Octopus Anatomy: Three Hearts, Blue Blood, Eight Arms and How an Octopus Compares to a Squid

Octopus anatomy is among the strangest of any animal. The octopus (order Octopoda) circulates blue, copper-based blood through three separate hearts, distributes two-thirds of its neurons into eight arms that move under separate neural control, sees in greyscale yet matches the colour of any background within a second, and squeezes its entire body through any opening that fits its single hard part: a parrot-like beak. The largest species, the giant Pacific octopus (Enteroctopus dofleini), reaches an arm span of 6 meters (20 ft) and a body mass of 50 kg or more.

The order Octopoda contains around 300 living species, ranging from the 2.5 cm star-sucker pygmy octopus to the giant Pacific. Octopuses sit within the class Cephalopoda alongside squid, cuttlefish, and the chambered nautilus, all descended from a shelled common ancestor that lived over 500 million years ago. Of the cephalopods, octopuses have shed the ancestral shell, traded body symmetry for radial flexibility, and pushed the cephalopod nervous system further than any of their relatives. Squid keep an internal pen, a residual shell remnant, and a torpedo-shaped body for fast pelagic life. Cuttlefish keep an even larger calcified internal shell. Octopuses have abandoned the shell altogether, which is what lets them squeeze through any hole that fits their beak.

Octopus Body Size and Build

Octopus body size varies across four orders of magnitude. The star-sucker pygmy octopus (Octopus wolfi) reaches a total length of 2.5 cm (1 in) and weighs less than a gram. The common octopus (Octopus vulgaris) carries a mantle 25 cm long and total arms around 1 meter (3.3 ft). The giant Pacific octopus tops the order at a verified 71 kg (156 lbs) and an arm span of 6 meters, with anecdotal reports of 9-meter individuals in deep water off Alaska and British Columbia. The blue-ringed octopuses (Hapalochlaena) sit at the small end at 12 to 20 cm despite carrying tetrodotoxin venom potent enough to kill an adult human within hours.

The body plan is built around a soft, sac-shaped mantle that sits behind a fused head, with eight arms attached to the head in a ring around the central mouth. There is no internal skeleton and no shell. The only hard structure in the entire body is the chitinous beak, a parrot-shaped pair of mandibles inside the mouth. This means an adult giant Pacific octopus can compress itself through a hole the size of a small orange, the diameter of its own beak. Lifespan is short across the order: most species live 1 to 2 years, the common octopus around 18 months, the giant Pacific 3 to 5 years. Females die within days of their eggs hatching.

Three Hearts and Blue Copper-Based Blood

An octopus has three separate hearts. A single systemic heart sits at the centre of the mantle and pumps oxygenated blood around the body. Two smaller branchial (gill) hearts, one on each side, push deoxygenated blood through the gill capillaries, where it picks up oxygen before returning to the systemic heart. The systemic heart stops beating when an octopus swims, which is one reason most species prefer to crawl across the seabed rather than jet through open water for more than short bursts.

Octopus blood is blue. The oxygen-carrying molecule is haemocyanin, a copper-containing protein dissolved in the plasma rather than packed into red blood cells. Haemocyanin holds oxygen with weaker affinity than haemoglobin at warm temperatures but works far better at the cold, low-oxygen conditions of the deep sea, which is why most cephalopods carry it. The blood turns deep blue in oxygenated form and goes nearly clear when the oxygen is released to the tissues. Two extra branchial hearts compensate for the lower oxygen-carrying capacity of haemocyanin compared to mammalian blood, so an octopus moves enough oxygen to support its active brain and arms despite the chemistry trade-off.

Eight Arms and the Distributed Nervous System

An octopus has eight arms, each lined with two rows of suckers. A common octopus carries around 240 suckers per arm for a total of roughly 1,920; a giant Pacific octopus reaches 280 per arm. Each sucker is a muscular cup that grips by adhesion and suction, and each one carries chemoreceptors that taste the surface it touches. An octopus tastes whatever its arms are holding. Recent work by the Bellono lab at Harvard has identified specific receptor proteins on octopus suckers that detect waterborne molecules from prey, allowing the animal to identify a crab or mussel through the sucker before bringing it to the mouth.

The nervous system is the most distributed of any animal. An octopus carries around 500 million neurons, comparable to a dog. Two-thirds of those neurons sit not in the central brain (which wraps around the oesophagus) but in the eight arms themselves, with around 40 million neurons per arm bundled into ganglia that control local movement. An arm can reach into a crevice, identify prey by taste and touch, and pass it back along the rows of suckers toward the mouth without instruction from the central brain. Severed octopus arms continue to crawl, grip, and respond to stimuli for some minutes after separation. The central brain coordinates large-scale tasks such as deciding where to go and what colour to display, while the arm ganglia handle the moment-to-moment movement of the appendage itself.

Skin: Chromatophores, Iridophores and Camouflage

Octopus skin is the most sophisticated colour-changing surface in the animal kingdom. Three layers work together. The outermost layer carries chromatophores, sacs of pigment (red, brown, yellow, or black depending on species) attached to radial muscle fibres. When the muscles contract, the sac expands across the skin and the pigment becomes visible. When the muscles relax, the sac shrinks to a dot and the pigment disappears. A single octopus can have several million chromatophores under independent muscular control, switching from grey to mottled red within 200 milliseconds.

Beneath the chromatophores sit iridophores, stacks of platelets that reflect specific wavelengths of light through thin-film interference, producing greens, blues, and metallic sheens that the chromatophore pigments cannot generate. A third layer of leucophores reflects all wavelengths and provides white. The combination of pigment chromatophores and structural iridophores gives an octopus the full visible spectrum despite carrying only two or three pigment colours in its sacs. Skin texture is also under muscular control: papillae across the mantle and arms can be raised into spikes, ridges, or small bumps within seconds, allowing an octopus to mimic the surface of seaweed, coral, or sand on top of matching its colour.

Beak, Mantle, Funnel and Internal Anatomy

The mouth sits at the centre of the ring of arms and contains the chitinous beak, the only hard structure in the body. The beak shears flesh, cracks crustacean shells, and chips into bivalve hinges. Inside the mouth a tongue-like organ called the radula carries rows of small teeth that scrape food off the beak and into the oesophagus. Octopus saliva contains paralysing toxins, which is why every octopus species (not only the blue-ringed) carries some level of venom; the venom of most species is too mild to harm humans.

The mantle is the muscular body sac that encloses most of the internal organs: the three hearts, the gills, the digestive tract, the gonads, and the ink sac. Two slits at the front of the mantle let water flow in across the gills, and a single muscular tube called the funnel (or siphon) directs the spent water out. The funnel is steerable and acts as the octopus’s main mode of fast travel: contracting the mantle drives water out through the funnel and jets the body in the opposite direction at up to 40 km/h (25 mph) over short distances. The funnel also expels eggs, faeces, and a cloud of ink during a startle response.

Senses: Eyes, Photoreceptive Skin, and Touch

Octopus eyes are large, camera-style eyes built on a similar plan to the vertebrate eye, with a cornea, lens, and retina, but with the photoreceptors facing forward toward the light rather than backward as in mammals. The lens does not change shape; the entire lens slides forward and back to focus, like a camera body. Octopus retinas carry only one type of photopigment, which means an octopus is colour-blind by the usual measure. Two hypotheses explain how octopuses match background colours despite this: chromatic aberration through their unusual horizontal-slit pupils may split light into wavelengths that the single photopigment can sample at different focal distances, and opsins (light-sensitive proteins) embedded across the skin sense ambient colour without going through the eyes.

Touch and chemical senses are exceptional. Each sucker carries hundreds of mechanoreceptors and chemoreceptors. An octopus exploring a new environment touches almost everything, building a detailed picture of textures and tastes through the arms. Hearing is rudimentary at best; the cephalopod statocyst, a balance organ in the head, responds to low-frequency vibration but the animal does not hear in any conventional sense. Smell is handled by olfactory pits on the head behind the eyes. Octopus eyes also include a horizontal slit pupil that maintains the same orientation regardless of how the body tilts, an active mechanism by which a small ocular muscle compensates for body roll and keeps the visual world level.

Defence: Ink, Jet, Mimicry and Venom

An octopus has four overlapping defensive systems. The first is camouflage: chromatophore colour change combined with skin papillae texture lets the octopus blend into a surface within a second. The second is mimicry. Several species, including the famous mimic octopus (Thaumoctopus mimicus) of the Indo-Pacific, can imitate the body shape and movement of toxic flatfish, sea snakes, lionfish, and jellyfish, switching disguise depending on the predator at hand.

The third defence is the ink cloud. The ink sac sits between the gills and contains melanin, mucus, and tyrosinase enzymes. When an octopus startles, it contracts the mantle, fires water and ink through the funnel, and leaves behind a dark cloud roughly the size and shape of its own body. Predators strike at the cloud while the octopus jets away. The ink also disrupts the predator’s chemoreception, blocking the trail. The fourth defence is venom. All octopus species deliver paralytic saliva from glands at the back of the mouth, but only the four blue-ringed species carry tetrodotoxin powerful enough to kill a human; their bite is painless and the toxin causes paralysis within minutes.

Octopus vs Squid: Anatomy Comparison

Octopuses and squid are both cephalopods, share the three-hearts-and-blue-blood plan, and use chromatophore-based colour change. The body plans diverge because the two groups have specialised for different niches. Most octopuses are benthic ambush hunters of reef and sea floor. Squid are pelagic predators that cruise open water in fast schools. The differences are visible at a glance once you know the markers.

For more on individual species, see the Common Octopus profile, the Giant Pacific Octopus, and the venomous Blue-Ringed Octopus. For squid relatives, see the Giant Squid, Colossal Squid, Bigfin Reef Squid, and Bigfin Squid. The blog covers 8 incredible octopus facts and a colossal squid vs giant squid comparison. Browse the Invertebrates hub for related species, or the Animal Anatomy hub for comparable body-plan guides.

Sources & References

• IUCN Red List, Octopus vulgaris
• Smithsonian Ocean, Octopuses and Squid
• Monterey Bay Aquarium, Giant Pacific Octopus
• Animal Diversity Web, Octopus vulgaris
• Wikipedia, Octopus