A squid () is a mollusc with an elongated soft body, large eyes, eight cephalopod limb, and two in the orders Myopsida, Oegopsida, and Bathyteuthida (though many other molluscs within the broader Neocoleoidea are also called squid despite not strictly fitting these criteria). Like all other cephalopods, squid have a distinct head, bilateral symmetry, and a mantle. They are mainly soft-bodied, like , but have a small internal skeleton in the form of a rod-like gladius or pen, made of chitin.
Squid diverged from other cephalopods during the Jurassic and radiated at the beginning of the Late Cretaceous, and occupy a similar Ecological niche to teleost fish as open-water of similar size and behaviour. They play an important role in the open-water food web. The two long tentacles are used to grab prey and the eight arms to hold and control it. The beak then cuts the food into suitable size chunks for swallowing. Squid are rapid swimmers, moving by jet propulsion, and largely locate their prey by sight. They are among the most intelligent of invertebrates, with groups of Humboldt squid having been observed Pack hunter. They are preyed on by , other fish, sea birds, Pinniped and , particularly .
Squid can change colour for camouflage and signalling. Some species are bioluminescent, using their light for counter-illumination camouflage, while many species can eject a cloud of cephalopod ink to distract predators.
Squid are used for human consumption with commercial fisheries in Japan, the Mediterranean, the southwestern Atlantic, the eastern Pacific and elsewhere. They are used in cuisines around the world, often known as "calamari". Squid have featured in literature since classical times, especially in tales of giant squid and .
The cladogram, not fully resolved, is based on Sanchez et al., 2018. Their molecular phylogeny used and nuclear DNA marker sequences; they comment that a robust phylogeny "has proven very challenging to obtain". If it is accepted that Sepiidae cuttlefish are a kind of squid, then the squids, excluding the vampire squid, form a clade as illustrated. Orders are shown in boldface; all the families not included in those orders are in the paraphyletic order "Oegopsida", except Sepiadariidae and Sepiidae that are in the polyphyletic order "Sepiida",
The ancestral coleoid was probably nautiloid-like with a straight septate shell that became immersed in the mantle and was used for buoyancy control. Four lines diverged from this, Spirulida (with one living member), the , the squids and the . Squid have differentiated from the ancestral mollusc such that the body plan has been condensed antero-posteriorly and extended dorso-ventrally. What may have been the foot of the ancestor is modified into a complex set of appendages around the mouth. The sense organs are highly developed and include advanced eyes similar to those of .
The ancestral shell has been lost, with only an internal gladius, or pen, remaining. The pen, made of a chitin-like material, is a feather-shaped internal structure that supports the squid's mantle and serves as a site for muscle attachment. The cuttlebone or sepion of the Sepiidae is calcareous and appears to have evolved afresh in the Tertiary.
Due to their soft bodies and tendency to easily decay, fossil remains of true squid are very rare in the Mesozoic fossil record, in contrast to those of the superficially similar Belemnitida. Even their Cephalopod beak, the only hard part of them that can sometimes fossilize, tend to be extremely fragile and can be easily destroyed during preparation of the surrounding rock. The lack of squid remains in the Mesozoic fossil record has skewed calibrations over the phylogenetic divergence times of squid, with some studies suggesting that they may have only evolved during the Cenozoic, although more recent discoveries have clarified and refuted this. In 2006 and 2015, the fossil squid genera Yezoteuthis and Haboroteuthis were identified from the Late Cretaceous-aged Yezo Group of Japan based on fossil beaks.
In 2025, multiple fossil squid taxa belonging to both the Oegopsida and Myopsida were described based on fossil beaks from the Yezo Group, with records of both found dating back to the earliest Cenomanian. The authors resolved the fragility of the fossil beaks by scanning them and the surrounding matrix with grinding tomography. The abundance of squid beaks in carbonite concretions, which would have only formed within a few weeks, suggests that squid had likely already formed large populations by the mid-Cretaceous despite their paucity in the fossil record. Their total biomass would have likely already exceeded that of marine fishes and even Ammonoidea by that point.
The suckers may lie directly on the arm or be stalked. Their rims are stiffened with chitin and may contain minute toothlike denticles. These features, as well as strong musculature, and a small ganglion beneath each sucker to allow individual control, provide a very powerful adhesion to grip prey. Hooks are present on the arms and tentacles in some species, but their function is unclear. The two tentacles are much longer than the arms and are retractile. Suckers are limited to the spatulate tip of the tentacle, known as the manus.
In the mature male, the outer half of one of the left arms is hectocotylus – and ends in a copulatory pad rather than suckers. This is used for depositing a spermatophore inside the mantle cavity of a female. A ventral part of the foot has been converted into a funnel through which water exits the mantle cavity.
The main body mass is enclosed in the mantle, which has a Cephalopod fin along each side. These fins are not the main source of locomotion in most species. The mantle wall is heavily muscled and internal. The visceral mass, which is covered by a thin, membranous epidermis, forms a cone-shaped posterior region known as the "visceral hump". The mollusc shell is reduced to an internal, longitudinal chitinous "pen" in the functionally dorsal part of the animal; the pen acts to stiffen the squid and provides attachments for muscles.
On the functionally ventral part of the body is an opening to the mantle cavity, which contains the (ctenidia) and openings from the excretory, digestive and reproductive systems. An inhalant siphon behind the funnel draws water into the mantle cavity via a valve. The squid uses the funnel for locomotion via precise jet propulsion. In this form of locomotion, water is sucked into the mantle cavity and expelled out of the funnel in a fast, strong jet. The direction of travel is varied by the orientation of the funnel. Squid are strong swimmers and certain species can "fly" for short distances out of the water.
The skin is covered in controllable of different colours, enabling the squid to match its coloration to its surroundings. The play of colours may in addition distract prey from the squid's approaching tentacles. The skin also contains light reflectors called and that, when activated, in create changeable skin patterns of polarized light. Such skin camouflage may serve various functions, such as communication with nearby squid, prey detection, navigation, and orientation during hunting or seeking shelter. Neural control of the iridophores enabling rapid changes in skin iridescence appears to be regulated by a cholinergic process affecting reflectin .
Some mesopelagic squid such as the firefly squid ( Watasenia scintillans) and the midwater squid ( Abralia veranyi) use counter-illumination camouflage, generating light to match the downwelling light from the ocean surface. This creates the effect of countershading, making the underside lighter than the upperside.
Counter-illumination is also used by the Hawaiian bobtail squid ( Euprymna scolopes), which has symbiotic bacteria ( Aliivibrio fischeri) that produce light to help the squid avoid nocturnal predators. This light shines through the squid's skin on its underside and is generated by a large and complex two-lobed light organ inside the squid's mantle cavity. From there, it escapes downwards, some of it travelling directly, some coming off a reflector at the top of the organ (dorsal side). Below there is a kind of iris, which has branches (diverticula) of its ink sac, with a lens below that; both the reflector and lens are derived from mesoderm. The squid controls light production by changing the shape of its iris or adjusting the strength of yellow filters on its underside, which presumably change the balance of wavelengths emitted. Light production shows a correlation with intensity of down-welling light, but it is about one third as bright; the squid can track repeated changes in brightness. Because the Hawaiian bobtail squid hides in sand during the day to avoid predators, it does not use counter-illumination during daylight hours.
The paired eyes, on either side of the head, are housed in capsules fused to the cranium. Their structure is very similar to that of a fish eye, with a globular lens that has a depth of focus from to infinity. The image is focused by changing the position of the lens, as in a camera or telescope, rather than changing the shape of the lens, as in the human eye. Squid adjust to changes in light intensity by expanding and contracting the slit-shaped pupil. Deep sea squids in the family Histioteuthidae have eyes of two different types and orientation. The large left eye is tubular in shape and looks upwards, presumably searching for the silhouettes of animals higher in the water column. The normally-shaped right eye points forwards and downwards to detect prey.
The statocysts are involved in maintaining balance and are analogous to the inner ear of fish. They are housed in cartilaginous capsules on either side of the cranium. They provide the squid with information on its body position in relation to gravity, its orientation, acceleration and rotation, and are able to perceive incoming vibrations. Without the statocysts, the squid cannot maintain equilibrium. Squid appear to have limited hearing, but the head and arms bear lines of hair-cells that are weakly sensitive to water movements and changes in pressure, and are analogous in function to the lateral line system of fish.
The female has a large translucent ovary, situated towards the posterior of the visceral mass. From here, eggs travel along the gonocoel, where there are a pair of white , which lie anterior to the gills. Also present are red-spotted accessory nidamental glands containing Symbiosis bacteria; both organs are associated with nutrient manufacture and forming shells for the eggs. The gonocoel enters the mantle cavity at the gonopore, and in some species, receptacles for storing spermatophores are located nearby, in the mantle wall.
In shallow-water species of the continental shelf and epipelagic or , it is frequently one or both of arm pair IV of males that are modified into hectocotyli. However, most deep-sea squid lack hectocotyl arms and have longer penises; Ancistrocheiridae and Cranchiinae are exceptions. Giant squid of the genus Architeuthis are unusual in that they possess both a large penis and modified arm tips, although whether the latter are used for spermatophore transfer is uncertain. Penis elongation has been observed in the deep-water species Onykia ingens; when erect, the penis may be as long as the mantle, head, and arms combined. As such, deep-water squid have the greatest known penis length relative to body size of all mobile animals, second in the entire animal kingdom only to certain sessile .
The food bolus is moved along the gut by waves of muscular contractions (peristalsis). The long oesophagus leads to a muscular stomach roughly in the middle of the visceral mass. The Hepatopancreas, which is equivalent to a vertebrate liver, diverticulates here, as does the pancreas, and both of these empty into the Cecum, a pouch-shaped sac where most of the absorption of nutrients takes place. Indigestible food can be passed directly from the stomach to the rectum where it joins the flow from the caecum and is voided through the anus into the mantle cavity. Cephalopods are short-lived, and in mature squid, priority is given to reproduction; the female Onychoteuthis banksii for example, sheds its feeding tentacles on reaching maturity, and becomes flaccid and weak after spawning.
In 1978, sharp, curved claws on the suction cups of squid tentacles cut up the rubber coating on the hull of the USS Stein. The size suggested the largest squid known at the time.
In 2003, a large specimen of an abundant but poorly understood species, Mesonychoteuthis hamiltoni (the colossal squid), was discovered. This species may grow to in length, making it the largest invertebrate. In February 2007, a New Zealand fishing vessel caught the largest squid ever documented, weighing and measuring around off the coast of Antarctica. Dissection showed that the eyes, used to detect prey in the deep Southern Ocean, exceeded the size of footballs; these may be among the largest eyes ever to exist in the animal kingdom.
In a well-studied bioluminescent species, the Hawaiian bobtail squid, a special light organ in the squid's mantle is rapidly colonized with Aliivibrio fischeri bacteria within hours of hatching. This light-organ colonization requires this particular bacterial species for a symbiotic relationship; no colonization occurs in the absence of A. fischeri. Colonization occurs in a horizontal manner, such that the hosts acquires its bacterial partners from the environment. The symbiosis is obligate for the squid, but facultative for the bacteria. Once the bacteria enter the squid, they colonize interior epithelium in the light organ, living in crypts with complex Microvillus protrusions. The bacteria also interact with , macrophage-like blood cells that migrate between epithelial cells, but the mechanism and function of this process is not well understood. Bioluminescence reaches its highest levels during the early evening hours and bottoms out before dawn; this occurs because at the end of each day, the contents of the squid's crypts are expelled into the surrounding environment. Approximately 95% of the bacteria are voided each morning before the bacterial population builds up again by nightfall.
Slow jetting is used for ordinary locomotion, and ventilation of the gills is achieved at the same time. The circular muscles in the mantle wall contract; this causes the inhalant valve to close, the exhalant valve to open and the mantle edge to lock tightly around the head. Water is forced out through the funnel which is pointed in the opposite direction to the required direction of travel. The inhalant phase is initiated by the relaxation of the circular muscles causes them to stretch, the connective tissue in the mantle wall recoils elastically, the mantle cavity expands causing the inhalant valve to open, the exhalant valve to close and water to flow into the cavity. This cycle of exhalation and inhalation is repeated to provide continuous locomotion.
Fast jetting is an escape response. In this form of locomotion, radial muscles in the mantle wall are involved as well as circular ones, making it possible to hyper-inflate the mantle cavity with a larger volume of water than during slow jetting. On contraction, water flows out with great force, the funnel always being pointed anteriorly, and travel is backwards. During this means of locomotion, some squid exit the water in a similar way to flying fish, gliding through the air for up to , and occasionally ending up on the decks of ships.
Although squid can catch large prey, the mouth is relatively small, and the food must be cut into pieces by the chitinous beak with its powerful muscles before being swallowed. The radula is located in the buccal cavity and has multiple rows of tiny teeth that draw the food backwards and grind it in pieces. The deep sea squid Mastigoteuthis has the whole length of its whip-like tentacles covered with tiny suckers; it probably catches small organisms in the same way that flypaper traps flies. The tentacles of some bathypelagic squids bear which may bring food within its reach by attracting prey.
Squid are among the most intelligent invertebrates. For example, groups of Humboldt squid hunt cooperatively, spiralling up through the water at night and coordinating their vertical and horizontal movements while foraging.
The pair adopt a head-to-head position, and "jaw locking" may take place, in a similar manner to that adopted by some cichlid fish. The hectocotylus of the male is used to transfer the spermatophore and deposit it in the female's mantle cavity in the position appropriate for the species; this may be adjacent to the gonopore or in a seminal receptacle.
The sperm may be used immediately or may be stored. As the eggs pass down the oviduct, they are wrapped in a gelatinous coating, before continuing to the mantle cavity, where they are fertilised. In Loligo, further coatings are added by the nidimental glands in the walls of the cavity and the eggs leave through a funnel formed by the arms. The female attaches them to the substrate in strings or groups, the coating layers swelling and hardening after contact with sea water. Loligo sometimes forms breeding aggregations which may create a "community pile" of egg strings. Some pelagic and deep sea squid do not attach their egg masses, which float freely.
Different species of squid vary wildly in size, and even giant squid young are rice-grain-sized at hatching. Throughout their life cycle, they can serve as a food source for many sizes of predator. Juvenile squid provide part of the diet for Polychaete and small fish. Larger squid are food for larger predators, including sharks, sea birds, seals and whales.
When researchers studied the contents of the stomachs of in South Georgia, they found 96% squid by weight. In a single day, a sperm whale can eat 700 to 800 squid, and a Risso's dolphin entangled in a net in the Mediterranean was found to have eaten Ancistroteuthis, umbrella squid, reverse jewel squid and European flying squid, all identifiable from their indigestible beaks.
Ornithoteuthis volatilis, a common squid from the tropical Indo-Pacific, is predated by yellowfin tuna, longnose lancetfish, Mahi-mahi and swordfish, the tiger shark, the scalloped hammerhead shark and the smooth hammerhead shark. Sperm whales also hunt this species extensively as does the brown fur seal. In the Southern Ocean, and wandering albatrosses are major predators of Gonatus antarcticus.
The risk comes from eating raw or undercooked seafood that can cause severe symptoms such as abdominal pain, nausea, vomiting, fever, and diarrhea. The best way to prevent infection is to avoid eating raw or undercooked squid seafood; cook squid to at least 60 degrees Celsius (140 degrees Fahrenheit) or freeze squid at -20 degrees Celsius (-4 degrees Fahrenheit) for a minimum of 24 hours.
In literature, H. G. Wells' short story "The Sea Raiders" featured a man-eating squid species Haploteuthis ferox. The science fiction writer Jules Verne told a tale of a kraken-like monster in his 1870 novel Twenty Thousand Leagues Under the Seas.
In English-speaking countries, squid as food is often called calamari, adopted from Italian into English in the 17th century. Squid are found abundantly in certain areas, and provide large catches for fishery. The body can be stuffed whole, cut into flat pieces, or sliced into rings. The arms, tentacles, and ink are also edible; the only parts not eaten are the beak and gladius (pen). Squid is a good food source for zinc and manganese, and high in copper, selenium, vitamin B12, and riboflavin.
The squid giant axon inspired Otto Schmitt to develop a comparator circuit with hysteresis now called the Schmitt trigger, replicating the axon's propagation of .
Description
Camouflage
Predator distraction with ink
Nervous system and sense organs
Reproductive system
Digestive system
Cardiovascular and excretory systems
Buoyancy
Largest and smallest
Development
Behaviour
Locomotion
Feeding
Reproduction
Ecology
Parasites
Other common squid parasites
Human uses
In literature and art
As food
Commercial fishing
+ World squid catch in 2002 Loligo gahi or Doryteuthis gahi Loliginidae Patagonian squid 24,976 1.1 Loligo pealei Loliginidae Longfin inshore squid 16,684 0.8 Common squid nei Loliginidae 225,958 10.3 Ommastrephes bartramii Ommastrephidae Neon flying squid 22,483 1.0 Illex argentinus Ommastrephidae Argentine shortfin squid 511,087 23.3 Dosidicus gigas Ommastrephidae Humboldt squid 406,356 18.6 Todarodes pacificus Ommastrephidae Japanese flying squid 504,438 23.0 Nototodarus sloanii Ommastrephidae Wellington flying squid 62,234 2.8 Squid nei Various 414,990 18.6 Total squid 2,189,206 100.0
In biomimicry
See also
Notes
Sources
External links
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