Overview
The Seven-arm Octopus (Haliphron atlanticus) is one of the largest and most enigmatic cephalopods on Earth, rivaling the giant Pacific octopus in total mass. Inhabiting the deep, sunless depths of the world’s open oceans, this colossal pelagic octopus derives its common name from a curious anatomical quirk in males: the specialized reproductive hectocotylus arm is coiled invisibly within a sac below the right eye, giving the external illusion of having only seven arms.
Biology and Physical Characteristics
Unlike benthic octopuses that possess muscular, firm bodies built for crawling over seafloors, Haliphron atlanticus features a soft, gelatinous body plan with low-density tissues that confer neutral buoyancy in deep water. Its wide arms are connected by an extensive, delicate web of translucent tissue that can expand into a massive umbrella. The suckers are arranged in two rows along each arm. Its skin is pale pinkish-red to translucent white, interspersed with delicate chromatophores. In females, total lengths can approach four meters, while dwarf males reach only about thirty centimeters.
Behavior and Ecology
Haliphron atlanticus is a pelagic cruiser that drifts quietly through the water column between several hundred and thousands of meters deep. Rather than expending heavy energy hunting fast-swimming fish, it specializes in capturing giant coronate jellyfish (such as Periphylla periphylla) and salps. Deep-sea submersibles have observed individuals clutching large medusae in their arms, consuming the nutrient-dense digestive gonads and utilizing the stinging tentacles as a defensive shield against predators. Major predators of adult seven-arm octopuses include sperm whales, swordfish, and blue sharks.
Conservation and Human Impact
Assessed as Least Concern by the IUCN, Haliphron atlanticus is protected by its immense circumglobal oceanic range and deep-water habitat far removed from coastal industrial activity. While occasionally captured as bycatch in deep-water commercial trawl nets and longlines, there is no targeted commercial fishery. Long-term environmental challenges center around ocean deoxygenation and changing mid-water prey dynamics linked to global ocean warming.
Interesting Factoid
Despite being nicknamed the ‘seven-arm’ octopus, it does indeed possess eight arms; the male simply keeps its eighth arm curled up inside a discreet pouch under its right eye until ready to mate!
Last updated: October 2026