Kingdom: Animalia

Phylum: Mollusca

Class: Cephalopoda

Cephalopods are any member of the molluscan class Cephalopoda (Greek plural κεφαλόποδα (kephalópoda); "head-feet"). 


A common cuttle fish is a a type of cephalopod. image: Hans Hillewaert /wikipedia

These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, and a set of arms or tentacles (muscular hydrostats) modified from the primitive molluscan foot. 

Fishermen sometimes call them inkfish, referring to their common ability to squirt ink. The study of cephalopods is a branch of malacology known as teuthology.

Evolutionary history and classification

Cephalopods became dominant during the Ordovician period (beginning 488.3 million years ago and ending 443.7 million years ago), represented by primitive nautiloids. 

The class now contains two, only distantly related, extant subclasses:

  • Coleoidea, which includes octopuses 🐙, squid 🦑, and cuttlefish; and 
  • Nautiloidea, represented by Nautilus and Allonautilus. 


A swimming octopus vulgaris image: albert kok/wikipedia

The differences between the two are:

In the Coleoidea, the molluscan shell has been internalized or is absent, whereas in the Nautiloidea, the external shell remains.


A Nautilus belauensis has an external shell it grows.  image:wikipedia/Manuae


About 800 living species of cephalopods have been identified. 

Two important extinct taxa are the Ammonoidea (ammonites) and Belemnoidea (belemnites).

Cephalopods include the order octopoda, commonly referred to as octopus.

Light and colouration of cephalopods 

Most cephalopods possess chromatophores - that is, coloured pigments - which they can use in a startling array of fashions. 

As well as providing camouflage with their background, some cephalopods bioluminesce, shining light downwards to disguise their shadows from any predators that may lurk below.

The bioluminescence is produced by bacterial symbionts; the host cephalopod is able to detect the light produced by these organisms. 

 Bioluminescence may also be used to entice prey, and some species use colourful displays to impress mates, startle predators, or even communicate with one another. It is not certain whether bioluminescence is actually of epithelial origin or if it is a bacterial production.

How they change colour

Cephalopods can change their colours and patterns in milliseconds, whether for signalling (both within the species and for warning) or active camouflage by contracting or expanding their chromatophores.  

How much colouration depends on where they live

Colouration is typically stronger in near-shore species than those living in the open ocean, whose functions tend to be restricted to disruptive camouflage.

A cuttlefish mimicking a hermit crab.via National Geographic.