Footage from remote-controlled submersibles reveals that five-and-a-half-inch deep-sea male squid which dwells a depth of 400 to 800m, commonly try to mate equally with their own gender as they do with females.

Although instances of male-male mating has been previously observed in many other species, including giant squid, this is the first time such a high proportion of same-sex mating as frequent as male-female mating, according to researchers.

A team led by Henk-Jan Hoving, a marine biologist at the Monterey Bay Aquarium Research Institute in California, found evidence of the squid's approach to sex when they examined screen grabs from video recorded at depths of 400 to 800 metres in the Monterey canyon off the coast of California. Remotely operated vehicles were used to explore the deep waters.

The report, published in the journal Biology Letters, focused on a squid species called Octopoteuthis deletron.

Scientists report that factors such as bad light, similar male and female body size and the rarity of encounters are to be blamed for this indiscriminate behavior, resulting in marks slapped on the bodies of unsuspecting males in the form of spent sperm sacs.

Scientists suspect that male squid may mate with any member of their species they encounter, which helps these rare animals take advantage of any possible chance to reproduce.

Hoving suggests that this squid species does not spend much effort identifying the sex or courting of potential mates, but puts its energy into generating large quantities of sperm, for all possible encounters.

This behavior reflects the limited window of opportunity available to deep-sea squid for reproduction.

“This is a solitary species that is not very abundant; it lives in deep, dark waters where opportunities for reproduction are few and far between. In response to that challenge, this reproductive strategy ensures that no opportunity for successful mating is lost,” said Co-author Bruce Robison.

During mating among squids, the male releases a sperm-filled bag that discharges into the female's tissues. Males use a long tentacle-like appendage - a penis of sorts - to deposit small sperm-laden sacs, called spermatangia, on to females. Though the act is fast, it leaves empty sperm sacs on the female body as an outward sign of recent mating.

In the study, of the 108 squid filmed, it was possible to determine the sex of 19 females and 20 males. Among them, 10 females and nine males had visible sperm sacs on their bodies. Those left on the males were such that they must have been left by other males, the researchers have reported.