Do Octopuses Have Bones? How They Squeeze Through Gaps

What hard parts does an octopus have?

Octopuses are mollusks, relatives of snails, slugs and clams, and like all mollusks they are invertebrates, with no backbone. True bone, a living tissue of calcium phosphate and collagen, is found only in vertebrates. An octopus has just a few firm parts:

• The beak: two sharp, curved jaws hidden where the arms meet, made of chitin (the same tough material found in insect shells) toughened with proteins. It bites and tears prey and is the single largest hard structure in the body.
• The radula: a ribbon covered in rows of tiny teeth, which works like a rasping tongue to move food toward the gut.
• Cartilage: a capsule of cartilage, a firm but flexible tissue, surrounds and protects the brain and the balance organs.
• Stylets: many octopuses have a pair of tiny rods buried in the muscle of the mantle, the last remnants of an ancient shell. Some species lack even these.

Everything else, from the mantle to the tip of each arm, is soft tissue: muscle, connective tissue, organs and water.

How does an octopus move without a skeleton?

Instead of bones, octopuses use water and muscle. Their arms are muscular hydrostats, the same design as a human tongue or an elephant’s trunk. Muscle fibers run lengthwise, across and around each arm. Because muscle tissue keeps the same volume when it contracts, squeezing in one direction makes the arm longer, thicker or bend in another. This lets an octopus arm stretch, shorten, twist, bend at any point or turn stiff.

The mantle works in a similar way. When its muscles contract, it pushes water out through the funnel for breathing or jet propulsion. On the seabed, octopuses mostly crawl by gripping with their suckers and pulling themselves along, and a few species can even walk on two arms while the others are tucked up.

Water supports the body. Out of water, an octopus sags and struggles to hold its shape, although some species crawl short distances between tide pools.

How small a gap can an octopus squeeze through?

As a rule of thumb, if the beak fits, the octopus fits. Everything else in the body can flatten, stretch or flow. The common octopus, which can weigh several kilograms, routinely hides in cracks and holes that look far too small for it.

Even the largest species is no exception. A giant Pacific octopus weighing 15 kg (33 lb) or more can pass through an opening just a few centimeters across, and figures of about 2.5–5 cm (1–2 in) are often given for big individuals. An octopus usually sends one arm through first to explore, then pours the rest of its body after it.

This talent explains why octopuses are famous escape artists in aquariums and why they can live in such tight dens: rock crevices, empty shells, bottles and cans. In one well-known case in 2016, an octopus named Inky escaped from a New Zealand aquarium down a drainpipe about 15 cm (6 in) wide.

Where did the octopus’s shell go?

Octopuses descend from shelled animals. The first cephalopods, more than 500 million years ago, lived inside hard external shells divided into gas-filled chambers that helped them float, much like the living chambered nautilus.

Around 400 million years ago, the ancestors of today’s octopuses, squid and cuttlefish began to carry the shell inside the body, where it became a stiffening rod or buoyancy organ covered by muscle. Fossils suggest that octopuses arose during the Jurassic period, more than 150 million years ago, and over time their lineage reduced the inner shell to almost nothing.

Traces remain. Common octopuses and their relatives keep the tiny stylets, while the deep-sea dumbo octopuses have a small internal shell that supports their fins. Female argonauts, open-ocean octopuses also called paper nautiluses, make a paper-thin, shell-like case for their eggs with two special arms, but it is a new invention, not a true mollusk shell. Losing the shell is not unique to octopuses: land slugs have done the same, as explained in slug vs snail.

Do squid, cuttlefish and nautiluses have bones?

None of them has true bone, but each keeps more of the ancestral shell than an octopus does.

The common cuttlefish adjusts the amount of gas and liquid in its cuttlebone to rise or sink, which is why dried cuttlebones often wash up on European beaches.

The pros and cons of being boneless

Giving up the shell brought freedom. Without heavy armor, octopuses became flexible, agile hunters that can probe deep into reef crevices for crabs and clams, hide in tiny dens, and change their shape as well as their color for camouflage.

The cost is vulnerability. A soft octopus is a meal for eels, fish, seals, dolphins and seabirds, so it relies on hiding, camouflage, ink clouds and quick escapes. Many scientists think this pressure helped drive the evolution of the octopus’s large brain and flexible behavior: an animal with no armor has to outthink its enemies instead.