Squid, cuttlefish, octopus and nautiluses are all members of a group collectively known as cephalopods. This name refers to the strange shape of these animals, where all their limbs come directly off their head. The name cephalopod comes from the Greek words kephalos for head and podos for foot. They are “head-footed”. The equivalent form in a human would be having our arms and legs attached around the edges of our mouth.
We have limited contact with them because they are so good at hiding and escaping. These animals are some of the most vulnerable to predators, with most lacking a shell in which to hide, lacking armour, they have no spines and very few are poisonous. Their bodies are all muscle, making them an ideal meal. As a consequence, they have evolved many ways to avoid detection, remain hidden or make quick escapes. Most species only come out under the cover of darkness.
These animals are some of the most complex, diverse and fascinating creatures on earth. They are voracious top-level predators. Many have large brains and complex behaviours. Their skills include rapid colour changes, shape changing, squirting ink decoys or smoke screens, dropping and regrowing limbs, swimming through sand and flying through the water.
Cephalopods are only found in marine and estuarine waters, there are no freshwater species. They occur in all oceans of the world from the hot tropical waters of the equator to the frigid waters of both the Arctic and Antarctica, where they can live in temperatures as low as -2 degrees Celsius. They are found at all depths, from intertidal reed flats to at least 7km deep where they live in permanent darkness. Many species are bottom dwelling (benthic), while others spend their entire lives in surface waters or midwater (pelagic).
Cephalopods come in all shapes and sizes but most share a number of common features; click on the headings below to find out more.
All cephalopods are carnivores, most taking live prey. None feed on plant matter.
Fast growing and short-lived
Most cephalopod species can be considered fast growing and short-lived, many for less than 1 or 2 years. The exceptions seem to be the deep water and polar species as well as the nautiluses, which may live longer.
All cephalopods are “head-footed”. The limbs of these animals come off the head as a ring around the mouth. In one way the limbs could be considered as “superlips”. In most groups the limbs bear rows of gripping suckers.
All the body organs are stored in a bag or tubelike body, known as a mantle. When you eat squid (calamari) rings, these are cross sections of the tubular mantle.
Beak and radula
All cephalopods possess a hardened beak, similar to the beak of a parrot. Most species also have a special toothed tongue (known as a radula), a long thin strip bearing rows of sharp teeth which is typically used to tear up food. The beak and radula are used together to chew food into small pieces.
All cephalopods have a donut shaped brain developed around the food tube (oesophagus).
All cephalopods have two eyes. In most species these are well developed consisting of retina, lens and iris, showing many similarities to the eyes of vertebrate animals including humans.
The blood of cephalopods is transparent blue. This colour is caused by a respiratory pigment based on a copper atom compared with the iron respiratory pigment that makes human blood red.
Most species have three hearts, one over each gill (branchial hearts) and one central (systemic) heart.
Cephalopods have two to four gills suspended within a cavity under the body. Each consists of numerous leaflike plates. Rhythmic pumping of the mantle draws seawater into the gill cavity via its wide opening. The expired water is channelled through the funnel.
All cephalopods have a muscular tubelike funnel which squirts water out of the gill cavity when the body is contracted. The funnel can be aimed in different directions allowing cephalopods to use these strong jets of water for fast propulsion, to squirt ink or as a threat to potential predators.
Most cephalopods can produce ink from a special gland embedded in their liver. This highly concentrated pigment is released with jets of water through the funnel to form decoys or smokescreens.
Most cephalopod species have some form of shell. It is external in nautiluses, internal in cuttlefish and many squid, and reduced to two tiny rods in octopus. Some cephalopods have lost their shell completely.
Many cephalopods have fins for locomotion, including cuttlefish, squid and deep sea finned octopus.
Cephalopods are probably most famous for their amazing skin, capable of rapidly changing colour and shape in many species. These changes are used for camouflage, mimicry, alarm displays and different forms of communication.
For most cephalopods reproduction and life cycle follow the same basic system. The sexes are separate and fixed from birth. Males produce special packages in which they store sperm. These are known as spermatophores and they can be considered waterproof containers for passing packets of sperm to the female. Males usually have one or two modified arms which are used to hold and pass the spermatophores to the female. She takes the sperm and uses it immediately or stores it until ready to fertilise and lay her eggs. Most cephalopods are “big bang” spawners, meaning they have a single breeding season then die of old age.