Today we're returning to the deep to meet an octopus that, at first glance, hardly seems to earn that eight-limbed designation.

Its very name sounds like an oxymoron—or a cautionary tale from a fishing accident. But the seven-armed octopus (Haliphron atlanticus) is a real, bonafide octopod—if a little misleading in its appellation.

This deep-ocean octopus does have the full eight arms, but the males keep one of their arms—which also serves as their sexual organ—tucked away mostly out of sight. Where? In a little sac, in front of its right eye.

Although these octopuses might be modest, they are anything by discrete in size. The seven-armed octopus is possibly the largest octopus species in the world, rivaling even the well known giant Pacific octopus (Enteroctopus dofleini). One dead female H. atlanticus specimen, caught by a fisheries trawler off the coast of New Zealand in 2002, came up missing part of its body and arms. Despite being incomplete, it was still just under three meters (about nine and a half feet) long and weighed 61 kilograms (135 pounds). Estimates put her potential full length—from mantle tip to arm tip—at four meters (13 feet) with a weight of 75 kilograms (170 pounds)!

The oddness of this octopod extends beyond its coy arm concealing and tremendous size. It is not your standard leggy cephalopod. In fact, it is almost all gelatinous body. The arms are short—especially on the front side. And the gooey web stretches far down the arms, making the limbs appear even stumpier than they are.

Not much is known about this massive species. The seven-armed octopus lives far from shore, in deep waters. But it seems to occupy a variety of underwater habitats. The species has been pulled up from bottom trawls on the deep offshore slopes of continents, and scientists have captured video of it swimming along these dark floors. However, other samplings have located them in the vast open ocean, thousands of meters from the floor—and far from any underwater slope. Unlike most octopuses, these pelagic creatures have fish-like swim bladders that help them stay aloft in the water column. They also seem to brood their eggs in midwater, rather than laying them in dens. One female was observed off the coast of Hawaii, some 270 meters (886 feet) below the surface, carrying her eggs among her arms. Her brood appeared to be attached to the top, inside area around her mouth, safely inside the protective web.

In a rare video, scientists captured one of these octopuses, swimming sleepily along, a huge gelatinous orb in the deep.

And, in case you were wondering how the male uses that carefully guarded eighth arm, he detaches it at mating, allowing the female it take it, along with his sperm. So in the end, the males of this species live up to their name.

Illustration courtesy of Ivan Phillipsen