Cephalopoda, squid, octopus, cuttle fish and nautilus

On this page, Cephalopoda overview - cephalopod body - Coleoidea (squid cuttlefish and octopus) - eyes - chromatophores - mating

Class Cephalopdoda overview

These are considered to be the most sophisticated molluscs, and possibly the most intelligent invertebrates. There are about 660 species and a large size range, the Giant squid (Architeuthis sp.), can be over 20 m long.

The cephalopod fossil record goes back to the Cambrian, and were similar to the nautilus (see below) extant today. They have a wide range of behaviours, a relatively large and complicated brain, and a great capacity for learning. All are predators.

Cephalopod body

The foot has developed into a number of prehensile arms with suckers around the mouth, with one or two modified for sperm transfer (see mating below); and a muscular funnel in the mantle cavity. This funnel is used in movement when water is forcibly expelled through it, a form of jet propulsion.

They have a radula and a pair of beak-like jaws. They also have an ink gland which releases a cloud of ink through the anus when the animal is alarmed. The ink cloud can act as a decoy to a predator allowing the cephalopod to escape.

There are two Sub-classes, the Nautiloidea with six species, and Coleoidea which includes the squids, octopuses and cuttlefishes. All are marine.

Coleoidea (squid, cuttlefish and octopus)

The Coleoidea have no external shell. Cuttlefish have an internal calcareous shell, squids have a thin cartilaginous pen, and in octopods the shell is absent, see Eledone sp. bottom left.

Octopuses tend to be solitary, but squid are often found in shoals making them a commercially viable species for fishermen to catch. Squid and cuttlefish have 10 arms - 8 short and 2 long. Octopods have 8 arms; all of the same length.

Coleoidea (squid, cuttlefish and octopus) eyes

Cepalopod eye cross section

The eyes are large, sensitive and share many features with vertebrate eyes (see above), e.g. iris, cornea, lens focused by muscles and retina. The pupil is slit-shaped and the slit is aligned so that it is kept horizontal.

The vertebrate eye arises from the development of the brain, whereas the cephalopod eye arises from the development of the skin. The eyeball of the giant squid is nearly 50 cm across!


The cephalopod chromatophore cell is a small sac filled with pigment and surrounded by muscle cells. These muscle cells are capable of stretching the pigment cell out so that it displays the colour. When the muscles cells are relaxed the pigment is more-or-less invisible. This type of cell allows for rapid colour changes.

Mating in the cephalopoda

Cephalopod males fertilize the female by inserting the hectocotylus (located at the end of one of their arms) into the female. In some cases the hectocotylus breaks off and stays in the female.

Before the mating habits were understood females found with a hectocotylus embedded in them were thought to have been parasitised.

Many species breed just once then die.

Squid have their own page


The blue-ringed octopus (Hapalochlaena lunata) is just 10 cm long. It is a very pretty animal and swims in an elegant manner among the coral reefs in the Pacific and Indian oceans. However it has a venomous bite potent enough to kill an adult human in just 15 minutes.

The giant Pacific octopus can be 5 m long and weigh up to 50 kg. The female lays her thousands of eggs in a sealed up burrow or crevice and looks after them washing them with a stream of water and grooming them to keep them free of parasites. The eggs can take eight months to hatch depending on the water temperature, and during all this time she does not feed. Soon after the eggs have hatched she dies. Although her size is large her life span is usually less than four years.

Eledone sp. octopus

Above is Eledone sp., note that it has a single row of suckers, some octopuses have a double row.

Japetella diaphna

Japetella diaphna, octopus

Japetella diaphna, above, is a deep-water octopus found from 700 - 3500 metres deep in the Northern Pacific.

It uses bioluminescence to attract mates, and a fully grown adult is just 10 centimetres long.

Graneledone boreopa, a common deep-water octopus found in the north-east Pacific can brood her eggs for four and a half years! When brooding she uses her siphon to blow water over the eggs to keep them oxygenated, and she protects them from predation. During brooding the egg size increases, but the female does not feed, so she loses weight. She dies soon after the eggs hatch.

Octopus by A C Hilton

Strange beauty, eight-limbed and eight-handed,
Whence camest to dazzle our eyes?
With thy bosom bespangled and banded
With the hues of the seas and the skies;
Is thy home European or Asian,
O mystical monster marine?
Part molluscous and partly crustacean,
Betwixt and between.

Wast thou born to the sound of sea trumpets?
Hast thou eaten and drunk to excess
Of the sponges -- thy muffins and crumpets,
Of the seaweed -- thy mustard and cress?
Wast thou nurtured in caverns of coral,
Remote from reproof or restraint?
Art thou innocent, art thou immoral,
Sinburnian or Saint?

Lithe limbs, curling free, as a creeper
That creeps in a desolate place,
To enroll and envelop the sleeper
In a silent and stealthy embrace,
Cruel beak craning forward to bite us,
Our juices to drain and to drink,
Or to whelm us in waves of Cocytus,
Indelible ink!

O breast, that 'twere rapture to writhe on!
O arms 'twere delicious to feel
Clinging close with the crush of the Python,
When she maketh her murderous meal!
In thy eight-fold embraces enfolden,
Let our empty existence escape,
Give us death that is glorious and golden,
Crushed all out of shape!

Ah! thy red lips, lascivious and luscious,
With death in their amorous kiss,
Cling round us, and clasp us, and crush us,
With bitings of agonised bliss;
We are sick with the poison of pleasure,
Dispense us the potion of pain;
Ope thy mouth to its uttermost measure
And bite us again!

Related pages, main Mollusca page, nautilus page

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