Image caption Footage revealed that the deep-sea squid is both promiscuous and bisexual

Scientists have gained a rare glimpse into the sex life of the mysterious deep-sea squid.

By studying footage taken by underwater vehicles, US researchers have found that this rarely seen creature will often engage in same-sex mating.

They believe this is because encounters with potential mates in the dark depths are rare, and the squid may be unable to tell the sexes apart.

The study is published in the Royal Society journal Biology Letters.

The researchers looked at video footage taken over 20 years by Remotely Operated Vehicles (ROVs), most of which was recorded in the Monterey Submarine Canyon, off the coast of California.

We concluded that males mate with males and females Hendrik Hoving, MBARI

The species that was observed is called Octopoteuthis deletron, a tentacled beast that measures about 12cm-long (5in), with impressive hook-lined arms. It was recorded between depths of 400m and 800m (1,300-2,600ft).

Until now, little was known about this creature's sex life, apart from the fact that the male uses a long, penis-like organ to deposit spermatophores, complex structures containing millions of sperm, onto a female, which are then absorbed into her tissue.

But by studying footage of this deep-sea squid, they were able to find out much more.

Indiscriminate insemination

Lead author Hendrik Hoving, from the Monterey Bay Aquarium Research Institute (MBARI), explained: "We did not observe two animals mating, but we found evidence of mating - sperm packages on males and females.

"Going through hours of video, we found that both males and females carry sperm packages.

Image caption The sperm packages can be seen as white dots on the the dorsal arms of this female

"As the locations of sperm packages were similar in both sexes, we concluded that males mate with males and females."

The finding surprised the team, said Dr Hoving.

The researchers found equal numbers of female and male squid that had had sperm packages deposited on them, indicating that same-sex mating was as frequent as encounters between squid of the opposite sex.

The number of sperm packages that had been deposited also suggested that these animals were promiscuous, the researchers said.

This unusual behaviour, they said, may be explained by the fact the squid is boosting its chances of successfully passing on its genes in the challenging environment it lives in.

In the Royal Society paper the team writes: "In the deep, dark habitat where O. deletron lives, potential mates are few and far between.

"We suggest that same-sex mating behaviour by O. deletron is part of a reproductive strategy that maximises success by inducing males to indiscriminately and swiftly inseminate every [squid] that they encounter."

Following the study, the researchers now hope to use genetic techniques to determine the paternity of the sperm packages, so as to find out whether more than one male is trying to deposit its sperm on the males and females.