Octopus Facts: Three Hearts, Blue Blood, and More

What exactly is an octopus?

An octopus is a mollusk, a relative of snails and clams, that gave up its shell. A common octopus has eight arms lined with suckers and no internal shell, which lets it squeeze into tight gaps ^[1]. The giant Pacific octopus carries about 2,240 suckers on its arms if it is a female, and roughly 100 fewer if it is a male ^[3].

Many people call these animals "octopi", and that plural is common in everyday speech. The plural "octopuses" is the standard English form, and the rare "octopodes" is the one that follows the Greek root. We use "octopuses" throughout WildFacts. If you enjoy this kind of detail, our fun facts collection has more short surprises from the animal world, and you can browse every profile on the animals page.

Octopuses belong to a group of animals called cephalopods. Different species vary a lot, so the numbers below name the species they describe. Nobody should expect one number to fit every octopus. The two we use most are the common octopus (Octopus vulgaris) and the giant Pacific octopus (Enteroctopus dofleini).

Why do octopuses have three hearts and blue blood?

An octopus does not circulate blood the way a person does. The octopus oxygen supply system is powered by three hearts: two branchial hearts that push blood through the gills, and a systemic heart that sends blood around the body. Researchers describe this as a closed circulatory system, which means the blood stays inside vessels ^[4].

The blue color comes from the oxygen carrier. Humans use hemoglobin, which contains iron and looks red. Octopuses use hemocyanin, a protein in which each oxygen-binding unit holds one oxygen molecule on a pair of copper atoms ^[4]. Octopus blood turns blue when hemocyanin is carrying oxygen, because of the copper ions in the molecule, and it becomes nearly clear when the oxygen is released ^[5].

A quick comparison helps keep the two systems straight:

Feature	Octopus	Human
Hearts	3	1
Oxygen-carrying protein	Hemocyanin	Hemoglobin
Metal at the center	Copper	Iron
Color of oxygen-rich blood	Blue	Red

How big do octopuses get?

It depends heavily on the species. The common octopus is a modest animal that reaches 0.3 to 0.9 m (1 to 3 ft) in length including its arms ^[1]. The giant Pacific octopus is the largest in the world. Its arm span is at least 2 m (7 ft) and often more than 4 m (13 ft), and the largest on record measured nearly 6 m (20 ft) across ^[3].

Weight tells the same story. Monterey Bay Aquarium reports that full-grown giant Pacific octopuses can weigh more than about 23 kg (50 lb), and that the heaviest recorded individual weighed about 91 kg (200 lb) ^[3]. Animal Diversity Web gives a much higher average of 60 kg (132 lb) and a maximum of 272 kg (600 lb) for the same species ^[2]. The two sources disagree, so treat giant Pacific octopus weights as a rough range, not a fixed number.

Measurements also differ by method. Animal Diversity Web lists an average dorsal mantle length of 50 to 60 cm (20 to 24 in) for the giant Pacific octopus, which is the body only, without the arms ^[2]. The arm span figures above include the arms, so a mantle length should never be compared directly with an arm span.

To put that in perspective, a giant Pacific octopus with its arms stretched out can be wider than a tall adult is high. Want to compare any animal with your own height? Try our size comparison tool.

How long do octopuses live?

Not long. The common octopus has a life span of 12 to 24 months ^[1]. The giant Pacific octopus lives about three to five years, which is long for an octopus, since octopuses in general usually live no more than a year or so ^[3]. Animal Diversity Web puts the same species at 4.5 to 5 years in the wild and in captivity ^[2]. Our sources do not split the common octopus figure into wild and aquarium lifespans, so we show it as a single reported range.

Octopus life starts small. A giant Pacific octopus hatchling is about a quarter-inch (6 mm) long and weighs 22 milligrams, roughly the size of a grain of rice, and it starts with about 14 suckers on each arm ^[3]. Common octopus hatchlings spend several weeks as part of the plankton, drifting in open water, before they settle to the bottom at about 0.2 grams ^[1].

The end of life is tied to reproduction. A female giant Pacific octopus lays 18,000 to 74,000 eggs, guards them for up to six months without eating, and usually dies shortly after they hatch ^[3]. A female common octopus lays far more, 100,000 to 500,000 eggs, and also stops feeding while she tends them. She dies shortly after the last young hatch, having lost about one third of her pre-spawning weight ^[1]. Before that, giant Pacific octopuses mate in deeper water, beyond 50 m (164 ft), and the female returns to shallower water to lay her eggs ^[3].

How smart are octopuses, and how do they hide?

Octopuses can learn. Animal Diversity Web notes that they can tell objects apart using visual, touch and chemical cues ^[1]. Giant Pacific octopuses in aquariums have learned to open jars, solve puzzles and interact with their caretakers ^[3]. Most of their lives are solitary, so these skills are not about group living.

Their camouflage is just as remarkable. An octopus can match nearly any surface by changing color, skin texture and posture ^[1]. The color change comes from millions of elastic color cells called chromatophores that are controlled by muscles and nerves. Experiments suggest the giant Pacific octopus is color-blind, which makes its matching even more surprising ^[3]. Color is also a signal. Monterey Bay Aquarium suggests that changing color may help an octopus communicate with a mate or warn off a rival, though the full meaning is still being studied ^[3]. If camouflage fails, an adult may release a cloud of ink that acts as a temporary visual barrier while it escapes (same source).

What do octopuses eat?

Octopuses are active hunters. The common octopus feeds mostly on snails and clams ^[1]. The giant Pacific octopus eats mainly crabs, clams and other mollusks, with other animals now and then ^[3]. They can pull shells apart, bite them open or drill through them. Baby octopuses start life drifting in the plankton and eat different food than adults. For the full menu, how they open shells and what changes with age, read what octopuses eat.

Where do octopuses live?

Octopuses live in oceans, and each species has its own range. The common octopus has a worldwide distribution in tropical, subtropical and temperate waters, from the surface down to a depth of 100 to 150 m (330 to 490 ft) ^[1]. The giant Pacific octopus prefers cold water of 16 °C (60 °F) or colder across the North Pacific, from Korea and Japan to Alaska, Canada, the United States and Mexico. It is found in shallow water and as deep as 1,500 m (4,900 ft) or more ^[3]. Common octopuses are absent from polar and subpolar regions ^[1]. Adult giant Pacific octopuses are commonly found in tide pools and as deep as 110 m (360 ft), though the species ranges much deeper ^[2]. Explore more animals of the open water in our ocean habitat guide, or see what an octopus actually puts on its plate in what octopuses eat.