This rare footage of what is possibly the ocean's largest octopod species – Haliphron atlanticus – was captured 300 metres beneath the waves by researchers on a mission to spy on cephalopods in the Philippines. 

Back in 2014, the Save the Nautilus Foundation team deployed a series of camera traps throughout the South Pacific in order to gather information about elusive marine molluscs known as nautiluses. The appearance of the seven-arm was an unexpected bonus – and sightings like this one made the project all the more interesting for the team. 

"It was an amazing find!" recalls lead scientist Dr Gregory Barord. "Our research is not only helping to protect nautiluses. It is also providing data on many other species – like this wonderful octopus friend – [as well as] environmental problems in the deep sea."

Despite its name, the seven-arm octopus does actually have eight arms, and they're easy to see in the females:

In males, however, one of the arms is much smaller and harder to spot – and like many quirks in the animal kingdom, it all comes down to sex. The small arm on a male H. atlanticus also serves as the penis-like sex organ, known as a hectocotylus.

Of course, dangling your member in the water column makes it a prime target for predators, so the male octopuses have devised a crafty workaround. Is there a better place for stashing one's sex organs than a handy pouch on one's face? The seven-arm octopus thinks not. Once coiled, the hectocotylus can be neatly tucked away in a special sac located just in front of the right lens. 

And it doesn't stop at "ocular tucking". During mating, the hectocotylus detaches – something of a takeaway sperm packet for the female. This strategy is shared by a few cephalopods, and in some species, like the chambered nautilus, the appendage can swim freely on its own in search of a mate.   

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The seven-arm is a far cry from the muscular octopuses we're more familiar with. The body is almost entirely gelatinous, and the web extends nearly halfway down the animal's stubby arms – but don't let that fool you. These lumbering animals are heavy. Really heavy.

The species was officially described only in 2004, after a female H. atlanticus specimen was caught by a fisheries trawler off New Zealand's coast. Though she was missing part of her web and several arms, that octopus tipped the scales at a whopping 61 kilograms (135lbs). Scientists estimate that a more complete specimen would have been an even heftier 75 kilograms (165 lbs), measuring nearly 3.5 metres (11ft) end to end. 

Before the New Zealand find, the seven-arm octopus was known only from anecdotal reports and collected beaks, and we've only just cracked the surface of its ecology – so each sighting brings important information about these mysterious creatures of the deep.