Out of all 8,000 living species of  marine invertebrates in the oceans,  

by far the largest, most deadly and  most intelligent are the cephalopods.  

A class of highly developed mollusks that includes  octopuses, squid, cuttlefish and the mysterious  

nautilus. Their graceful fluid movements, vibrant  colour changes and complex brains make them one  

of the most peculiar and important groups of  organisms in our oceans. The largest - the  

giant squid - measures longer than a school bus,  while the smallest could sit atop your finger.  

Let’s take a closer look at these diverse  and often alien-looking invertebrates.

First, let’s establish what  makes an animal a cephalopod.  

It is the most morphologically and behaviourally  complex class of Mollusks. Cephalopoda means  

"head foot”, on account of their completely  merged head and foot. The basic cephalopod  

body plan includes a pair of large, complex  eyes, a mantle, a funnel used for propulsion,  

and a ring of arms at the centre of  which is a mouth, armed with a sharp,  

parrot-like beak. It is these arms that  earn cephalopods such a fearsome reputation.

The eight arms of octopuses are lined with  hundreds of suckers, which can be moved  

independently. These allow the animal to touch,  smell and manipulate objects. Octopuses have been  

observed using them to open clamshells, and pick  up objects to use as protection. The only example  

of invertebrates using tools. There are no  bones in the arms or body of an octopus.  

So they are very flexible, allowing  them to fit into very small crevices.  

Similar to the octopus, many squid possess 8 arms  but with two extra appendages, known as tentacles.  

These tend to be far longer than the rest of the  arms, allowing squid to capture far-away prey  

using their unique ability to  extend and retract the tentacles.

In addition to possessing useful appendages,  cephalopods exhibit a number of other adaptations  

to help them survive. In the deep sea, many  are able to light up in vibrant colours  

via the fascinating process of bioluminescence.  Their eerie glow, startling flashes,  

or syncopated blinking open up a world of  advantages, allowing them to communicate,  

find partners, distract predators and lure prey.  For example, firefly squid contain a chemical  

called luciferin, which lights up when it comes  into contact with oxygen. Thus, they can carefully  

monitor their chemistry to control when they  light up, as well as the intensity of the light.  

In doing so, they exhibit counter-illumination,  producing light at a brightness that matches  

the ambient light around them. This  makes them invisible to predators.

A similar adaptation is found in cephalopods with  the ability to change the colour of their skin  

in the blink of an eye. This is made  possible by the thousands of pigment-filled  

cells that cover the entire body, called  chromatophores. Within each chromatophore,  

muscles and nerves control elastic pigment-filled  sacs. When muscles contract, the sacks expand,  

revealing vibrant pigments. When the muscles  relax, the sacks shrink once more, concealing  

these pigments. Like counter-illumination,  this is used as an advanced form of camouflage.  

Octopuses can even change the texture of their  skin by controlling the size of projections on  

their skin (called papillae), allowing them to  resemble algae or coral and hide in plain sight.

Far stranger is the behaviour known as mimicry,  when certain cephalopods like the mimic octopus  

imitate other animals. This species  has been observed disguising itself  

as up to 15 different animals, from  crinoids to lion fish and sea snakes.  

When it swims along the sea floor, it adopts the  flounder position, imitating the pale flatfish  

in both appearance and behaviour. This is a  clever trick. By mimicking its own predators,  

the octopus will be left alone by those animals,  and its chances of survival are increased.  

Sometimes, cephalopods use their colour-changing  powers not to blend in but to stand out.  

Cuttlefish engage in a courtship  display that rivals birds of paradise.

Cephalopods are an incredibly diverse group of  organisms, and there are a number of species  

with unique morphologies that stand out among  the rest. Most peculiar is the nautilus, the  

only living cephalopod whose bony body structure  is externalised as a shell. A feature that endures  

from an ancient lineage of shelled cephalopods  that ruled the seas 200 million years ago.

In deeper waters, a phenomenon known as deep  sea gigantism is prevalent in cephalopods.  

A feature that has inspired myths of sea  monsters like the mighty tentacled kraken.  

This is the Magnapinna squid. Seen here in  footage recorded at a deep oil drilling site,  

it is an elusive and otherworldly creature.  

The specimens in the few recordings captured look  very distinct from all previously known squids.  

Their arms and tentacles are of the  same length, like in extinct belemnites.  

Their appendages are held perpendicular to the  body, creating peculiar elbow-like features.  

And the length of their tentacles is unparalleled,  reaching up to 20 times the length of their  

mantle. Based on video evidence, Magnapinna  squid can grow to 8 metres or 26 feet long.

Another example of gigantism is the  2 metre (6 ft) long Humboldt squid,  

hunting in packs at 700 metres  (2,300 ft) below the surface.  

The giant squid, at 18 metres (59 feet) long, is  one of the largest invertebrates on the planet,  

exceeded only by the colossal squid . Growing  so large is an advantage in the nutrient-poor  

deep sea, for it allows these animals to be  more efficient and lose less energy to their  

surroundings. An in-depth look at abyssal  gigantism can be found on our channel.

Cephalopods have a key role to play in the  

marine ecosystem. Their intelligence  allows them to be cunning predators,  

consuming anything from crustaceans to fish and  other cephalopods. They are well-adapted to such  

a lifestyle, having evolved special tools like  a sharp beak used to chop prey into tiny pieces.  

Within the beak, a tongue-like radula lined  with tiny teeth can break open shellfish.  

But in addition to their role as predators in  the food web, the soft bodies of cephalopods  

make them ideal prey for an abundance of  creatures, including sharks, bony fish,  

and the deep-diving sperm whale which hunts  the elusive giant squid in the deep sea.

Overall, cephalopods are truly remarkable  organisms. Excellent vision and stealth  

make them master predators. With their  flexible tentacles, advanced eyesight,  

and camouflage abilities more advanced than any  other creature’s, their abundance and diversity  

owes itself to their intelligence that  is unrivalled in the invertebrate world.

To find out more about the deep  sea, and to visualise just how deep  

some of these creatures live, visit  the ‘exploring the zones’ page over  

on our Deep Sea Hub, where you can scroll  down through a scale model of our oceans.