Temporal range: (at least) Late Cretaceous–Recent
|European squid (Loligo vulgaris)|
A. Naef, 1916
Squid are cephalopods in the order Teuthida with elongated bodies, large eyes, eight arms and two tentacles. Like all other cephalopods, squid have a distinct head, bilateral symmetry, and a mantle. They are mainly soft-bodied, like octopuses, but have a small internal skeleton in the form of a rod-like gladius or pen, made of chitin.
- 1 Taxonomy and phylogeny
- 2 Description
- 3 Development
- 4 Behaviour
- 5 Human uses
- 6 See also
- 7 Notes
- 8 References
- 9 Sources
- 10 External links
Taxonomy and phylogeny
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, however, is more closely related to the octopuses than to any squid.
The cladogram, not fully resolved, is based on Sanchez et al, 2018. Their molecular phylogeny used mitochondrial 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".
Crown Coleoids (the ancestors of octopuses and squid) diverged at the end of the Paleozoic, in the Permian. Squid diverged during the Jurassic, but many squid families appeared in or after the Cretaceous.
Squid have differentiated from their ancestral molluscs 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 tentacles and highly developed sense organs, including advanced eyes similar to those of vertebrates.
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.
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. The foot has been transformed into a set of eight arms and two tentacles, which surround the mouth; each takes the form of a muscular hydrostat. These are flexible and prehensile, and usually bear disc-like suckers. In the mature male, the outer half of one of the left arms is hectocotylised – 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 swimming fin along each side. These fins are not the main source of locomotion in most species. The mantle wall is heavily muscled and inside, 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 gills (ctenidia) and openings from the excretory, digestive and reproductive systems. An inhalant siphon behind the funnel draws water into the mantel 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, 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 skin is covered in controllable chromatophores of different colours, enabling the squid to match its coloration to its surroundings. The play of colours may also distract prey from the squid's approaching tentacles. The skin also contains light reflectors called iridophores and leucophores which, when activated over milliseconds, create changeable skin patterns of polarized light. Such skin camouflage may serve various functions, such as contrast enhancement and 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 proteins.
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.
Squid distract attacking predators by ejecting a cloud of ink, 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 of melanin particles.
Cephalopods have the most highly developed nervous systems among invertebrates. Squids have a complex brain in the form of a nerve ring encircling the oesophagus, enclosed in a cartilaginous cranium. Paired cerebral ganglia above the oesophagus receive sensory information from the eyes and statocysts, and further ganglia below control the muscles of the mouth, foot, mantle and viscera. Giant axons up to 1 mm in diameter convey messages with great rapidity to the circular muscles of the mantle wall allowing a synchronous, powerful contraction and maximum speed via 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 globular lens that has a depth of focus from 3 cm (1 in) 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 Histioteuthidae family 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.
The sexes are separate in squid, there being a single gonad in the posterior part of the body with fertilisation being external, and usually taking place in the mantle cavity of the female. The male has a testis from 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" which 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 a hectocotylus, and placed inside the mantle cavity of the female during mating.
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 nidamental glands, which lie anterior to the gills. Also present are red-spotted accessory nidamental glands containing symbiotic 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 mesopelagic 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 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 barnacles.
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, including marine species. 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. In some species, toxic saliva helps 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 food bolus is moved along the gut by peristalsis. The long oesophagus leads to a muscular stomach which is found 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. 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.
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 which pumps blood to the general circulatory system, and two branchial hearts which pump specifically to the gills. The systemic heart consists of three chambers, a lower ventricle and two upper atria, which are all contractile. The blood contains the copper-rich protein, hemocyanin, which is used for oxygen transport at low ocean temperatures and low oxygen concentrations, and which makes the oxygenated blood a deep, blue color. As systemic blood returns via two vena cavae to the branchial hearts, excretion of urine, carbon dioxide, and waste solutes occurs through outpockets (called nephridial appendages) in the vena cavae walls which enable gas exchange and excretion via the mantle cavity seawater.
In 2003, a large specimen of an abundant but poorly understood species, Mesonychoteuthis hamiltoni (the colossal squid), was discovered. This species may grow to 14 m (46 ft) in length, making it the largest invertebrate. Squid have the largest eyes in the animal kingdom. The kraken is a legendary tentacled monster possibly based on sightings of real giant squid.
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.
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. Cleavage is superficial and a germinal disc develops at the pole. During gastrulation, the margins of the germinal 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.
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. 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 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.
According to the FAO, the cephalopod catch for 2002 was 3,173,272 tonnes (6.995867×109 lb). Of this, 2,189,206 tonnes, or 75.8 percent, was squid. The following table lists the squid species fishery catches which exceeded 10,000 tonnes (22,000,000 lb) 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 form a major food resource and are used in cuisines around the world, notably in Japan where it is eaten as ika-somen, sliced into vermicelli-like strips; as sashimi; and as tempura. Three species of Loligo are used in large quantities, L. vulgaris in the Mediterranean (known as Calamar in Spanish, Calamaro in Italian); L. forbesii in the Northeast Atlantic; and L. pealei on the American East Coast. Among the Ommastrephidae, Todarodes pacificus is 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 called calamari, adopted from Italian into English in the 17th century. 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.
In literature and art
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; the science fiction writer Jules Verne told a tale of a kraken-like monster in his 1870 novel Twenty Thousand Leagues Under the Sea.
Prototype chromatophores, mimicking 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 will provide rapid active camouflage.
- Common name is however shared with Mastigoteuthidae.
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