Cephalopoda, squid, octopus, and cuttle fish

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.


Loligo opalescens, squid internal anatomy

Squid have a pair of long, club-ended arms that can be shot out with both great speed and precision to capture prey. The Giant squid (Architeuthis) are the largest invertebrates with a body length that can exceed 4 m and arms of 20 m. However, they, like all squid, will die after reproducing just once. Squid are a very important prey of sperm whales.

Above is Loligo opalescens showing the major internal organs.

Loligo sp. feed on shrimp and fish, and cuttlefish feed mainly on shrimp and crabs.

Histioteuthis boniellii, the Umbrella squid, or cock-eyed squid

Histioteuthis boniellii, the Umbrella squid

Histioteuthis boniellii, the Umbrella squid, above, is found in the Atlantic and Indian Oceans, and in the Mediterranean Sea at 400 - 500 m deep. Its eyes are of different sizes, and it is thought that it uses the larger eye to look up, while the smaller eye can detect the glow and flashes of light emitted from light organs on other animals. Its own smaller eye is ringed by light-emitting cells, so could be used rather like a search light. This squid is preyed on by whales.

Vampire squid

Vampyroteuthis infernalis, vampire squid

Aboveis the vampire squid, Vampyroteuthis infernalis. It lives in tropical and sub-tropical oceans in water between 700 - 3 500 m deep, and is purple/black in colour. It has light emitting organs (photophores) on its body, eyes and tentacles. Its arms are joined by a web of skin.

It is a very fast swimmer, and feeds on copepods, prawns and Cnidarians. Its Latin name translates as "vampire squid from hell"! More recently this squid has been compared to bankers. This, I feel, is unfair to the squid. It is true that both eat prawns, but only one has managed the seemingly difficult task of eating our future prawns, and getting us to pay for them as well. Prawns today, prawns tomorrow (apologies to Lewis Carroll). Any bankers who disagree can do so through the donation page.

Taonius pavo, a glass squid

Taonius pavo, glass squid

Taonius pavo, on of the glass squids is shown above. It is found in the North Atlantic, North Pacific and Antarctic Oceans in water deeper than 700 m.

Cranchia scabra, a Glass squid

Cranchia scabra, a glass squid

Cranchia scabra, above, is one of the Glass squids and can be found in all oceans from the surface down to 1000 metres.

It has red pigment cells all over its body, and reaches about 15 cm long. When alarmed it can retract its head and shorter arms into the mantle cavity leaving its longer arms and eyes outside and forming itself into a tight ball. And as its body is covered by cartilaginous tubercles this will give it some protection against predators as it is difficult for them to bite it.

Bathyteuthis abyssicola

Bathyteuthis abyssicola, squid

Bathyteuthis abyssicola, above is found in all the oceans of the world between 700 - 3000 m deep. Its colour is a very dark red.

Mastigoteuthis flamea, whip-lash squid

Mastigoteuthis flamea, whip-lash squid

Mastigoteuthis flamea, whip-lash squid (above) is found in the North Atlantic and around New Zealand at depths of 700 - 3500 m.

Lycoteuthis diadema, Lycoteuthis lorigera

Lycoteuthis diadema, also known as Lycoteuthis lorigera, on the right, is found in the Indian and Atlantic oceans at depths of 200 - 700 metres.

It has light-producing organs on its body, eyes and tentacles, and is considered quite a rare squid. In adults the fin is diamond or heart-shaped. It is thought that the light-producing organs below its eyes and on its tentacles are used in courtship.

Lycoteuthis doadema, squid

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.

Pterygioteuthis giardi

Pterygioteuthis giardi

Pterygioteuthis giardi, above, is a squid found in temperate and tropical oceans at depths of 200 - 700 metres. It is unusual in that some of its suckers have hooks that enable it to capture prey more easily.

Onychoteuthis banksii, the Common clubhook squid

Onychoteuthis banksii, Common clubhook squid

Onychoteuthis banksii also known as Loligo banksii, above, is found world wide. The long arms have a double row of hooks as well as the more usual suckers, giving this squid its common name.


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.

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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