Seven-arm octopus

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.^[5]

Unlike most octopods, H. atlanticus has a swim bladder, an organ evolved from its digestive system. The swim bladder allows it to maintain neutral buoyancy, neither sinking nor rising in the water column.^[6]

Older specimens have been found to have shorter bodies and longer arms than juveniles.^[7]

While juveniles tend to be translucent, one study in 1985 in Norway studied the morphology between 2 female H. Atlanticus specimens, with one being a dark purple and the other was a pale blue/white.^[7] Video evidence from the Monterey Bay Aquarium Research Institution (MBARI) was released showing a red specimen as well^[8], giving the species a color range of blue to red when adults.

H. atlanticus is found worldwide in tropical and temperate waters.^[9]

The type specimen was collected in the Atlantic Ocean at 38°N 34°W (west of the Azores). It is deposited at the University of Copenhagen Zoological Museum.^[10]

Since then, several specimens have been caught throughout the Atlantic, as far as the Azores archipelago^[11] and near South Georgia Island.^[12]

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. 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]

Another specimen was reported from Skjørafjorden, Norway, where a female was captured in a trawl net at approximately 100 m (328 ft) depth in November 2000. The individual measured about 96 cm (3.1 ft) in total length, though the arm tips were damaged, and weighed 8.5 kg (18.7 lb) when received by researchers. The specimen had a soft, gelatinous body with a reddish coloration and rows of pale suckers, and was later preserved and deposited at the NTNU Museum of Natural History and Archaeology in Trondheim.^[13]

Evidence of the species in northern waters is also supported by dietary studies of sperm whales, which have contained remains of H. atlanticus in their stomach contents. Because these whales are believed to feed in Arctic and northern boreal waters before migrating south, these findings suggest that the species likely occurs more widely in the Norwegian Sea and nearby Arctic regions than the relatively small number of direct captures of indicate.^[13]

There have also been multiple sightings of H. atlanticus in Monterey Bay in the Northeast Pacific Ocean.^[14]

The mating of H. atlanticus is poorly researched, but it is predicted to occur sporadically on the continental coast.^[15]

Dwarf males of H. atlanticus have been discovered. These males have invested energy for growth into reproductive traits such as a spermatophore and a muscular penis. They also possess a detachable hectocotylus which is used to transfer the spermatophore into their mates. These males qualify as dwarf males as they are only 5-15% of the size of a maximum female specimen of H. atlanticus.^[6] The organism's swim bladder helps them maintain buoyancy in the deep sea without expending large amounts of energy searching for mates.^[6]

Isotopic,^[12] photographic and video evidence^[11] have shown complex interactions between H. atlanticus and jellyfish and other gelatinous zooplankton, from feeding to protection, respectively. H. atlanticus specimen have been observed feeding in mesopelagic areas, particularly at the ridges of seamounts.^[15] Stable isotope analyses indicate that H. atlanticus occupies an intermediate trophic level among deep-sea cephalopods, consistent with feeding on gelatinous zooplankton as well as other prey types.^[16]

Predators of H. atlanticus include the blue shark, sperm whale, and swordfish.^{[15][17][18][19][20][21]}

• Cephalopod size