What’s the Difference Between Squid and Octopuses?

Squid and octopuses are both brainy, soft-bodied sea dwellers with many arms, shade-shifting skin, and a tendency to spew ink into the paths of their predators. They're remarkably similar—closely related, even—and share much of their range across every ocean in the world. Despite their likenesses, there are a few ways to tell the two leggy invertebrates apart if you happen to run across one thrusting its baglike body through the water.

Learn about their physical, biological, and behavioral differences, their conservation statuses, and more.

Squid and Octopus Classification

Squid and octopuses are both members of the class Cephalopoda, which also includes cuttlefish and nautiluses, but they taxonomically part ways at their orders. Unlike octopuses, squid belong to the superorder Decapodiformes, derived from the Greek word for "10-legged." Under Decapodiformes, there is the order Teuthida, the squid order, comprising the suborders Myopsida (covered-eye squids) and Oegopsida (open-eyed squids). Octopuses, rather, belong to the order Octopoda. Teuthida and Octopoda contain about 300 known species, respectively.

Characteristics of Squid vs Octopuses

It's tough to tell the difference between squid and octopuses just by size and location. Body sizes vary widely, from less than an inch (example: pygmy squid) all the way up to 40-plus feet in length. The giant squid is known to be the largest living cephalopod, reaching recorded lengths of 42 feet, while the largest octopus, the giant Pacific octopus, grows to only about 16 feet.

Both squid and octopuses inhabit all the world's oceans. They prefer warmth, but some species of both can be found in the subzero waters off Antarctica. Here are some surefire ways to tell the sea mollusks apart.

Limbs

Octopuses are called so—with the prefix "octo"—because they have eight arms. Each flexible arm is covered in suckers that help the animal taste, grip, and smell. Squid have eight sucker-covered arms, too, but they also have two additional tentacles, both equipped with muscular "hooks" instead of suckers, that are longer than their eight arms and used for grabbing prey.

Octopus arms are more flexible than squid's, which allows them to "walk" on the seafloor and manipulate objects. They've been called "the most flexible limbs in nature."

Head Shape

One of the most obvious physical distinctions is the difference in head shape. Octopuses have characteristically round heads and mantles whereas squid's are more triangular and flanked by fins. Squid flap these winglike appendages for locomotion; most octopuses—except a few deep-water species including the dumbo octopus—do not have fins, getting around almost exclusively by jet propulsion.

Habitat

Although some species are pelagic, meaning they hang around the surface of the water, most octopuses stick—literally, with their adherent appendages—to the seafloor. They hide away in deep, dark waters, holing up in the crevices of rocks and coral when they aren't hunting crustaceans at the bottom of the ocean. Squid, on the contrary, tend to float about the open ocean where their preferred food—shrimp and small fish—is most abundant.

Sociality

If you're lucky enough to see an octopus in the wild, you'll likely catch it alone, inside or near its den on the ocean floor. Squid, on the other hand, swim continuously, always filling their bellies with small fish in the open water. Both are mostly solitary, but some squid species are extremely social, traveling in schools of millions.

Conservation Status

Of the 278 squid species assessed by the International Union for Conservation of Nature (IUCN), half are data deficient. The rest are of least concern.

Of the 271 octopus species assessed, one is critically endangered, two are endangered, two are vulnerable, and one is near threatened. The remaining 265 are of least concern (52%) or data deficient (48%).

For the endangered (Cirroctopus hochbergi and Opisthoteuthis mero) and critically endangered (Opisthoteuthis chathamensis) octopus species, all inhabiting the coast of New Zealand and none of which have been given common names, IUCN lists the primary threat to all as commercial deep-water fishing.

How Is Climate Change Impacting Cephalopods?

Don't be fooled by the conservation data: Just because a meager few are in danger doesn't mean squid and octopuses have evaded the consequences of climate change. Cephalopods are some of the few species experiencing population growth as a result of rising ocean temperatures. The warm water accelerates their development so that they reach maturity more quickly and breed earlier.

Squid and octopuses are voracious predators, meaning the climate-driven cephalopod population boom could result in dangerously low numbers of the fish they eat, in turn throwing the whole marine food chain off balance.