"Walnut-Size" Male Octopus Seen Alive for First Time

John Pickrell
for National Geographic News
February 12, 2003

Imagine if your spouse was 40,000 times heavier and a hundred times larger than you. This is reality for the male blanket octopus, which was recently spotted alive for the first time off Australia's northern coast.

Australian and British marine biologists stumbled across the diminutive male octopus while scuba diving in the Great Barrier Reef. Divers spotted a male 2.4 centimeters (0.9 inches) long. By contrast, females of the species grow up to two meters (6.6 feet) long.

"Imagine a female the size of a person and the male a size of a walnut," said Tom Tregenza, an evolutionary biologist at the University of Leeds in England. Tregenza co-authored the study that reported the find in the latest issue of the New Zealand Journal of Marine and Freshwater Research.

Dead blanket octopus males (Tremoctopus violaceus) have been collected before in plankton nets. But the recent observation marks the first time one has been spotted swimming about in its natural environment.

Evolutionary Oddity

"This is certainly the most dramatic example of sexual dimorphism in any large animal," said Tregenza. So-called sexually dimorphic species are those, like humans, whose males and females are of different average sizes.

Blanket octopuses are rarely seen. They spend their entire life drifting in the open oceans of warm regions worldwide. Females have the odd appearance of a "big pink drifting blanket" said Tregenza, explaining the origins of the octopus's name.

Among their more unusual behavior, the octopuses employ a unique defense mechanism by tearing off the tentacles of passing Portuguese man-of-war jellyfish. The octopuses are immune to the tentacle's painful sting. When they encounter potential predators, the octopuses waft the captured man-of-war tentacles in two pairs of its upper arms as an effective deterrent.

The male blanket octopus recently photographed by researchers was shown to clutch tentacle segments in his suckers, said Tregenza.

Blackwater Hang

Study co-authors Mark D. Norman of the Museum Victoria in Melbourne, Australia, and David Paul at the University of Melbourne photographed the animal while on a type of late-night diving expedition known as a blackwater hang.

Such expeditions involve taking a boat to sea on moonless nights and suspending a diver up to 20 meters (66 feet) into the pitch-black water. The researchers then waited in the eerie silence and used torches as light sources in the hope of attracting interesting passers-by, said Tregenza.

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