Squid

Squid are members of the class Cephalopoda, subclass Coleoidea. The squid orders Myopsida and Oegopsida are in the superorder Decapodiformes (from the Greek for 'ten-legged'). Two other orders of decapodiform cephalopods are also called squid, although they are taxonomically distinct from squids and differ recognizably in their gross anatomical features. They are the bobtail squid of order Sepiolida and the ram's horn squid of the monotypic order Spirulida. The vampire squid (Vampyroteuthis infernalis), however, is more closely related to the octopus than to any squid. The cladogram, not fully resolved, is based on Sanchez et al., 2018. True squid diverged from other cephalopods during the Jurassic, but only emerged to prominence at the boundary between the Early and Late Cretaceous, around 100 million years ago. 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 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 belemnites. Even their beaks, 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 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 ammonoids by that point. File:Rhomboteuthis lehmani (cropped).jpg|Fossil Rhomboteuthis from the Lower Callovian (, middle Jurassic) of La Voulte-sur-Rhône, France File:Plesioteuthis prisca 01.jpg|Fossil Plesioteuthis from the Tithonian (, upper Jurassic), Solnhofen, Germany ==Description==

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 functionally anterior, 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 a muscular hydrostat and is flexible and prehensile, usually bearing disc-like suckers. 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. Camouflage Squid make use of different kinds of camouflage, namely active camouflage for background matching (in shallow water) and counter-illumination. This helps to protect them from their predators and allows them to approach their prey. The play of colours may in addition distract prey from the squid's approaching tentacles. The skin also contains light reflectors called iridophores and leucophores that, when activated, in milliseconds 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. This creates the effect of countershading, making the underside lighter than the upperside. 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. 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 of melanin particles 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 their chemoreceptors. 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. 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. However, most deep-sea squid lack hectocotyl arms and have longer penises; Ancistrocheiridae and Cranchiinae are exceptions. 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 barnacles. Digestive system ''|alt=Diagram labeling siphon, intestine, nidamental gland, accessory nidamental gland, renal pore, and branchial heart Like all cephalopods, squids are predators and have complex digestive systems. The mouth is equipped with a sharp, horny beak mainly made of chitin and cross-linked proteins, 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 the radula, the rough tongue common to all molluscs except bivalvia, which is equipped with multiple rows of teeth. 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 digestive gland, which is equivalent to a vertebrate liver, diverticulates here, as does the pancreas, and both of these empty into the caecum, a pouch-shaped sac where most of the absorption of nutrients takes place. the female Onychoteuthis banksii for example, sheds its feeding tentacles on reaching maturity, and becomes flaccid and weak after spawning. Cardiovascular and excretory systems The squid mantle cavity is a seawater-filled sac containing three hearts and other organs supporting circulation, respiration, and excretion. Squid have a main systemic heart that pumps blood around the body as part of the general circulatory system, and two branchial hearts. The systemic heart consists of three chambers, a lower ventricle and two upper atria, 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. Largest and smallest . The bars are a metre (3 feet) apart.|alt=Photo of squid with prominent eye The majority of squid are no more than long, although the giant squid may reach . The smallest species are probably the benthic pygmy squids Idiosepius, which grow to a mantle length of , 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 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. ==Development==

The eggs of squid are large for a mollusc, containing a large amount of yolk to nourish the embryo as it develops directly, without an intervening veliger larval stage. The embryo grows as a disc of cells on top of the yolk. During the gastrulation stage, 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; Loligo for example lives from one to three years according to species, typically dying soon after spawning. Approximately 95% of the bacteria are voided each morning before the bacterial population builds up again by nightfall. ==Behaviour==

Locomotion 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. '' are covered with tiny suckers to catch small organisms like flypaper. 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. 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. Reproduction (Sepioteuthis sepioidea) employs a complex array of colour changes during courtship and social interactions. 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. The Caribbean 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 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. ==Ecology==

Squid mostly have an annual life cycle, growing fast and dying soon after spawning. The diet changes as they grow but mostly consists of large zooplankton and small nekton. In Antarctica for example, krill is the main constituent of the diet, with other food items being amphipods, other small crustaceans, and large arrow worms. Fish are also eaten, and some squid are cannibalistic. 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 worms 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 elephant seals in South Georgia, they found 96% squid by weight. Ornithoteuthis volatilis, a common squid from the tropical Indo-Pacific, is predated by yellowfin tuna, longnose lancetfish, common dolphinfish 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, penguins and wandering albatrosses are major predators of Gonatus antarcticus. == Parasites ==

Squid can be hosts to a variety of different parasites that make them unsafe for human consumption. The most harmful is a zoonotic Anisakis nematode also known as herring worm disease. The larvae of this parasite use the squid as an intermediate host and typically do not harm the squid, but still propose a large risk if consumed by humans. 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. Other common squid parasites • Dicyemids: Wormlike parasites from the phylum Rhombozoan that are typically found in the renal sac of cephalopods. To young, healthy squid they are harmless and can be symbiotic, but to older, stressed squids they cause tissue damage, inflammation, and hypertrophy. • Ciliates: Chromidina ciliates are the species most known to infect cephalopods like pelagic squid and octopus. • Parasitic Copepods: Copepods are a group of free-living organisms known either for forming symbiotic relationships with other marine animals or using them as parasitic hosts. In 2024, a new species of parasitic copepods was discovered named Ikanecator primu. This parasite is responsible for the infestation of a large portion of squid egg clutches in squids being held at the Physics and Biology Unit at OIST. They pose an extreme health threat to copepods living in aquaculture containments. ==Human uses==

In literature and art , by Alphonse de Neuville to illustrate Jules Verne's Twenty Thousand Leagues Under the Seas, 1870 Giant squid have featured as monsters of the deep since classical times. Giant squid were described by Aristotle (4th century BC) in his History of Animals and Pliny the Elder (1st century AD) in his Natural History. The Gorgon of Greek mythology may have been inspired by squid or octopus, the animal itself representing the severed head of Medusa, the beak as the protruding tongue and fangs, and its tentacles as the snakes. The six-headed sea monster of the Odyssey, 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 species Haploteuthis ferox. Chapter 59 of Herman Melville's novel Moby-Dick is devoted to an encounter with a large, white squid, "furlongs in length and breadth," which is taken as an ill harbinger, frightening Starbuck; this description may be ascribed to poetic license or hyperbole, but naturalists have pointed out that no squids fitting this description are known to exist. The science fiction writer Jules Verne told a tale of a kraken-like monster in his 1870 novel Twenty Thousand Leagues Under the Seas. Three species of Loligo are used in large quantities: L. vulgaris in the Mediterranean (known as in Spanish, in Italian); L. forbesii in the Northeast Atlantic; and L. pealei on the American East Coast. Squid are found abundantly in certain areas, and provide large catches for fisheries. 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. Commercial fishing According to the FAO, the cephalopod catch for 2002 was . Of this, 2,189,206 tonnes, or 75.8 percent, was squid. The following table lists squid species fishery catches that exceeded in 2002. In biomimicry the squid giant axon removes noise from noisy analog input (U), where ordinary comparator (A) does not. Green dashed lines are thresholds. Prototype chromatophores that mimic the squid's adaptive camouflage have been made by Bristol University researchers using an electroactive dielectric elastomer, 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 inspired Otto Schmitt to develop a comparator circuit with hysteresis now called the Schmitt trigger, replicating the axon's propagation of nerve impulses. ==See also==