Researchers from the Monterey Bay Aquarium Research Institute (MBARI) have reported a rare sighting of the seven-arm octopus, Haliphron atlanticus, offering new insight into the feeding habits of one of the deep ocean’s least-seen giants.
The species lives in the ocean’s twilight zone, hundreds of metres down, and encounters are extraordinarily uncommon. MBARI has observed Haliphron only four times in nearly 40 years of remotely operated vehicle (ROV) expeditions.
One of those sightings — recorded in 2017 — involved a large female carrying an egg-yolk jelly, Phacellophora camtschatica, in her arms. The behaviour raised questions about whether the octopus was feeding on the jelly or using it in another way.
Subsequent examination of museum specimens confirmed that the seven-arm octopus does indeed prey on gelatinous animals.
MBARI’s most recent observation in 2025 showed another Haliphron individual consuming a helmet jelly, Periphylla periphylla.
Although jellies are not especially nutritious, their sheer abundance appears to make them a reliable food source for a range of deep-sea predators.
Researchers note that Haliphron first consumes the tissues in the jelly’s bell, then trails the stinging tentacles behind it — likely using them defensively. Similar behaviour has been seen in other deep-sea animals that repurpose jellyfish tentacles after feeding.
Female seven-arm octopuses can reach enormous sizes, weighing up to 75 kilograms (165 pounds), while males remain much smaller, typically about 21 centimetres (eight inches) long.
Despite the name, the species has eight arms like all octopuses. Males keep their specialised reproductive arm — the hectocotylus — tucked beneath the right eye, giving the impression of having only seven.
Haliphron atlanticus plays a much larger ecological role than its rarity suggests. It is eaten by sperm whales, blue sharks and swordfish, linking the upper ocean to the depths, and underscoring how gelatinous animals support a wide range of marine predators.
Understanding how species like Haliphron feed and interact is key to predicting how the deep sea will respond to changes in ocean temperature and other pressures such as overfishing and habitat destruction.
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