Predators of the Open Ocean: Sea Slugs?

Man O’ War (Photograph by Enrique Talledo)

The Portuguese man o’ war (Physalia physalis) is not a jellyfish, in fact it is not a discreet animal at all, but instead a siphonophore—a colonial medusoid made up of specialized animal polyps working together as an organism.   These siphonophores have stinging tentacles which typically measure 10 metres (30 ft) in length but can be up to 50 metres (165 ft) long.  Being stung by a man o’ war does not typically cause death, but sailors and mariners who have survived the experience assert that it taught them a new definition of agony.

Glaucus atlanticus

But the fearsome man o’ war is not the subject of this post.  Instead we are concentrating on the animal which feeds on the man ‘o war (as well as other siphonophores which drift in the great blue expanses of the open ocean).  One is inclined to imagine that men o’ war are eaten only by armored giants with impervious skins and great shearing beaks (and indeed the world’s largest turtles, the loggerheads, are the main predators of siphonophores), however another much less likely predator is out there in the open ocean gnawing away at the mighty stinging colonies.   Glaucus atlanticus, the blue sea slug, is a tiny shell-free mollusk which lives in the open ocean.  The little nudibranch only grows up to 3 cm in length but it hunts and eats a variety of large hydrozoans, pelagic mollusks, and siphonophores (including the man o’ war).

Glaucus atlanticus eating velella colony animal

Although not quite as gaudy as its lovely cousins from tropical coral reefs, Glaucus atlanticus is a pretty animal of pale grey, silver, and deep blue with delicate blue appendages radiating out from its six appendages.  The little mollusks live in temperate and tropical oceans worldwide. They float at the top of the water thanks to a swallowed air bubble stored in a special sack in their gastric cavity. Because of this flotation aid, the slug is able to cling upside down to the surface tension of the waves.  Since it is entirely immune to the venomous nematocysts of the man o’ war, the sea slug can store some of the man o’ wars venom for its own use.   The tendrils at the edge of Glaucus atlanticus’ body can produce an extremely potent sting (so it is best to leave the tiny creatures alone, if you happen to somehow come across them).

Glaucus atlanticus inshore

Each and every Glaucus atlanticus is a hermaphrodite with a complete set of sex organs for both genders.  Incapable of mating with themselves they ventrally (and thoroughly) embrace another blue sea slug during breeding, and both parties then produce strings of eggs.  The hatchling nudibranchs have a shell during their larval stages, but this vestige quickly disappears as they mature into hunters of the open ocean.

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Siphonophorae

A physconect siphonophore, Marrus sp., photographed during NOAA's Arctic "Hidden Ocean" expedition in support of the Census of Marine Life. ©2005, Kevin Raskoff.

The Siphonophorae are a group of marine animals closely related to jellyfish and corals.  Like jellyfish, siphonophores are free to move around the ocean.  They hunt and capture fish and crustaceans by means of stinging tentacles.  However, like adult coral, siphonophores are colony animals.  A single siphonophore consists of multiple living animals–some of which are quite different from each other, since they serve different functions in the overall colony.  Their unusual nature makes them a focus of the scientific (and philosophical) question of what constitutes an individual organism as opposed to a group of organisms.

The best known siphonophore, the fearsome Portuguese man-of-war, possesses a gas filled bladder from which long colonies of stinging animals hang.   Most siphonophores however are not found near the surface.  Siphonophore colonies form delicate chains which can be quite long.  Some have been recorded to be 130 feet or more in length—substantially longer than the mighty blue whale.  Additionally some siphonophores are bioluminescent.

A Prayid Siphonophore in its Contracted Form

Marine biologists are beginning to think that siphonophores are more prevalent and important than initially believed.  Scientists once used nets and tows to capture specimens and calculate overall biomass.  These methods broke delicate siphonophores into unidentifiable pieces.  Now that biologists are using unmanned submarines to study the ocean, they have been finding many more siphonophores than they expected.

Siphonophorae illustrated by Haeckel