Squid are a large, diverse group of marine cephalopods. Like all cephalopods, squid are distinguished by having a distinct head, bilateral symmetry, a mantle, and arms. Squid, like cuttlefish, have eight arms and two tentacles arranged in pairs.
Modification from ancestral forms
Squid have differentiated from their ancestral molluscs in such a way that the body plan has been condensed antero-posteriorly and extended dorso-ventrally. What before may have been the foot of the ancestor is now modified into a complex set of tentacles and highly developed sense organs, including advanced eyes similar to those of vertebrates.
The shell of the ancestor has been lost, with only an internal gladius, or pen, remaining.
European Squid (Loligo vulgaris)
The main body mass of the squid is enclosed in the mantle, which has a swimming fin along each side. It should be noted that these fins, unlike in other marine organisms, are not the main source of ambulation in most species. The skin of the squid is covered in chromatophores, which enable the squid to change color to suit its surroundings. The underside of the squid is also found to be lighter than the topside, in order to provide camouflage from both prey and predator.
Under the body are openings to the mantle cavity, which contains the gills (ctenidia) and openings to the excretory and reproductive systems. At the front of the mantle cavity lies the siphon, which the squid uses for locomotion via precise jet propulsion. This is done by sucking water into the mantle cavity and quickly expelling it out of the siphon in a fast, strong jet. The direction of the siphon can be changed in order to suit the direction of travel.
Inside the mantle cavity, beyond the siphon, lies the visceral mass of the squid, which is covered by a thin, membranous epidermis. Under this are all the major internal organs of the squid.
Ventral view of the viscera of Chtenopteryx sicula.
In female squid, the ink sac is hidden from view by a pair of white nidamental glands, which lie anterior to the gills. There are also red-spotted accessory nidamental glands. Both of these organs are associated with manufacture of food supplies and shells for the eggs. Females also have a large translucent ovary, situated towards the posterior of the visceral mass.
Male squid do not possess these organs, but instead have a large testis in place of the ovary, and a spermatophoric gland and sac. In mature males, this sac may contain spermatophores, which are placed inside the mantle of the female during mating.
Squid, like all cephalopods, have complex digestive systems. Food is transported into a muscular stomach, found roughly in the midpoint of the visceral mass. The bolus is then transported into the caecum for digestion. The caecum, a long, white organ, is found next to the ovary or testis. In mature squid, more priority is given to reproduction and so the stomach and caecum often shrivel up during the later stages of life. Finally, food goes to the liver (or digestive gland), found at the siphon end of the squid, for absorption. Solid waste is passed out of the rectum. Beside the rectum is the ink sac, which allows a squid to discharge a black ink into the mantle cavity at short notice.
Squid have three hearts. Two branchial hearts, feeding the gills, each surrounding the larger systemic heart that pumps blood around the body. The hearts have a faint greenish appearance and are surrounded by the renal sacs – the main excretory system of the squid. The kidneys are faint and difficult to identify and stretch from the hearts (located at the posterior side of the ink sac) to the liver. The systemic heart is made of three chambers, a lower ventricle and two upper auricles.
The head end of the squid bears 8 arms and two tentacles, each a form of muscular hydrostat containing many suckers along the edge. These tentacles do not grow back if severed. In the mature male squid, one basal half of the left ventral tentacle is hectocotylised — and ends in a copulatory pad rather than suckers. It is used for intercourse between mature males and females.
The mouth of the squid is equipped with a sharp horny beak mainly made of chitin  and cross-linked proteins, and is used to kill and tear prey into manageable pieces. The beak is very robust, but does not contain any minerals, unlike the teeth and jaws of many other organisms, including marine species. Captured whales often have squid beaks in their stomachs, the beak being the only indigestible part of the squid. The mouth contains the radula (the rough tongue common to all molluscs except bivalvia and aplacophora).
The majority of squid are no more than 60 cm long, although the giant squid may reach 13 m in length. 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 in length, making it the largest invertebrate. It also possesses the largest eyes in the animal kingdom. Giant squids are often featured in literature and folklore with a frightening connotation. The Kraken is a legendary tentacled monster possibly based on sightings of real giant squids.
In February 2007, a Colossal Squid weighing 495 kg (1,091 lb). and about 10 metres (33 ft) long, was caught by a New Zealand fishing vessel off the coast of Antarctica. The specimen is currently frozen while scientists plan how best to examine it.
Squid are members of the class Cephalopoda, subclass Coleoidea, order Teuthida, of which there are two major suborders, Myopsina and Oegopsina (including the giant squids like Architeuthis dux). Teuthida is the largest of the cephalopod orders, edging out the octopuses (order Octopoda) for total number of species, with 298 classified into 28 families.
The order Teuthida is a member of the superorder Decapodiformes (from the Greek for “ten legs”). Two other orders of decapodiform cephalopods are also called squid, although they are taxonomically distinct from Teuthida 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 of the squid.
Breaded calamari: Grilled and breaded squid stuffed with vegetable and rice
Fried baby squid is a dish commonly associated with Chinese cuisine.
Many species of squid are popular as food in cuisines as diverse and separated as Korean and Italian. In English-speaking countries, it is often known by the name calamari. Individual species of 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; in fact, the only parts of the squid that are not eaten are its beak and gladius (pen).
There are many ways in which squid is eaten worldwide.
- In Italy, Turkey and Greece, squid rings and arms are often coated in batter and fried in oil. Other recipes from these regions feature squid (or octopus) simmered slowly, often with tomatoes. When frying, the squid flesh is kept tender by keeping the cooking time as short as possible. When simmering, the flesh is most tender when the cooking time is prolonged and reduced in temperature.
- In Spain a similar recipe (Calamares a la romana, battered calamari, lit. roman-style calamari) has the calamari rings covered in a much thicker batter, deep fried and accompanied with lemon juice and mayonnaise or garlic mayonnaise.
- Also in Spain, cooked in its own black ink (Calamares en su tinta).
- In the Philippines squid is most often cooked as adobong pusit, squid in adobo sauce, along with the ink, so it resembles the spanish calamares en su tinta, but with a tangier flavour, especially when fresh chillies are added. In restaurants, the common style is battered calamari served with alioli, mayonnaise or chilli vinegar. In casual bars, beer gardens and on the beaches, squid is grilled on coals, brushed with a soy sauce-based marinade, and sometimes stuffed with a tomato-onion filling.
- In Korea, live squid is freshly taken from a tank, killed, cleaned and served quickly. Unlike octopus served in a similar fashion however, squid tentacles do not usually continue to move for long enough to reach the dinner table. This type of fresh squid is called 산 오징어 (‘san ojingo’) (also with small octopuses called nakji). The squid is served with wasabi/soy sauce, chili pepper sauce or sesame sauce with salt and often wrapped in lettuce or pillard leaves.
- In the Mediterranean, squid or cuttlefish ink is eaten in a variety of dishes such as paella, risotto, soups and pasta; Spaghetti al Nero di Seppia (cuttlefish) being an example.
- In Croatia squid are often eaten grilled and stuffed with pršut and cheese, accompanied by blitva (Swiss Chard)
- Seafood stews often contain squid.
- In Chinese and South East Asian cuisine, squid is a common ingredient in a variety of dishes such as stir-fries, rice and noodle dishes. It is often heavily spiced.
- Whole grilled squid is a common food item in Asia; they are popular at food stalls in China, Thailand, Japan and Taiwan. In Sardinia they are accompanied by a sauce made from lemon, garlic, parsley, and olive oil.
- Pre-packaged Dried Shredded Squid or cuttlefish are popular snack items in Hong Kong, Taiwan, Japan, Russia, and most other former USSR regions mostly sold in a shredded form due to its chewiness.
- Squid is a common sushi and sashimi item.
- In Japan and Korea, squid (usually Firefly Squid or Spear Squid) is often made into shiokara (in Japanese) or chotkal (in Korean). Heavily salted squid is left to ferment, sometimes with its innards, for up to a month, and is sold in small jars. This salty, strong flavoured item is served in small quantities as an accompaniment to white rice or alcoholic drinks.
- In Korea, dried squid is also a popular accompaniment for alcoholic beverages. Dried squid is often served with peanuts. Squid is also served roasted, with hot pepper paste and/or mayonnaise as a dip sauce. Steamed squid, or boiled squid, is also a delicacy.
- In Turkey, it can be an ingredient in dolma.
- In India and Sri Lanka, squid or cuttlefish is popular in coastal areas mainly in Tamilnadu and in Kerala. Squid are often eaten as deep fried or squid gravy. In Tamilnadu, squid are often called kanava or kadamba.
In popular culture
The giant squid is featured in Jules Verne‘s Twenty Thousand Leagues Under the Sea, in which one attacks Captain Nemo‘s ship, the Nautilus. It is also featured in the Harry Potter series, where it is friendly rather than dangerous.
Three species of future squid appeared in The Future Is Wild. They were the ocean-going Rainbow Squid and the two land-dwelling squid known as the Megasquid and the Squibbon.
Miserez, A (2007). “Jumbo squid beaks: Inspiration for design of robust organic composites”. Acta Biomaterialia. 3: 139–149. Unknown parameter |coauthors= ignored (|author= suggested) (help)