Rare, deep-sea encounter: California scientists observe 'extraordinary' seven-arm octopus

Grace Toohey, Los Angeles Times on Dec 12, 2025

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Almost a half-mile below the surface of Monterey Bay, California, scientists recently recorded rare footage of a seven-arm octopus— only the fourth time the same research team has spotted the species in about four decades.

In a new video posted online, scientists with the Monterey Bay Aquarium Research Institute shared footage of the giant octopus eating and holding onto a smaller red helmet jellyfish, a bioluminescent creature known to avoid sunlight.

The last time these researchers spotted this type of “extraordinary octopus,” the creature was also holding part of a jellyfish, which at the time helped scientists understand its surprising diet of gelatinous animals.

“To be able to confirm our first observation with this new sighting was informative because this octopus was holding a different, deeper-living type of jellyfish than we’d seen before,” said Steven Haddock, a senior scientist with the MBARI and a member of the team that observed the giant octopus Nov. 6. The footage of the latest octopus was captured by a remotely operated vehicle at about 700 meters, or about 2,300 feet, below sea level.

Scientifically known as the Haliphron atlanticus, the seven-arm octopus actually has eight arms, but males tend to keep their eighth, specialized arm — used for transferring sperm during mating — hidden, giving the appearance of only seven appendages, according to the MBARI.

Female seven-arm octopuses are significantly larger than males, growing up to 13 feet long and weighting 165 pounds, MBARI experts reported. They mostly live in the ocean’s twilight zone where light rarely reaches, typically considered 650 to 3,000 feet below the water’s surface.

“It was super exciting for me to see this species that I had only seen once before,” Haddock said. “It is also interesting that this octopus is one of the largest known species, yet it reaches these sizes (on) a gelatinous diet.”