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Squid, Octopus, Cuttlefish


Octopus / Octopus cyanea / Family Octopodidae
Cuttlefish / Sepia sp. / Family Sepiidae
Squid, octopus and cuttlefish [and nautilus] belong to the class of molluscs known as Cephalopods, from a Greek word meaning 'head-footed.'

The group is characterised by:

  • A shell that is often modified or absent
  • A well-developed head with large eyes
  • Feet that are modified into funnels
  • Tentacles
  • A parrot-like beak for feeding
  • Ink used for protection in all groups except the nautilus
  • All except the nautilus capable of colour changes
The cephalopods are the most highly advanced group of molluscs, and the most intelligent group among the invertebrates. Octopuses, for example, can be easily trained to open jars and to perform other simple tricks.


With their stream-lined, torpedo-shaped body, excellent eyesight and active swimming lifestyle, squid seem to be more similar to fish than to other molluscs. Unlike cuttlefish which are mostly solitary, squid often move about in shoals. They lack the internal chalky bone of the cuttlefish, relying instead on a thin membrane called the pen, for support. When disturbed, squid squirt a cloud of black ink into the water to help mask their escape.


Octopi use jet propulsion and may cloud the water with ink to escape predators. While most octopi do not present a threat, the blue-ringed octopus is lethal to humans. Several species occur in Australian waters, most of which do not exceed 20 cm in length. These octopi are normally drab in appearance, with the blue rings showing only when the animal feels threatened or senses the approach of a larger animal. Like all octopi, the mouth of the blue-ring has a parrot-like beak. When the beak pierces the skin, venom is injected. The venom of the blue-ringed octopus is the same venom found in the puffer fish, known as tetrodotoxin or TTX. It causes paralysis. There is as yet no antidote.


Cuttlefish are well known for spectacular colour and skin texture changes which can indicate their mood. Like other members of their family, cuttlefish have a relatively short lifespan of approximately 18 months. This is due to their unusual circulatory system which includes three hearts and copper, rather than iron, -based green blood. The system requires so much energy that the cuttlefish literally wears out after 18 months of life. Cuttlefish have eight arms and two tentacles. When feeding on crustaceans and fish, two tentacles quickly snatch the prey which is drawn towards the beak-like mouth beneath the arms. The cuttlebone, well-known to beach goers, is a porous internal structure used by the cuttlefish to control its buoyancy. When dried, the so-called 'cuttlebone' is sometimes given to caged birds as a source of calcium and essential salts and minerals.