A species of whip-lash squid
Squids are marine cephalopods (class Cephalopoda, phylum Mollusca) with ten arms and tentacles (at some point in life), secondary armature on their suckers, and lacking the internal shell specific to cuttlefish. This large, diverse group of invertebrates comprise the order Teuthida (sometimes listed as order or suborder Teuthoidea) or, in some classifications, the orders Oegopsida and Myopsida (listed as suborders of Teuthida in some taxonomies). Like all cephalopods, squids are characterized by bilateral symmetry, a prominent head, and a modification of the mollusk foot into the form of arms or tentacles surrounding the mouth, which has beak-like jaws.
Squids belong to the subclass Coleoidea along with octopuses, cuttlefish, and extinct belemites. They differ from octopuses in that octopuses have eight arms and no tentacles, while squids and cuttlefish, at some point in their life cycle, have eight arms and two tentacles. (Tentacles tend to be longer than arms and usually have suckers as their tips only.) The suckers of squids also have hooks and/or sucker rings, while octopuses have simple suckers without secondary armature (O'Shea 2006). Squids differ from the squid-like cuttlefish in that cuttlefish have an internal shell (cuttlebone) on their back. Some squid species lose their tentacles in post-larval stages, and thus the adult only has eight arms (O'Shea 2006).
Squids not only play a key role in the marine food chains—they prey on fish, crustaceans, and other mollusks, and in turn are preyed on by fish and aquatic mammals, such as whales—but they also serve as a popular source of food for humans. Squids also offer aesthetic value, as a popular item in public aquariums and as a focal point in movies and literature.
There are about 300 species of squid, classified into 28 families. Some cephalopods whose common name includes "squid" (bobtail squid, ram's horn squid, vampire squid) are taxonomically different.
The colossal squid (Mesonychoteuthis hamiltoni) and giant squids (Architeuthisare sp.) are the world's largest invertebrates. Their tentacles are particularly formidable, having powerful suckers and deadly teeth at the ends of the tentacles. The teeth of the giant squid are small, "bottle-cap" shaped circular saws, while the tentacles of the colossal squid wield two long rows of thick, sharp, finger-length screws of protruding bone.
The majority of squid are no more than 60 centimeters long, although the giant squid (Architeuthisare sp.) may reach 13 meters (40 feet) in length. In 2003, however, a large specimen of an abundant but poorly understood species Mesonychoteuthis hamiltoni (the colossal squid) was discovered. This species may grow to 14 meters in length, making it the largest invertebrate in the world.
There have been reports of the largest being over 60 feet and weighing over one ton (Vecchione 2006). O'Shea (2006), however, discounts reports of Architeuthisare sp. being 60 feet in length as a myth, noting exaggeration of size by lengthening of specimens—stretching their tentacle arms like rubber bands. He likewise discounts reports of specimens weighing up to one ton. The size of Mesonychoteuthis hamiltoni, considered to be the largest squid, is based on estimates. Giant squids are featured in literature and folklore, with a strongly frightening connotation. Even the adult giant squids are not immune from predators—they show up in the stomach of sperm whales.
The tentacles are a type of muscular hydrostat and have suckers. If severed, the tentacles do not grow back.
Squid have chromatophores (pigment-containing and light-reflecting cells) embedded in their skin in the outer layer of mantle and they can change color and readily blend in with their surroundings to avoid predators. Squids also have the ability to expel ink if threatened.
A squid's bony structure is internalized. There is a single flat plate structure buried within the soft tissue, referred to as the gladius or pen.
Squids have a specialized foot called the siphon, or hyponome, that enables them to move by expelling water under pressure. Squids are the most skilled of the coleoids at this form of jet propulsion. In this method, oxygenated water is taken into the mantle cavity. Through muscular contraction of this cavity, the spent water is expelled through the hyponome, created by a fold in the mantle. Motion of squids is usually backward as water is forced out anteriorly through the hyponome, but direction can be controlled somewhat by pointing it in different directions, and a typical squid has fins, sometimes more than two (O'Shea 2006). While jet propulsion requires more energy than the means used by fish, and is normally slower, squids rival fish in their speed of swimming (Young et al. 1996).
The mouth of the squid is equipped with a sharp horny beak made of chitin, used to kill and tear prey into manageable pieces. Captured whales, which consume squid, 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 mollusks except bivalvia and aplacophora).
Squids are exclusively carnivorous, feeding on fish and other invertebrates. Squids usually have two elongated tentacles especially for the capture of food. They are voracious, fast-moving, and fast-growing predators, and can be hugely abundant in productive seas. Most live for just one year, dying after spawning, although some of the giant species may live for two years or more.
Many species of squid are popular as food in cuisines as widely separated as Korean and Italian. In English-speaking countries, it is often known by the name calamari, from the Greek-Italian word for these animals.
Individual species of squid are found abundantly in certain areas and provide large catches for fisheries.
When cooking, it is important to keep the cooking time brief as the flesh tends to toughen if overcooked. The body can be stuffed whole, cut into flat pieces, or sliced into rings. The tentacles and the ink are also edible; in fact, the only parts of the squid that are not eaten are its beak and the internal bony structure.
There are myriads of ways in which squid is eaten worldwide.
Squids are members of the class Cephalopoda, subclass Coleoidea, and order Teuthida, of which there are two major suborders, Myopsina and Oegopsina (which in some classifications are raised to the level of orders). 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 single species order Spirulida. The vampire squid is more closely related to the octopuses than to any of the squid, and is generally placed in its own order, Vampyromorphida, in the superorder Octopodiformes.
Young et al. (1996) recognize orders Oegopsida and Myopsida rather than suborders Oegopsina and Myopsina under Order Teuthida, among other changes.
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