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“Blanket octopus” sounds like an endearing nursery game, but the blanket octopuses are actually pelagic hunters which have adapted to living in the ultra-competitive environment of the open ocean. There are four species of blanket octopuses (Tremoctopus) which can be found ranging from the surface to medium depths of open tropical and subtropical seas worldwide. Because they often live far from any land, some of the methods which other octopuses use to escape predators do not work very well for them. Fortunately Blanket octopuses have adapted in their own unique bag of tricks.
Blanket octopuses are named after the distinctive appearance of adult female octopuses which grow long transparent/translucent webs between their dorsal and dorsolateral arms. Blanket octopuses use these webs as nets for hunting fish, but they can also unfurl and darken their nets in order to appear much larger than they actually are. Since blanket octopuses do not produce ink and can not camouflage themselves as rocks, coral, or sand, they rely heavily on their blankets. As a last resort they can jettison the blankets as a decoy and jet away while the confused predator attacks the highly visible membranes.
Blanket octopuses exhibit extreme sexual dimorphism. Whereas the female octopus can grow up to 2 meters (6 feet) in length, the male octopus is puny and does not grown longer than a few centimeters (1 to 2 inches). Males store their sperm in a modified quasi-sentient third right arm, known as a hectocotylus. During mating this arm detaches itself and crawls into the female’s reproductive vent. As soon as the hectocotylus is detached the male becomes unnecessary and dies.
Tiny males and immature females do not have blankets, but they utilize another trick to protect themselves. Because they hunt jellyfish and other hydrozoans, the little octopuses are immune to the potent venom of the Portuguese man o’war. The octopuses tear off stinging tentacles from the man o’war and wield them in their tentacles like little whips to ward off predators.
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.
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).
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).
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.