MBARI researchers recorded a rare seven-arm octopus (Haliphron atlanticus) on Nov. 6 at roughly 2,300 feet in Monterey Bay — the institute's fourth confirmed sighting in nearly 40 years. The animal was observed clutching a crimson-red helmet jelly, supporting evidence that the species feeds on gelatinous prey. Females can reach about 75 kg (165 lb), while males are much smaller; the "seven-arm" name comes from males tucking a reproductive arm under the right eye. High-quality video and behavior notes from this sighting add important data to the limited knowledge of this elusive deep-sea species.
Rare 'Seven-Arm' Octopus Spotted in Monterey Bay — MBARI's 4th Sighting in Nearly 40 Years
Monterey Bay Aquarium Research Institute/YouTubeSeven-arm octopus spotted in Monterey Bay on Nov. 6
Researchers from the Monterey Bay Aquarium Research Institute (MBARI) observed a rarely seen deep-sea octopus on Nov. 6 in Monterey Bay, marking only the fourth confirmed encounter with the species by MBARI in almost four decades.
Details of the Sighting
The animal, commonly called the seven-arm octopus or septopus and known scientifically as Haliphron atlanticus, was recorded at about 2,300 feet (≈700 meters) below the surface. MBARI scientists captured high-quality video and detailed behavioral notes during the encounter, calling the individual a "rarely seen giant."
Behavior and Diet
During the observation the octopus was seen clutching a crimson-red helmet jelly, reinforcing previous evidence that this species feeds on gelatinous animals. A prior MBARI encounter in 2017 also showed a septopus holding an egg-yolk jelly, and analyses of museum specimens’ stomach contents further support a gelatinous diet.
GettyA stock image of the Monterey Bay coast
"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 MBARI senior scientist Steven Haddock, who participated in the Nov. 6 cruise.
Size, Sex Differences, and the Name
Female seven-arm octopuses are among the largest octopus species: MBARI reports females can reach up to about 75 kilograms (165 pounds). Males are much smaller, typically around 8 inches in length. The species’ common name—"seven-arm octopus"—stems from a male reproductive adaptation: males possess a specialized arm called a hectocotylus used to transfer sperm and often tuck it into a sac beneath their right eye, making only seven arms appear visible.
Why This Matters
Each rare observation yields valuable behavioral and ecological data about an elusive deep-sea species. MBARI’s Nov. 6 footage and notes add to the limited body of knowledge about Haliphron atlanticus, helping scientists better understand its feeding habits, habitat depth, and life-history differences between sexes. Continued documentation of such encounters advances our understanding of deep-ocean biodiversity.
Source: Monterey Bay Aquarium Research Institute (MBARI) observation; quotes as reported by The Los Angeles Times.
