Cephalopods are a group of molluscs and very highly evolved animals. They include the squid (pictured
here), octopus, cuttlefish (sometimes called a 'squid'), nautilus and argonauts and many extinct forms,
including the giant nautiloids. 'Cephalopod' literally means 'head-foot' and refers to the tentacles (arms)
borne on the head, which characterise these animals. Cephalopods have a number of special features
that we look at in turn:
Intelligence & communication
Phenomenal control over their skin colour and sometimes texture
Arms/tentacles and suckers
This 3D Pov-Ray model of a squid has the usual 8 suckered arms (short tentacles) and the two
retractile fishing tentacles, which only bear suckers on their terminal claspers. The Giant Squid
(Architeuthis) can reach a length of 5 m (2 m for the main body or mantle) plus 8 m for the two long
fishing tentacles, for a total length of 13 m. The Antarctic or Colossal Squid (Mesonychoteuthis) is
even larger at up to about 14 m and is probably the largest known invertebrate (the largest caught
specimen, at 10m, weighed about the same as the largest jellyfish - 500 kg or half a tonne). These
giants eluded science until only very recently. They probably gave rise to many of the legends of the
Kraken and the word 'kraken' is used synonymously with 'giant squid'. These large squid have
sometimes been reported floating at the water's surface and will lash out with their tentacles if
accidentally rammed by a boat or ship. In the centre of the tentacle crown is a very hard, sharp and
powerful chitinous beak, which is reportedly capable of biting through steel cable. The fact that such
large creatures could evade science for so long makes one wonder what else lives in the Earth's
oceans ... .
Squid come in a tremendous variety of forms. Some are transparent, some are spherical, some
conical and some have large leaf-like tails. The Colossal Squid has the largest known eyes in the
animal kingdom - at 28 cm diameter. Squid are able to communicate with one-another by remarkable
and spectacular colour displays - their skin flushing a myriad of colours and patterns as
chromatophore cells/organs (external link: chromatophore organ) in their skin contract and expand,
under nervous control. Squid can also use a form of semophore - signalling by changes in arm
positions. Many squid are bioluminescent - producing their own light (with the help of bacterial
symbionts) with photophore organs. Filters can change the colour of the light produced and shutters
can switch the lights on or off, whilst reflectors and lenses intensify the light emitted. These lights
may be used in communication as well as to lure prey animals toward the squid. Each species has a
distinct pattern of lights, allowing squid to recognise their own species and shoal together. Humboldt
squid form shoals (packs?) of up to 1000 or so individuals that communicate by displays of colour
and coordinate their hunting.
The Colossal Squid
Diagram reproduced under the GNU Free Documentation
License from Wikipedia. External link: Colossal Squid.
the squid body can be divided into three sections - the arm/tentacle crown, the head and the
mantle. The mantle (or pallium) is like a 'cloak' that covers the body of the squid behind the head.
It is an extension of the posterior body wall that extends forward, in a cone, to the head. Between
the mantle and the body proper (visceral mass) is the mantle cavity. When the mantle expands,
water is sucked into the cavity behind the squid's head. The mantle can then seal itself by means
of valves and the squid can expel a jet of water through the ventral movable siphon (funnel),
propelling it backwards at speed. This is important in escape responses when the squid evades
a potential predator. The mantle will also pulse periodically to flush the gills with fresh oxygenated
water. A pair of gills extend as outgrowths of the body wall that project into the mantle cavity. They
are attached to the mantle by supporting suspensory membranes.
The rectum also opens in the mantle cavity via the anus and the pair of renal organs each opens into
the mantle cavity via a renal pore. The pen is an internal skeletal rod made of chitin. This extends along
the length of the body along the back of the squid. It protects the internal organs on the upper surface.
The pen expands into a chitinous shield (pro-ostracum) at its anterior end, to which the mantle tissues
The gills, suspended from the body wall by the suspensory gill membranes, are bipectinate - they have
filaments on both sides of their axis. An extra heart, the branchial heart, is present at the base of each
gill, ensuring that they have an efficient blood supply.
The ink-sac discharges via its own duct, which runs along the rectum, just behind the anus and is a
black ovoid sac.
The renal sacs open either side of the anus. There is also a single genital opening on the left, just
behind the renal openings.
Above: a diagram of a dissection of a male squid (Loligo) with a median cut in the ventral side of the
mantle to reveal the organs of the mantle cavity. The cartilages (and corresponding sockets that
they fit into) constitute the 'resisting apparatus' which seal the mantle when 'valve' muscles contract.
The funnel can be directed, performing steering movements.
Above: a 3D computer model of a squid (generated using computer script in Pov-Ray).
Buoyancy Mechanisms in Cephalopods
Squid, like some other marine animals can rely on speed of movement to keep them afloat, or they
can achieve neutral bouyancy - that is they can have more-or-less the same density as sea-water so
that they able simply to float. To achieve perfect neutral bouyancy is difficult, proteins are dense and
make animal tissue denser than water. An active animal needs plenty of muscle protein which makes
its body denser, but then it can swim when needed to readjust its height as it sinks. Some animals
opt for a very different strategy, they have lower protein-content muscles which are more watery and
These animals might not have the power to be continuously active, but then they don't need to be -
they may move in short bursts when catching prey or wait in ambush for prey to find them. There is
another complication, however, muscle protein quite severely impedes the diffusion of oxygen
through tissues. However, not all swimming muscles need a rich oxygen supply - the white muscle
which makes up the bulk of the body in bony fish is anaerobic muscle used in sprinting, it is strong
and fast but fatigues quickly and has a poor oxygen supply. This kind of muscle is used in burst
swimming or sprinting, as when catching prey and avoiding predators and so does not need to be
used continuously for any length of time. The red muscle of bony fish has a rich oxygen supply and
although much smaller and less powerful than the white muscle, the red muscle is able to move
continuously and is used for cruising.
Fish with low protein content are often deep-sea fish that dwell at abyssal depths where food is
scarce and they will wait in ambush, often luring prey with bioluminescent lures meant to resemble
the prey food of the intended victim. The giant squid, Architeuthis, has a dense muscle structure
indicative of an active animal. However, the story is not so simple. The giant squid uses another
technique (used by many other sea creatures) to reduce its density. Its tissues have a high
ammonium ion content and low sodium ion content - ammonium is replacing sodium (both ions have
a single plus charge and so have similar properties). Ammonium has a much lower relative atomic
mass than sodium and so, it is reasoned, tissues with a high ammonium/sodium ratio have low
density. Indeed they do, but for a slightly different reason - it is not so much ionic mass that matters,
but the way in which the ions interacts with water molecules around it as this has the greater effect
on density of the final solution. A solution of ammonium ions is less dense than a solution of sodium
ions of the same concentration. Interestingly, the giant squid has a much higher ammonium/sodium
ratio (about 2) in its mantle than in its head and arms. This suggests that it hangs with the head and
tentacles angled downwards, perhaps passively fishing for food that passes beneath it. This
suggests that it might be a passive ambush hunter.
Cuttlefish resemble squid (and are often referred to as 'squid') but are quite different - their tail fin is
extended forwards laterally to form an undulating skirt around the whole mantle, allowing these
cephalopods to hover in a way that seems almost effortless and is quite fascinating to watch.
Cuttlefish also have internal floats in the form of cuttlebone. The cuttlebone is a porous calcareous
structure that fills much of the body of the cuttlefish and is a gas-filled internal shell that gives the
cuttlefish buoyancy. This float will implode at around 200-600 m, so cuttlefish are shallow-water
creatures largely confined to the continental shelf. The internal shell of squids is the skeletal pen
(gladius) made of chitinous material and has no special function in buoyancy.
The Nautilus is the only living cephalopod to retain an external
shell. Many ancient and extinct cephalopods, like the Nautiloid
shown below, had shells. The Nautiloids, some of which had
shells up to at least 8 feet in length, had either straight conical
shells, coiled shells or shells that were partially coiled like the
one pictured below. Like the Nautilus the rear chambers of the
shell were probably gas-filled for flotation.
The giant squid, Architeuthis dux, was denied by science
for a long time, despite the fact that many seamen had
reported seeing such creatures (kraken) and also despite
the fact that some tentacle remains had been pickled in a
museum collection. When it was finally ascertained that
these creatures really existed, the question remained as
to how big they could grow. Specimens are known in
which the mantle length is over 2 m. The arms and fishing
tentacles add to their length, for a total verifiable
maximum length of about 13 m for females and 10 m for
Recently, the even larger colossal squid was realised to
exist (although tentacles had been known for some time,
it was not until 2003-2007 that the full size of these
creatures was realised). This leads naturally to the
question: how large is the largest squid? Studies of the
stomach contents of sperm whales, which feed on giant
squid (and being some 100 times heavier than the
colossal squid can probably manage even the largest
squid) which yield squid beaks, suggests that giant squid
do not commonly exceed these proportions. This,
however, still does not rule out the possibility of their
existing rare mega-giants. Giant cephalopods were
dominant predators in the Ordovician seas, some 450
million years ago. How big was the largest cephalopod of
all time? This question remains unanswered.
With their large heads and large eyes, cephalopods look intelligent and indeed they are. They probably
have the most complex brains among the invertebrates and some octopus are known to collect a tool
for later use - they will pick-up half a coconut shell and carry it with them until they find another half and
then they will make a shelter from them. This is a recent discovery, but there are old fables of
octopuses crawling up onto the beach at night and climbing palm trees to steal coconuts (presumably
for food) before scurrying back into the sea when approached in the morning! True or not, these old
stories are certainly possible. So, cephalopods exhibit social behaviour, they have intelligent brains and
they have suckered arms capable of complex manipulations, so why have they not evolved civilisation?
One possibility is their lack of longevity. Cephalopods tend to die after spawning and so are short-lived.
Even captured giant squids are only about 5-6 years old (a phenomenal growth rate!). Perhaps these
creatures never live long enough to make discoveries and then pass them on to their offspring, indeed
they die before their young grow and develop.
Nevertheless, the possibilities have engaged the human imagination for decades. Cephalopods
inspired H. G. Wells' Martians and hence probably the famous Daleks, as well as the Illithid mindflayers
of Dungeons & Dragons. In all cases these aliens were super-intelligent. H. G. Wells added a final twist
in speculating that the Martians evolved from creatures more like us, humanoids with bones, but having
adapted to pushing buttons and pulling levers on a low-gravity world they lost their limbs, with the two
hands remaining as paired clusters of tentacles on either side of the face, and the bones disappearing.
They had classic cephalopod looks, however, with snakelike tentacles and large, luminous and disc-like
eyes! Similarly the Daleks evolved from a race of humans by a combination of radiation-induced
mutation and genetic engineering. They too became little more than tentacled brains that operate
fighting-machines. The cephalopod has certainly captivated many creative minds in the world! Martians,
Illithids and Daleks happen to be some of my favourite monsters!
Is this a message from the future?
The shell of Nautilus is divided into chambers, with the oldest and smallest chamber at the apex of the
spiral and the body of Nautilus only occupies the most recent and largest chamber. The chambers are
separated by walls of shell called septa (sing. septum). A tube of tissue called the siphuncle traverses
the length of the animal, passing through each dividing wall (septum) in each shell chamber. The
siphuncle takes up salts from a water-filled chamber and water follows by osmosis and then gas
(mostly nitrogen) replaces the water. This filling of a chamber with gas occurs whenever the animal
grows by secreting a new chamber into which the animal moves, leaving behind a water-filled chamber
in place of its previous living compartment. Once the newly secreted septum is strong enough to
withstand the pressures, the vacated chamber is filled with gas - counteracting the increase in mass of
the growing animal and keeping it buoyant. Extinct nautiloids probably maintained buoyancy in a
similar way. Cuttlefish also maintain a shell comprising a mixture of fluid-filled and gas-filled chambers,
but this shell is the internal cuttlebone. Cuttlefish can regulate the fluid/gas ratio in their cuttlebones.
During the day they lie buried in the sea bottom and emerge at night to hunt for food. Light regulates
this, with buoyancy of the cuttlefish decreasing on exposure to light. Spirula is a cuttlefish whose
internal shell is coiled in a spiral, unlike typical cuttlebones which are more-or-less straight
Some squid, like Grimalditeuthis below, have secondary fins - an extra tail fin behind the main
locomotory fin. The secondary fin lacks muscle and is thinner and more sheet-like and functions
primarily as a flotation device.
Cephalopods, like many marine creatures, are equipped with lights. These lights have a number of
Counterillumination: an animal that lives near to the surface of the sea will be well-hidden if it stays dark
when seen from above, against the dark depths, but will appear dark if viewed from underneath against
the downwelling light. To counter this, many fish and squid have dark backs but illuminated
undersurfaces. Some squid are translucent, but their eyes and ink sacs are opaque, so they may have
downward-pointing lights positioned underneath their eyes and ink sacs.
Communication: female squids tend to have lights at the tips of their arms which are presumably used
to signal to the males. Squid may have complex species-specific patterns of lights on their bodies, and
this may aid species identification in shoal formation or in other forms of communication.
Prey-capture: waving or flashing lights around to mimic smaller bioluminescent organisms is a good way
to attract food. A smaller fish or crustacean swims toward the lights, expecting a feast, but is eaten
Defense: a sudden flashing of lights can distract, confuse and possibly temporarily blind an attacker in
the dark depths. Some squid can release luminescent ink (which may be in addition to the ink proper
which is used in well-lit waters) as a decoy.
Path-finding: some animals use their lights to see where they are going! This is especially handy if your
lights work at a wavelength that you can see but your potential prey cannot! Some fish exploit this
mechanism in hunting.
Light-producing organs are called photophores. Photophores in cephalopods vary widely in design and
a single animal may even have several different types. However, a fairly typical squid photophore is
shown in section in the diagram below:
(Adapted from Pringgenies and Jorgensen, 1994).
This photophore contains a lens (made of modified muscle cells) to focus the light beam. A reflector
intensifies the light. This reflector is made up of a stack of very thin plates or lamellae, spaced 200 nm
apart from one another. This reflects and diffracts the light in such a way as to intensify it by
constructive interference. The central chamber contains the light-emitting tissue, in this case canals
and pockets of tissue which house masses of bacteria. These bacteria are of a specific type and they
produce the light. A canal, which opens into the mantle, circulates a current of water through the organ
and through the interconnected chambers, by means of tiny beating hairlike cilia. This presumably
provides the bacteria with oxygen and perhaps other substances.
Most cephalopods (excluding Nautilus and some octopuses) squirt ink as a defense mechanism. The
ink sac is situated between the gills in the mantle cavity and ink is released into the exhalent jet of the
siphon. The first tactic is to release large quantities of ink whilst jetting rapidly away (backwards). This
generates a smokescreen to cover the cephalopod's escape. It is also possible that the ink contains
chemicals that numb the sense of smell of the attacker (?). The second tactic is to generate decoys -
less ink is released along with lots of mucus to entrap it. This creates a dark mass roughly the same
size and shape as the squid and predators often mistakingly attack the decoy. The squid may release
several such decoys, firing them to a small distance away from itself, and changes colour (turning pale)
as it does so, adding to the attacker's confusion, and then attempts to escape. Some cephalopods
release curtains of 'luminous ink' - light-emitting fluid when in the dark depths. Some can release both
dark ink in well-lit waters and luminous ink in dark waters. The luminous 'ink' is released from a
separate chromatophore gland rather than by the ink sac.
Arms/tentacles and Suckers
Squid and cuttlefish have ten 'tentacles' - 8 arms and two fishing tentacles, whilst the octopus has the 8
arms only. The arms/tentacles are made of solid blocks of muscle and connective tissues - there are
no skeletal parts, not even the fluid-filled chambers of a hydrostatic skeleton. Instead, groups of
muscles oppose (antagonise) one-another - longitudinal muscles shorten the tentacles, circular and
transverse muscles make them narrower and more elongated, while helical muscles are presumably
responsible for twisting movements. Thus, the muscles serve as both motors and in skeletal support.
The arms and the tentacle clubs possess suckers and/or hooks. The suckers may be toothed and in
some squid they are replaced by hooks, or both hooks and suckers may be present. In octopus there
are suckers but no teeth or hooks. In squid and cuttlefish the suckers are borne on stalks, whilst in
octopus they are stalkless, although the base may extend to twice its normal length. The suckers can
be rotated and tilted. The structure of an octopus sucker is shown below (redrawn from Kier and Smith
The sucker is divided into three parts - the sucker itself or infundibulum (I) and the sac-like
acetabulum (A) and the mobile base. The acetabulum wall (AW) and wall of the
infundibulum contain radial muscle fibres (R), circular muscle fibres (C) and meridional
muscles (M). these structures are enclosed in a tough outer connective tissue sheath (OS).
Connective tissue fibres also criss-cross in the acetabulum wall. A pair of circular sphincter
muscles (S) - one large and one small - control the diameter of the narrow orifice between
the infundibulum and acetabulum. Extrinsic muscles (E) and extrinsic circular muscle (EC)
ensheath the acetabulum. D, dermis; Ep, epithelium.
The sucker secretes mucus, allowing it to create a water-tight seal against a surface when suction is
applied. The suckers of cephalopods are capable of generating 1-3 atmospheres of pressure and are
also highly dextrous, being able to pass food along the arms, for example. The most powerful suckers
seem to belong to fast-swimming squid, presumably since these catch fast-moving prey. The tentacles
are rapidly extended and if the strike is successful then the clubs will hit the target and the suckers will
attach. The tentacles of the giant squid can be over 10 m in length and are tipped with 10 cm diameter
suckers. In small squid, the speed of tentacle extension during prey-capture has been filmed at over 2
m/s. Octopus will use their webbed arms for gliding and for walking along the sea bottom, as well as for
food capture and to manipulate objects.
Although the giant squid looks as if it could swallow a human whole in seconds, most squid are actually
shy creatures and unlikely to avoid rather than to bother humans, though the colossal squid does feed
largely on fish such as the Antarctic cod, which can reach 1.5 metres in length and 150 kg in mass, so a
colossal squid could probably easily devour a human if it was inclined to do so! There are many accounts
of people being attacked by cephalopods, not all of them easily dismissed, but usually only after
accidentally treading on one or perhaps if a boat accidentally collides with one floating in the water.
Nevertheless, all large and/or armed animals should be treated with caution as individuals can be
unpredictable and fail to follow textbook behaviour. In addition colossal squids have very sharp chitinous
beaks to rip apart their prey, though their bodies are soft and gelatinous.
The question has been raised - how do such gelatinous animals bite prey without ripping themselves
apart. Could it be that an arrangement of taught muscles and connective tissue spreads and dissipates
the strain? (We have seen how jellyfish can anchor powerful swimming muscles to special cartilaginous
plates that act as a skeleton). Perhaps the alleged razor sharpness of the beaks negates the need for
power? There are reports of squids biting through steel cables; I don't know if these are authentic, but
chitin does have a hardness comparable to copper. A good review of the what little is known of their
eating habits can be found on the Discovery News site colossal squid article (external link). The colossal
squid has the largest eyes in the animal kingdom, each the size of a basket ball and each accompanied
by a bioluminescent organ. Some have suggested this helps it locate prey in murky waters, others that
their use is primarily to detect the squid's main enemy - sperm whales which predate large squids.
Left: Is this an alien from outer
space? Not quite, its a 3D computer
model (Pov-Ray) of the Google-eyed
glass squid, Teuthowenia pellucida.
This squid occurs at depths of
1600-2500 m (adult stage). When
threatened, these normally narrow
squid inflates with water. If the threat
persists then they retract their head
and tentacles and if this is insufficient,
then they fill the body cavity with ink,
causing the squid to apparently
disappear in the darkness. These
squid can also use jet-propulsion to
escape from danger.
Glass squids come in a remarkable
variety of forms and some are so
fragile that they are very hard, if not
impossible, to capture intact by
conventional means and so are
poorly studied, but observations of te
ecreatures in their natural habitats
are adding valuable data.
Article last updated: 10/5/14