of its prey by the strong hold of its chitinous jaws (Savilov 1956). Within the jaws, there are rows of pointed “denticles” which are probably used to hold on to prey even in unsteady surroundings (Roginskaya 2006. Aug 2).

      G. atlanticus was commonly named the blue ocean slug for good reason. Its vibrantly blue coloration is probably the most visually striking feature of the animal. This characteristic wasn’t just evolutionary selected upon for its good looks, it provides the animal with protection from predators such as birds flying above the waters it inhabits (Rudman 1998. Nov 6). G. atlanticus exhibits a perfect example of counter-shading. Its ventral side which floats on the sea surface due to a gas-filled float in its stomach is the blue and white color which matches the color of the ocean, while its dorsal side which faces downward into the water is a silvery-grey providing protecting from predatory fish (Rudman 1998. Nov 6). While camouflage is one way the blue ocean slug defends itself in the dangerous open waters, another mode is to steal their prey’s defenses and use it for their own protection as previously mentioned. Like most aeolids, they do this by ingesting the nematocysts and storing them in special sacs called cnidosacs, which transport them to the tips of their limb-like outgrowths called cerata (Rudman 1998. Nov 6). A study by Thompson and Bennett (1969) found through microscopy that the nematocysts in G. atlanticus were the same nematocysts found in P. physalis proving that the slugs do in fact obtain these nematocysts from others.

      G. atlanticus possess an interesting feature of its male reproductive part. The penis of this organism is large and hooked, probably to get around the dangerous cerata which could present difficulty in mating when animals are forced to be close with one another (Rudman 2002. Oct 21). Although there is relatively little literature on the mating behaviors of G. atlanticus, it is known that sea slugs are hermaphroditic and lay eggs (Rudman 2003. May 27). There is some information reported on their egg masses: G. atlanticus produces strings of eggs that are encased in a thin mucus membrane. Because of the lack of a hard substrate to attach the eggs to, the eggs often float freely in the water or are placed on the remains of prey (Savilov 1968) until the larvae hatch (Dakin 1953, Thompson and McFarlane 1967, Rudman 1999. Feb 25).

Recent Research

      There is little research on the blue ocean slug in general, and much less recent research. The most recent message I came across was a comment by Irina Roginskaya in the Sea Slug Forum. From analysis of SEM photos of the mouths and jaws of Glaucus she noticed some of the denticles were worn and broken. She suggest that this might mean the slug puts “considerable effort” into holding on to its prey, and that these denticles are critical for primary feeding by juveniles.

Commercial Importance

      The blue ocean slug is of no commercial importance. However, its close association with commonly known hydrozoans could present dangerous situations for beach-goers when the animals are blown inshore.

Bermuda Laws

      There are no specific Bermudian laws for the protection of the blue ocean slug. Since this species is circumglobal and resides in the open ocean, I can imagine it being difficult to enforce regulations of the collection of this species. However, Bermuda does have several marine protected areas where the collection of any animal is prohibited (Wood and Jackson 2005).

Personal Interest

      My personal interest in the blue ocean slug was provoked by the fascinating picture on the cover of Wolfgang Sterrer’s book, “Bermuda’s Marine Life”. The animal’s bright blue and silver coloration and unusual shape impelled me to open to the page, and once I found out it was actually a slug, I wanted to learn more about the creature. When I read that this small gastropod feeds on deadly hydrozoans such as the portuguese-man-o-war or by-the-wind-sailors, I knew I had to learn more about the life of this invertebrate!


Bieri R. 1966. Feeding preferences and rates of the snail Janthina prolongata, the barnacle Lepas anserifera, the nudibranch Glaucus atlanticus and Fiona pinnata and the food web in the marine pleiston. Publications Seto Marine Biological Laboratory. 14(2):161-170

Dakin, W. J. 1953.Australian Seashores. Angus and Robertson.

Forster, G. 1777. A voyage round the world in His Britannic Majesty's sloop, Resolution, commanded by Capt. James Cook, during the years 1772, 3, 4, and 5 by George Forster. 1:49.

Roginskaya, I.S. 2006 (Aug 2) Feeding and some trophic interactions of Glaucus atlanticus. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Rudman, W.B., 1998 (November 6) Glaucus atlanticus Forster, 1777. [In] Sea Slug Forum. Australian Museum, Sydney. Available from

Rudman, W.B., 1999 (Feb 25). Comment on Glaucus in New Zealand by Ross Armstrong.[Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Rudman, W.B., 2002 (Oct 21). Comment on Glaucus atlanticus - observations by A. Lovatt. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Rudman, W.B., 2003 (May 27). Comment on Glaucus in Hawaii by Chaunte Ling.[Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Savilov, A.I. 1956. Floating biocoenosis in the Pacific Ocean based on the material of the Expeditions of the Institute of Oceanology, Academy of Sciences USSR. Priroda. 45(3):62-68.

Savilov A.I. 1968. Pleuston of the Pacific Ocean.Pages 264-353 in the Monography "Pacific Ocean. Biology of Pacific Ocean". Part 2. Moscow. Academy of Sciences of the USSR. Institute of Oceanology."Nauka".

Sterrer, W. 1992. Bermuda's Marine Life. Bermuda Zoological Society. Flatts, Bermuda.

Thompson,TE. and Bennett,I., 1969. Physalia nematocysts: Utilised by mollusks for defense. Science. 166: 1532-1533.

Thompson,TE. and Bennett,I., 1970. Observations on Australian Glaucidae (Mollusca: Opisthobranchia). Zoological Journal of the Linnean Society, 49: 187-197.

Thompson, T.E. and McFarlane, I.D. 1967. Observations on a collection of Glaucus from the Gulf of Aden with a critical review of published records of Glaucidae (Gastropoda, Opisthobranchia). Proceedings of of the Linnean Society. 178: 10-123.

Wood, J.B. and K.J. Jackson. 2005. Bermuda In: Caribbean Marine Biodiversity: The known and unknown. Miloslavich, P. and E. Klein eds. DEStech Publications, Inc. 19-36.


Opisthobranchs of the Sunshine Coast
Sea Slug Forum
Bermuda Invertebrates