Squid

Asquid(pl.: squid) is amolluscwith an elongated soft body, large eyes, eightarms, and twotentaclesin the ordersMyopsida,Oegopsida, andBathyteuthida(though many other molluscs within the broaderNeocoleoideaare also calledsquiddespite not strictly fitting these criteria). Like all othercephalopods, squid have a distinct head,bilateral symmetry, and amantle. They are mainly soft-bodied, likeoctopuses, but have a small internal skeleton in the form of a rod-likegladiusor pen, made ofchitin.
Squid diverged from other cephalopods during theJurassicand occupy a similarroletoteleostfish as open waterpredatorsof 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 byjet propulsion, and largely locate their prey by sight. They are among the most intelligent ofinvertebrates, with groups ofHumboldt squidhaving been observedhunting cooperatively. They are preyed on bysharks, other fish, sea birds,sealsandcetaceans, particularlysperm whales.
Squid can change colour forcamouflageandsignalling. Some species arebioluminescent, using their light forcounter-illuminationcamouflage, while many species can eject a cloud ofinkto 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 andsea monsters.
Squid are members of the classCephalopoda, subclassColeoidea. The squidordersMyopsidaandOegopsidaare in the superorderDecapodiformes(from theGreekfor "ten-legged"). Two other orders of decapodiform cephalopods are also called squid, although they aretaxonomicallydistinct from squids and differ recognizably in their gross anatomical features. They are thebobtail squidof orderSepiolidaand theram's horn squidof the monotypic orderSpirulida. Thevampire squid(Vampyroteuthisinfernalis), however, is more closely related to the octopus than to any squid.
Thecladogram, not fully resolved, is based on Sanchez et al., 2018.Theirmolecular phylogenyusedmitochondrialandnuclear DNAmarker sequences; they comment that a robustphylogeny"has proven very challenging to obtain". If it is accepted thatSepiidaecuttlefish are a kind of squid, then the squids, excluding the vampire squid, form acladeas illustrated.Orders are shown in boldface; all the families not included in those orders are in theparaphyleticorder "Oegopsida", except Sepiadariidae and Sepiidae that are in the polyphyletic order "Sepiida",
Crowncoleoids (the common ancestor of octopuses and squid) diverged in the latePaleozoic(Mississippian), according to fossils ofSyllipsimopodi, an early relative of vampire squids and octopuses.True squid diverged during the Jurassic, but many squid families appeared in or after theCretaceous.Both the coleoids and the teleost fish were involved in much adaptive radiation at this time, and the two modern groups resemble each other in size, ecology, habitat, morphology and behaviour, however some fish moved into fresh water while the coleoids remained in marine environments.
The ancestral coleoid was probably nautiloid-like with a strait septate shell that became immersed in the mantle and was used for buoyancy control. Four lines diverged from this, Spirulida (with one living member), thecuttlefishes, the squids and theoctopuses. Squid have differentiated from the ancestralmolluscsuch that the body plan has been condensed antero-posteriorly and extended dorso-ventrally. What may have been thefootof 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 ofvertebrates.
The ancestral shell has been lost, with only an internalgladius, 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. Thecuttleboneor sepion of the Sepiidae iscalcareousand appears to have evolved afresh in theTertiary.
Squid are soft-bodied molluscs whose forms evolved to adopt an active predatory lifestyle. The head and foot of the squid are at one end of a long body, and this end is functionallyanterior, leading the animal as it moves through the water. A set of eight arms and two distinctive tentacles surround the mouth; each appendage takes the form of amuscular hydrostatand is flexible and prehensile, usually bearing disc-like suckers.
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 themanus.
In the mature male, the outer half of one of the left arms ishectocotylised– and ends in a copulatory pad rather than suckers. This is used for depositing aspermatophoreinside the mantle cavity of a female. A ventral part of the foot has been converted into afunnelthrough which water exits the mantle cavity.
The main body mass is enclosed in the mantle, which has aswimming finalong 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, membranousepidermis, 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 thegills(ctenidia) and openings from the excretory, digestive andreproductive 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.
Squid make use of different kinds of camouflage, namelyactive camouflagefor background matching (in shallow water) and counter-illumination. This helps toprotect them from their predatorsand allows them to approach their prey.
The skin is covered in controllablechromatophoresof 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 callediridophoresandleucophoresthat, when activated, inmillisecondscreate 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 skiniridescenceappears to be regulated by acholinergicprocess affectingreflectinproteins.
Somemesopelagicsquid such as thefirefly squid(Watasenia scintillans) and themidwater squid(Abralia veranyi) use counter-illumination camouflage, generating light to match the downwelling light from the ocean surface.This creates the effect ofcountershading, making the underside lighter than the upperside.
Counter-illumination is also used by theHawaiian bobtail squid(Euprymna scolopes), which hassymbiotic 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 ofiris, which has branches (diverticula) of itsink sac, with a lens below that; both the reflector and lens are derived frommesoderm. 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.
Squid distract attacking predators by ejecting a cloud ofink, giving themselves an opportunity to escape.The ink gland and its associated ink sac empties into the rectum close to the anus, allowing the squid to rapidly discharge black ink into the mantle cavity and surrounding water.The ink is a suspension ofmelaninparticles and quickly disperses to form a dark cloud that obscures the escape manoeuvres of the squid. Predatory fish may also be deterred by the alkaloid nature of the discharge which may interfere with theirchemoreceptors.
Cephalopods have the most highly developed nervous systems amonginvertebrates. Squids have a complex brain in the form of a nerve ring encircling theoesophagus, enclosed in a cartilaginouscranium. Paired cerebralgangliaabove the oesophagus receive sensory information from the eyes andstatocysts, and further ganglia below control the muscles of the mouth, foot, mantle and viscera.Giant axonsup to 1 mm (0.04 in) in diameter convey nerve messages with great rapidity to the circular muscles of the mantle wall, allowing a synchronous, powerful contraction and maximum speed in the jet propulsion system.
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 globularlensthat has adepth of focusfrom 3 cm (1.2 in) to infinity. The image is focused by changing the position of the lens, as in a camera ortelescope, rather than changing the shape of the lens, as in thehuman eye. Squid adjust to changes in light intensity by expanding and contracting the slit-shapedpupil.Deep sea squids in the familyHistioteuthidaehave 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 thewater 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 thelateral linesystem of fish.
The sexes are separate in squid, with a singlegonadin the posterior part of the body. Fertilisation is external and usually takes place in the mantle cavity of the female. The male has atestisfrom which sperm pass into a single gonoduct where they are rolled together into a long bundle, or spermatophore. The gonoduct is elongated into a "penis" that extends into the mantle cavity and through which spermatophores are ejected. In shallow water species, the penis is short, and the spermatophore is removed from the mantle cavity by a tentacle of the male, which is specially adapted for the purpose and known as ahectocotylus, and placed inside the mantle cavity of the female during mating.
The female has a large translucentovary, situated towards the posterior of the visceral mass. From here, eggs travel along the gonocoel, where there are a pair of whitenidamental glands, which lie anterior to the gills. Also present are red-spotted accessory nidamental glands containingsymbioticbacteria; both organs are associated with nutrient manufacture and forming shells for the eggs. The gonocoel enters the mantle cavity at thegonopore, and in some species, receptacles for storing spermatophores are located nearby, in the mantle wall.
In shallow-water species of thecontinental shelfandepipelagicormesopelagic zones, 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 squidof the genusArchiteuthisare 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 speciesOnykia 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 sessilebarnacles.
Like all cephalopods, squids are predators and have complex digestive systems. The mouth is equipped with a sharp, hornybeakmainly made of chitin andcross-linkedproteins,which is used to kill and tear prey into manageable pieces. The beak is very robust, but does not contain minerals, unlike the teeth and jaws of many other organisms; the cross-linked proteins are histidine- and glycine-rich and give the beak a stiffness and hardness greater than most equivalent synthetic organic materials.The stomachs of captured whales often have indigestible squid beaks inside. The mouth contains theradula, the rough tongue common to allmolluscsexceptbivalvia, which is equipped with multiple rows of teeth.In some species, toxicsalivahelps to control large prey; when subdued, the food can be torn in pieces by the beak, moved to the oesophagus by the radula, and swallowed.
The foodbolusis moved along the gut by waves of muscular contractions (peristalsis). The long oesophagus leads to a muscularstomachroughly in the middle of the visceral mass. Thedigestive gland, which is equivalent to a vertebrate liver, diverticulates here, as does thepancreas, and both of these empty into thecaecum, a pouch-shaped sac where most of the absorption of nutrients takes place.Indigestible food can be passed directly from the stomach to therectumwhere it joins the flow from the caecum and is voided through theanusinto the mantle cavity.Cephalopods are short-lived, and in mature squid, priority is given to reproduction;the femaleOnychoteuthis banksiifor example, sheds its feeding tentacles on reaching maturity, and becomes flaccid and weak after spawning.
The squid mantle cavity is a seawater-filled sac containing three hearts and other organs supporting circulation, respiration, andexcretion.Squid have a mainsystemicheart that pumps blood around the body as part of the generalcirculatory system, and twobranchial hearts. The systemic heart consists of three chambers, a lowerventricleand two upperatria, all of which can contract to propel the blood. The branchial hearts pump blood specifically to the gills for oxygenation, before returning it to the systemic heart.The blood contains thecopper-rich proteinhemocyanin, which is used foroxygen transportat low ocean temperatures and low oxygen concentrations, and makes the oxygenated blood a deep, blue color.As systemic blood returns via twovenae cavaeto the branchial hearts, excretion ofurine,carbon dioxide, and wastesolutesoccurs through outpockets (callednephridial appendages) in the venae cavae walls that enable gas exchange and excretion via the mantle cavity seawater.
Unlike nautiloids and cuttlefish which have gas-filled chambers inside their shells which provide buoyancy, and octopuses which live near and rest on the seabed and do not require to be buoyant, many squid have a fluid-filled receptacle, equivalent to theswim bladderof a fish, in thecoelomorconnective tissue. This reservoir acts as a chemical buoyancy chamber, with the heavy metallic cations typical of seawater replaced by lowmolecular-weightammoniumions, a product of excretion. The small difference in density provides a small contribution to buoyancy per unit volume, so the mechanism requires a large buoyancy chamber to be effective. Since the chamber is filled with liquid, it has the advantage over a swim bladder of not changing significantly in volume with pressure. Glass squids in the familyCranchiidaefor example, have an enormous transparent coelom containing ammonium ions and occupying about two-thirds the volume of the animal, allowing it to float at the required depth. About half of the 28 families of squid use this mechanism to solve their buoyancy issues.The familyBathyteuthidaeget their buoyancy from an oily substance found in their liver and around their mantle and head.
The majority of squid are no more than 60 cm (24 in) long, although the giant squid may reach 13 m (43 ft).The smallest species are probably the benthic pygmy squidsIdiosepius, which grow to a mantle length of 10 to 18 mm (0.4 to 0.7 in), and have short bodies and stubby arms.
In 1978, sharp, curved claws on the suction cups of squid tentacles cut up the rubber coating on the hull of theUSSStein. The size suggested the largest squid known at the time.
In 2003, a large specimen of an abundantbut poorly understood species,Mesonychoteuthis hamiltoni(thecolossal squid), was discovered. This species may grow to 10 m (33 ft) in length, making it the largest invertebrate.In February 2007, a New Zealand fishing vessel caught the largest squid ever documented, weighing 495 kg (1,091 lb) and measuring around 10 m (33 ft) 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.
The eggs of squid are large for a mollusc, containing a large amount of yolk to nourish the embryo as itdevelops directly, without an interveningveligerlarval stage. The embryo grows as adisc of cellsontop of the yolk. During thegastrulationstage, the margins of the disc grow to surround the yolk, forming a yolk sac, which eventually forms part of the animal's gut. The dorsal side of the disc grows upwards and forms the embryo, with a shell gland on its dorsal surface, gills, mantle and eyes. The arms and funnel develop as part of the foot on the ventral side of the disc. The arms later migrate upwards, coming to form a ring around the funnel and mouth. The yolk is gradually absorbed as the embryo grows. Some juvenile squid live higher in the water column than do adults. Squids tend to be short-lived;Loligofor example lives from one to three years according to species, typically dying soon after spawning.
In a well-studied bioluminescent species, the Hawaiian bobtail squid, a special light organ in the squid's mantle is rapidly colonized withAliivibrio fischeribacteria within hours of hatching. This light-organ colonization requires this particular bacterial species for a symbiotic relationship; no colonization occurs in the absence ofA. fischeri.Colonization occurs in a horizontal manner, such that the hosts acquires its bacterial partners from the environment. The symbiosis isobligatefor the squid, but facultative for the bacteria. Once the bacteria enter the squid, they colonize interiorepithelial cellsin the light organ, living incryptswith complexmicrovilliprotrusions. The bacteria also interact withhemocytes, 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.
Squid can move about in several different ways. Slow movement is achieved by a gentle undulation of the muscular lateral fins on either side of the trunk which drives the animal forward. A more common means of locomotion providing sustained movement is achieved using jetting, during which contraction of the muscular wall of the mantle cavity provides jet propulsion.
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 toflying fish, gliding through the air for up to 50 m (160 ft), and occasionally ending up on the decks of ships.
Squid arecarnivores, and, with their strong arms and suckers, can overwhelm relatively large animals efficiently. Prey is identified by sight or by touch, grabbed by the tentacles which can be shot out with great rapidity, brought back to within reach of the arms, and held by the hooks and suckers on their surface.In some species, the squid's saliva containstoxinswhich act to subdue the prey. These are injected into its bloodstream when the prey is bitten, along with vasodilators and chemicals to stimulate the heart, and quickly circulate to all parts of its body.The deep sea squidTaningia danaehas been filmed releasing blinding flashes of light from large photophores on its arms to illuminate and disorientate potential prey.
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 squidMastigoteuthishas the whole length of its whip-like tentacles covered with tiny suckers; it probably catches small organisms in the same way thatflypapertraps flies. The tentacles of somebathypelagicsquids bearphotophoreswhich may bring food within its reach by attracting prey.
Squid are among the most intelligent invertebrates. For example, groups ofHumboldt squidhunt cooperatively, spiralling up through the water at night and coordinating their vertical and horizontal movements while foraging.
Courtship in squid takes place in the open water and involves the male selecting a female, the female responding, and the transfer by the male of spermatophores to the female. In many instances, the male may display to identify himself to the female and drive off any potential competitors.Elaborate changes in body patterning take place in some species in both agonistic and courtship behaviour. TheCaribbean reef squid(Sepioteuthis sepioidea), for example, employs a complex array of colour changes during courtship and social interactions and has a range of about 16 body patterns in its repertoire.
The pair adopt a head-to-head position, and "jaw locking" may take place, in a similar manner to that adopted by somecichlidfish.The heterodactylus 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. InLoligo, 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.Loligosometimes 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.
Squid mostly have an annual life cycle, growing fast and dying soon after spawning. The diet changes as they grow but mostly consists of largezooplanktonand smallnekton. In Antarctica for example,krillis the main constituent of the diet, with other food items beingamphipods, other smallcrustaceans, and largearrow worms. Fish are also eaten, and some squid arecannibalistic.
As well as occupying a key role in the food chain, squid are an important prey for predators including sharks, sea birds, seals and whales. Juvenile squid provide part of the diet forwormsand small fish. When researchers studied the contents of the stomachs ofelephant sealsin South Georgia, they found 96% squid by weight.In a single day, a sperm whale can eat 700 to 800 squid,and aRisso's dolphinentangled in a net in the Mediterranean was found to have eatenangel clubhook squid,umbrella squid,reverse jewel squidandEuropean flying squid, all identifiable from their indigestible beaks.Ornithoteuthis volatilis, a common squid from the tropical Indo-Pacific, is predated byyellowfin tuna,longnose lancetfish,common dolphinfishandswordfish, thetiger shark, thescalloped hammerhead sharkand thesmooth hammerhead shark. Sperm whales also hunt this species extensively as does thebrown fur seal.In theSouthern Ocean,penguinsandwandering albatrossesare major predators ofGonatus antarcticus.
Giant squid have featured asmonsters of the deepsince classical times. Giant squid weredescribed by Aristotle(4th century BC) in hisHistory of AnimalsandPliny the Elder(1st century AD) in hisNatural History.TheGorgonofGreek mythologymay have been inspired by squid or octopus, the animal itself representing the severed head ofMedusa, the beak as the protruding tongue and fangs, and its tentacles as the snakes.The six-headed sea monster of theOdyssey,Scylla, may have had a similar origin. The Nordic legend of the kraken may also have derived from sightings of large cephalopods.
In literature,H. G. Wells' short story "The Sea Raiders" featured a man-eating squid speciesHaploteuthis ferox.Thescience fictionwriterJules Vernetold a tale of akraken-like monster in his 1870 novelTwenty Thousand Leagues Under the Seas.
Squid form a major food resource and are used in cuisines around the world, notably in Japan where it is eaten asika sōmen, sliced into vermicelli-like strips; assashimi; and astempura.Three species ofLoligoare used in large quantities:L. vulgarisin the Mediterranean (known asCalamarin Spanish,Calamaroin Italian);L. forbesiiin the Northeast Atlantic; andL. pealeion the American East Coast.Among the Ommastrephidae,Todarodes pacificusis the main commercial species, harvested in large quantities across the North Pacific in Canada, Japan and China.
In English-speaking countries, squid as food is often calledcalamari, adopted from Italian into English in the 17th century.Squid are found abundantly in certain areas, and provide large catches forfisheries. 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 forzincandmanganese, and high in copper,selenium,vitamin B12, andriboflavin.
According to theFAO, the cephalopod catch for 2002 was 3,173,272 tonnes (6.995867×10lb). Of this, 2,189,206 tonnes, or 75.8 percent, was squid.The following table lists squid species fishery catches that exceeded 10,000 tonnes (22,000,000 lb) in 2002.
Prototype chromatophores thatmimicthe squid's adaptive camouflage have been made byBristol Universityresearchers using an electroactivedielectricelastomer, a flexible "smart" material that changes its colour and texture in response to electrical signals. The researchers state that their goal is to create an artificial skin that provides rapid active camouflage.
The squid giant axon inspiredOtto Schmittto develop acomparatorcircuit withhysteresisnow called theSchmitt trigger, replicating the axon's propagation ofnerve impulses.