Seven-arm octopus
Ventral view of young female (70 mm ML)
Lateral view of young male
Conservation status
Least Concern (IUCN 3.1)[1]
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Mollusca
Class:	Cephalopoda
Order:	Octopoda
Family:	Alloposidae Verrill, 1881
Genus:	Haliphron Steenstrup, 1861
Species:	H. atlanticus
Binomial name
Haliphron atlanticus Steenstrup, 1861[2]
Synonyms
Alloposus mollis Verrill, 1880 ?Octopus alberti Joubin, 1895 Alloposus pacificus Ijima & Ikeda, 1902 Heptopus danai Joubin, 1929 Alloposus hardyi Robson, 1930 ?Alloposina albatrossi Robson, 1932

The seven-arm octopus (Haliphron atlanticus), also known as the blob octopus or sometimes called septopus, is one of the two largest known species of octopus; the largest specimen ever discovered had an estimated total length of 3.5 m (11 ft) and mass of 75 kg (165 lb).^[3][4] The only other similarly large extant species is the giant Pacific octopus, Enteroctopus dofleini.

The genera Alloposina Grimpe, 1922, Alloposus Verrill, 1880 and Heptopus Joubin, 1929 are junior synonyms of Haliphron, a monotypic genus in the monotypic family Alloposidae, part of the superfamily Argonautoidea in the suborder Incirrata of the order Octopoda.^[2]

Description

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. Due to this species' thick, gelatinous tissue, the arm is easily overlooked, giving the appearance of just seven arms. However, like other octopuses, it actually has eight.^{[citation needed]}

Distribution

The type specimen of H. atlanticus was collected in the Atlantic Ocean at 38°N 34°W / 38°N 34°W / 38; -34 (west of the Azores). It is deposited at the University of Copenhagen Zoological Museum.^[5]

Since then, several specimens have been caught throughout the Atlantic, as far as the Azores archipelago^[6] and near South Georgia Island.^[7]

In 2002, a single specimen of giant proportions was caught by fishermen trawling at a depth of 920 m off the eastern Chatham Rise, New Zealand. This specimen, the largest of this species and of all octopuses, was the first validated record of Haliphron from the South Pacific. It had a mantle length of 0.69 m (2.3 ft), a total length of 2.90 m (9.5 ft), and a weight of 61.0 kg (134.5 lb), although it was incomplete.^[3][4]

Ecology

Isotopic,^[7] photographic and video evidence^[6] 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.^{[8][9][10][11][12]}

Beak morphology

Lower (left) and upper beaks of female Haliphron atlanticus (estimated 150 mm ML) in lateral view
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