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Searching out the small - and his massive mate

January 31, 2003

The male half of one of the strangest marine couples has been seen alive for the first time. The blanket octopus shows the greatest size difference between sexes of any large animal, with the male 100 times smaller and 40,000 times lighter than the female.

University of Leeds researcher, Dr Tom Tregenza, saw the creature whilst diving off the Great Barrier Reef with colleagues from Melbourne and Tasmania. "The blanket octopus spends its entire life cycle in the open ocean, so it's rarely encountered," he said. "Dead males have been found in nets, but this is the first time a live one has been seen. Although mature, it is just 2.4cm long and weighs just 0.25g. In contrast, mature females can grow up to 2m long, and weigh around 10kg."

Male blanket octopuses develop an extra arm within a spherical pouch, used for reproduction. When they locate a female and mate, the arm is severed and passed to the female, remaining in the female's mantle cavity until used to fertilise her eggs. After mating, the male is believed to die.

"We don't really know why the octopus has developed with this huge difference in size," said Dr Tregenza. "One theory is that the male gains an advantage by remaining small, as it reduces the time taken to reach sexual maturity. We know that males are in competition with each other over females, as females are often found with several arms within their mantle cavity."

Leeds, University of