Squid
Squid Temporal range:
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Caribbean reef squid (Sepioteuthis sepioidea) | |
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Mollusca |
Class: | Cephalopoda |
Subclass: | Coleoidea |
(unranked): | Neocoleoidea |
Superorder: | Decapodiformes |
Groups included[1] | |
Cladistically included but traditionally excluded taxa | |
A squid (pl.: squid) is a
Squid diverged from other cephalopods during the
Squid can change colour for
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 sea monsters.
Taxonomy and phylogeny
Squid are members of the class
The
Cephalopoda
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Evolution
Crown coleoids (the common ancestor of octopuses and squid) diverged in the late Paleozoic (Mississippian), according to fossils of Syllipsimopodi, an early relative of vampire squids and octopuses.[3] True squid diverged during the Jurassic, but many squid families appeared in or after the Cretaceous.[4] 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.[5]
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), the
The ancestral shell has been lost, with only an internal gladius, or pen, remaining. The pen, made of a chitin-like material,[5][6] 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.[7]
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Fossil Rhomboteuthis from the Lower Callovian (c. 164 Mya, middle Jurassic) of La Voulte-sur-Rhône, France
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FossilSolnhofen, 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
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.[8] 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.[5]
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.[5]
The main body mass is enclosed in the mantle, which has a
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 mantle cavity via a valve. The squid uses the funnel for locomotion via precise jet propulsion.[9] 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.[5] Squid are strong swimmers and certain species can "fly" for short distances out of the water.[10]
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.[11][12]
The skin is covered in controllable
Some
Counter-illumination is also used by the
-
Controllable chromatophores of different colours in the skin of a squid allow it to change its coloration and patterns rapidly, whether for camouflage or signalling.
-
Principle ofpredator, the animal's light helps to match its brightness and colour to the sea surface above.
Predator distraction with ink
Squid distract attacking predators by ejecting a cloud of ink, giving themselves an opportunity to escape.[20][21] 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.[8] 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.[5]
Nervous system and sense organs
Cephalopods have the most highly developed nervous systems among
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
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.[5] Squid appear to have limited hearing,[23] 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.[5]
Reproductive system
The sexes are separate in squid, with a single
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.[5]
In shallow-water species of the
Digestive system
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,[27] 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.[28] 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.[5] 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.[29]
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.[5] 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.[5] Cephalopods are short-lived, and in mature squid, priority is given to reproduction;[30] the female Onychoteuthis banksii for example, sheds its feeding tentacles on reaching maturity, and becomes flaccid and weak after spawning.[31][32]
Cardiovascular and excretory systems
The squid mantle cavity is a seawater-filled sac containing three hearts and other organs supporting circulation, respiration, and
Buoyancy
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 the swim bladder of a fish, in the coelom or connective tissue. This reservoir acts as a chemical buoyancy chamber, with the heavy metallic cations typical of seawater replaced by low molecular-weight ammonium ions, 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 family Cranchiidae for 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.[5] The family Bathyteuthidae get their buoyancy from an oily substance found in their liver and around their mantle and head.[34]
Largest and smallest
The majority of squid are no more than 60 cm (24 in) long, although the giant squid may reach 13 m (43 ft).[35] The smallest species are probably the benthic pygmy squids Idiosepius, which grow to a mantle length of 10 to 18 mm (0.4 to 0.7 in), and have short bodies and stubby arms.[36]
In 1978, sharp, curved claws on the suction cups of squid tentacles cut up the rubber coating on the hull of the
In 2003, a large specimen of an abundant[38] but poorly understood species, Mesonychoteuthis hamiltoni (the colossal squid), was discovered. This species may grow to 10 m (33 ft) in length, making it the largest invertebrate.[39] 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.[40] 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.[41]
Development
The eggs of squid are large for a mollusc, containing a large amount of yolk to nourish the embryo as it
In a well-studied bioluminescent species, the Hawaiian bobtail squid, a special light organ in the squid's mantle is rapidly colonized with
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.[5]
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.[5]
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 50 m (160 ft), and occasionally ending up on the decks of ships.[5]
Feeding
Squid are carnivores, 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.[43] In some species, the squid's saliva contains toxins which 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.[5] The deep sea squid Taningia danae has been filmed releasing blinding flashes of light from large photophores on its arms to illuminate and disorientate potential prey.[44]
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.[45]
Reproduction
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.[46] 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.[47]
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.[48] 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.[5]
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.[5]
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.[49]
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 for
Human uses
In literature and art
Giant squid have featured as
In literature,
As food
Squid form a major food resource and are used in cuisines around the world, notably in Japan where it is eaten as
In English-speaking countries, squid as food is often called
Commercial fishing
According to the
Species | Family | Common name | Catch tonnes |
Percent |
---|---|---|---|---|
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[c] | 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[c] | Various | 414,990 | 18.6 | |
Total squid | 2,189,206 | 100.0 |
In biomimicry
Prototype chromatophores that
The squid giant axon inspired
See also
Notes
- Mastigoteuthidae
- ^ common name (whip-lash) is shared with Chiroteuthidae
- ^ a b Nei: not elsewhere included
References
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