Creature Feature: Blanket Octopus

Referred to by some as the "Caped Crusader" or the "Batman" of the underwater world, the blanket octopus is a strangely beautiful creature. This fascinating cephalopod was given the common name of blanket octopus because of its flowing webbing connected between its dorsal and dorsolateral arms, resembling a large blanket, as displayed by this adult female. As you can see in this photo, its remaining arms are much shorter and lack webbing.

These octopi are classified in the pelagic cephalopod genus tremoctopus. They are further broken down into four species Gelatinous Blanket Octopus (tremoctopus gelatus), Palmate Octopus (tremoctopus gracilis), Common Blanket Octopus or Violet Blanket Octopus (tremoctopus violaceus), and Tremoctopus Robsoni. Sightings of these elusive creatures are rare, but they are known to have been found in the subtropical and tropical waters of the Mediterranean Sea and the Atlantic Ocean, and along the northern coast of Australia,  preferring surface to mid-water depths.

This cephalopod does not use ink to defend against predators as other octopi do. Instead, it uses its blanket to intimidate predators with its size. It has also been observed to display another defense mechanism. These particular octopi are immune to the deadly sting of the Portuguese Man 'o War jelly. The males and young females who lack the enormous blanket instead rip off the poisonous tentacles of a man 'o war and use them as a weapon against their would-be attacker.

The Blanket Octopus is an extreme example of sexual dimorphism. This term refers to the differences between the size and various physical characteristics of the male and female in the same species. "Imagine if your spouse was 40,000 times heavier and a hundred times larger than you. This is reality for the male blanket octopus..." [1] A male may grow up to approximately an inch in length, whereas an adult female can reach a length of 6.5 feet. The males have a specifically modified third tentacle which it fills with sperm. During mating the male rips of this tentacle, presents it to the female, then drifts away to die. The female then stores this tentacle inside a large internal body cavity until she is ready to lay up to 100,000 tiny eggs. Interestingly enough, females have been found with multiple tentacles stored in this cavity. When the female is ready to fertilize her eggs, she will pull out this stored tentacle and squeeze it "like a tube of toothpaste" over the eggs which she will then carry at the base of her dorsal arm until they hatch.

Below is a short, albeit cheezy, video showing the fascinating beauty of a female unfurling her blanket.

This second video shows another octopus calmly cruising the depths. Watch closely around 1:40 where it appears one of her banners disconnects. I could find no explanation for this, but she appears to continue on unfazed.

To see a picture of a male blanket octopus go to :

http://www.selfishgene.org/Tom/Papers/MDNetal_NZJFMR02.pdf [2]

[1] Pickrell, John (Aug. 12, 2003)."Walnut-Size" Male Octopus Seen Alive for First Time. National Geographic News.

[2] Norman, M.D.; Paul, D.; Finn, J.; and Tregenza, T. (2002). First encounter with a live male blanket octopus: the world's most sexually size-dimorphic large animal. New Zealand Journal of Marine and Freshwater Research 36: 733-736.