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. Observed 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.