This octopus is a sucker for jellies.

The seven-arm octopus (Haliphron atlanticus) is a rarely seen giant that lives in the ocean’s twilight zone. In nearly four decades of deep-sea exploration with our remotely operated vehicles, MBARI has only encountered this species four times.

A chance encounter with a large female Haliphron atlanticus in 2017 provided important insight into the feeding behavior of this elusive species. 

This individual was clutching an egg-yolk jelly (Phacellophora camtschatica) in her arms, piquing our researchers’ curiosity. Analyzing the stomach contents of museum specimens confirmed our scientists’ suspicions that the seven-arm octopus feeds on gelatinous animals. In 2025, we logged our fourth sighting of Haliphron atlanticus, this time an individual eating a helmet jelly (Periphylla periphylla).

Jellies may not be very nutritious, but they are abundant, providing food for many ocean giants. Haliphron atlanticus first eats the tissues in the jelly’s bell, then trails the stinging tentacles behind, likely repurposing those for defense as a bonus to the meal.

While female seven-arm octopus can reach enormous sizes and can weigh up to 75 kilograms (165 pounds), the males are much smaller, typically only reaching 21 centimeters (about eight inches) long. 

Ironically, the seven-arm octopus actually does have eight arms, like other octopuses. As is typical of octopuses, male Haliphron atlanticus have a specialized hectocotylus arm for transferring sperm during mating. They tend to keep this arm tucked into a sac beneath their right eye, giving the appearance of only seven arms.

MBARI researchers are studying the complex connections across the ocean food web and the surprising importance of jellies to the diets of many animals. Many large predators, including sperm whales, blue sharks, and swordfish, feed on Haliphron atlanticus, underscoring the close link between the ocean’s surface and the inky depths below. 

The deep sea is closer than you think, and our actions on land can ripple across marine ecosystems. Overfishing, pollution, habitat destruction, and climate change also threaten the survival of deep-sea animals and their ecosystems. MBARI’s work is critical for understanding how a changing ocean will affect the denizens of the deep—and what individuals and communities can do to safeguard their future.