The creatures have proved difficult to study in the wild, leading to the mystery about their sex lives.

"They're obsessively secretive, solitary and pretty spooky," said Roy Caldwell, co-author of the study and Professor of Integrative Biology at the University of California in Berkeley.

"If you watch them, they watch you back."

"It's hard to study them."

Lead author and graduate student Christine Huffard chose to observe octopi by snorkelling in the waters off Indonesia to observe the Abdopus aculeatus octopus.

The creature is the size of a small orange and has arms that are 20-25 cm long.

"Each day in the water, we learned something new about octopus behavior, probably like what ornithologists must have gone through after the invention of binoculars," she said.

"We quickly realized that Abdopus aculeatus broke all the 'rules' -- doing the near opposite of every hypothesis we'd formed based on aquarium studies."

These new observations may help to illuminate the mating habits of other types of octopi.

"This is not a unique species of octopus, which suggests others behave this way," said Professor Caldwell.

Male octopi were observed selecting a sex partner as opposed to picking the first female that came along.

One a male had made a choice, he would jealously guard the female's den for several days.

Males would occasionally use their arms to strangle a rival to death.

Once the male impregnated the female with his sperm, she would then lay tens of thousands of eggs in her den.

After the eggs hatched, the weakened mother octopus died.

The father would also expire within a few months of mating -- leaving the newborn octopi to fend for themselves.

"It's not the sex that leads to death," said Huffard.

"It's just that octopuses produce offspring once during a very short life-span of a year."