Octopus vs Squid: Key Differences in Body and Behavior

Key differences at a glance

Octopuses and squid are both cephalopods, soft-bodied mollusks with a beak, large eyes, a ring of limbs around the mouth and the ability to change color. Beyond that, they lead very different lives:

• Body plan: octopuses have eight arms and no shell; squid have eight arms, two tentacles, two fins and an internal pen.
• Lifestyle: most octopuses live alone on the seabed; most squid swim in open water, often in groups.
• Hunting: octopuses hunt mainly by touch; squid hunt mainly by sight.
• Parenting: octopus mothers guard their eggs; most squid leave theirs.
• Relationships: squid are closest to cuttlefish; octopuses are closest to the vampire squid.

The two groups split hundreds of millions of years ago. Squid belong to the ten-limbed branch of cephalopods, while octopuses belong to the eight-limbed branch, which lost one pair of limbs.

Arms and tentacles

Both animals have eight arms, but squid add two specialized tentacles. These are longer and thinner than the arms, with a bare stalk and a widened club at the tip that carries the suckers. A squid shoots them out to snatch prey, then passes it to the arms. In the giant squid, the feeding tentacles can be about twice as long as the rest of the animal.

The suckers differ too. Octopus suckers sit directly on the arm along its full length and grip by suction alone. Squid suckers usually sit on short stalks and are ringed with sharp, toothed edges of chitin, and in some species, such as the colossal squid, many suckers are replaced by hooks. For more on why octopuses have no tentacles at all, see does an octopus have arms or tentacles?

Shell, pen and body shape

A squid is built for swimming. Its mantle is a long, muscular tube that tapers to a point, with a fin on each side near the rear for steering and slow cruising. Running along the inside of the mantle is the pen, or gladius, a thin, feather-shaped strip of chitin-like material that is all that remains of an ancestral shell. It stiffens the body and anchors muscles.

An octopus has given up even that. Its mantle is a soft, baglike sac with no fins in most species, and its only remnants of a shell are two tiny rods called stylets, or nothing at all. Without a rigid pen, the common octopus can squeeze into gaps barely wider than its beak, something a squid cannot do.

Where they live and how they move

Most octopuses are bottom dwellers. They crawl over rocks, sand and reefs using their suckers, hide in dens, and use jet propulsion mainly for short escapes. A few species, such as argonauts and blanket octopuses, live in open water.

Most squid live in the water column, from coastal shallows to the deep ocean. They swim almost constantly using jet propulsion, forcing water out of a funnel, helped by their fins. Some are among the fastest invertebrates in the sea, and a few oceanic species can launch themselves out of the water and glide short distances. Many coastal squid travel in groups or large schools, while octopuses are usually solitary.

How octopuses and squid hunt

Octopuses hunt largely by touch and taste. They probe crevices with their arms, whose suckers can taste what they touch, pounce on crabs and shrimp, trap prey under the web between their arms, and pull open clams or drill through shells. They usually carry their catch back to the den.

Squid are visual hunters. Their large eyes spot fish, shrimp and other squid in open water, and they close in at speed before firing out their tentacles. Some species, such as the Humboldt squid of the eastern Pacific, have been seen hunting in coordinated groups. Both octopuses and squid kill or subdue prey with their beak and, in many species, toxic saliva.

Eggs, lifespan and size records

Most octopuses and squid breed once and die soon after, but their parenting differs. A female octopus lays her eggs in a den and guards them, without eating, until they hatch. Most squid lay their eggs in gelatinous capsules attached to the seabed or in drifting masses and then leave, though a few deep-sea squid carry their eggs with them.

Lifespans are short in both groups. Most octopuses live one to two years, and the giant Pacific octopus three to five. Most squid live about a year, as shown by daily growth rings in the small balance stones (statoliths) inside their heads.

The largest squid dwarf the largest octopus. The giant Pacific octopus has been documented at 71 kg (157 lb) with an arm span of more than 4 m (13 ft). The colossal squid is the heaviest invertebrate known, with one caught in the Antarctic in 2007 weighing about 495 kg (1,091 lb), while the giant squid is the longest, at about 12–13 m (39–43 ft) including its tentacles. Those two giants are compared in giant squid vs colossal squid.

Where do cuttlefish fit?

Cuttlefish are often confused with both, but they are closer relatives of squid. Like squid, they have eight arms and two feeding tentacles, which they keep tucked into pockets beneath the eyes. Instead of a pen, they have a cuttlebone, a porous internal shell of calcium carbonate that they fill with gas or liquid to rise and sink.

Cuttlefish have a broad, flattened body with a narrow fin running along each side like a skirt, W-shaped pupils, and some of the most spectacular camouflage in the sea. Like octopuses, they spend much of their time near the seabed. The common cuttlefish of European waters is a good example.

What octopuses and squid have in common

For all their differences, octopuses and squid share a remarkable set of features. Both have three hearts and blue, copper-based blood. Both have a beak and a radula, squirt ink, change color with thousands of chromatophores in their skin, and swim by jet propulsion. Both have large brains and camera-like eyes much like ours, which evolved separately from the vertebrate eye.

Both are also highly intelligent for invertebrates. Octopuses are the better-studied problem solvers, as described in how smart are octopuses?, while squid such as the Caribbean reef squid communicate with a rich repertoire of color patterns during courtship and social encounters.