[3][4] The only other similarly large extant species is the giant Pacific octopus, Enteroctopus dofleini.

[2] The seven-arm octopus is so named because in males, the hectocotylus (a specially modified arm used in egg fertilization) is coiled in a sac beneath the right eye.

[8] In 2002, a single specimen of giant proportions was caught by fishermen trawling at a depth of 920 m (3,020 ft) off the eastern Chatham Rise, New Zealand.

[3][4] Isotopic,[8] photographic and video evidence[7] have shown complex interactions between H. atlanticus and jellyfish and other gelatinous zooplankton, from feeding to protection, respectively.

Predators of H. atlanticus include the blue shark, Hawaiian monk seal, sperm whale, and swordfish.