General · EdgepediaSeptember 17, 20266 min read

Squid

A squid is a marine mollusc with an elongated soft body, large eyes, eight arms, and two contractile tentacles, belonging to several orders of the class Cephalopoda, the group that also contains cuttlefish, octopuses, and nautiluses.¹^•² True squids were long treated as the order Teuthoidea (or Teuthida), divided into the suborders Myopsida, the "covered-eyed" nearshore squids, and Oegopsida, the "open-eyed" oceanic squids; current classifications instead recognize these as separate orders alongside the bobtail squids (Sepiolida) and pygmy squids (Idiosepiida), which are also called squid although taxonomically distinct.³^•⁴^•⁵

Key fact	Detail
Body plan	Ten circumoral appendages: eight arms plus a fourth pair of contractile (but not retractile) tentacles; suckers with chitinous rings and/or hooks³
Internal shell	A simple, rod- or feather-shaped chitinous pen (gladius); the cuttlebone of cuttlefish and the ram's horn squid's coiled shell are convergent elaborations of internal shell structures³^•⁶
Eye distinction	Myopsida eyes are covered by a transparent corneal membrane with a minute pore; Oegopsida eyes are completely open to the sea, without a pore³
Species diversity	The FAO's 2010 world catalog covered 28 families, 83 genera, and 295 named squid species; Oegopsida alone accounts for roughly 55% of known decapodiform species⁷^•⁸
Evolutionary origin	Crown-group decapodiform orders arose rapidly around 101 million years ago (95% CI: 99–122.6 Ma), near the Early–Late Cretaceous boundary⁶
Fisheries	World cephalopod catch was about 3.3 million tonnes in 2001, roughly 3.6% of the total marine catch; a handful of commercial flying squid species account for half of total cephalopod landings⁹^•¹⁰
Locomotion	Swimming by flapping the fins or by jet propulsion through the siphon¹¹

Anatomy and diagnostic features

Squids are decapodiform cephalopods, meaning they carry ten appendages around the mouth. The diagnostic set given in the FAO species catalog includes contractile tentacles that cannot be retracted into pockets, sucker ornamentation of chitinous rings or hooks, a buccal membrane, olfactory organs of two projecting papillae, gills with a branchial canal, a single-structure liver, and an internal chitinous shell.³ The pen, or gladius, looks like clear plastic and supports the streamlined mantle.¹¹

The two traditional suborders differ most visibly in the eye. In Myopsida ("covered-eyed") the eye is covered with a transparent membrane bearing a minute pore; in Oegopsida ("open-eyed") the eye is completely open to the sea with no pore. Female anatomy differs as well: myopsids have a single oviduct and accessory nidamental glands, while oegopsids have paired oviducts and lack the accessory glands.³

Classification

The higher-level classification of squids has been revised repeatedly as molecular data accumulated. The traditional order Teuthida (Myopsida plus Oegopsida, following Naef, 1921/23) was supported by some early mitochondrial analyses, but the authoritative MolluscaBase outline now discards the taxon Teuthida Naef, 1916 and treats Myopsida and Oegopsida as separate orders, with Oegopsida containing most squids except the Loliginidae.¹²^•¹³ Two other decapodiform orders, Sepiolida (bobtail and bottletail squids) and Idiosepiida (pygmy squids), are also called squid but differ recognizably in gross anatomy.⁴^•⁵

Phylogenomic results. A 2020 phylogenomic study using genome and transcriptome data consistently recovered a clade comprising Myopsida, Sepiida (cuttlefishes), and Oegopsida as sister to a Sepiolida clade, with Idiosepiida as the sister group to all sampled decapodiform lineages.¹⁴ Within the oceanic squids, the sister relationship between Oegopsida and Bathyteuthida has remained stable across studies, supported by morphology, multiple molecular datasets, and a shared duplication of six mitochondrial genes.⁶^•¹⁰

Oegopsida itself is the most morphologically diverse cephalopod group at the family level. A 2021 genome-skimming study of 35 oceanic squids recovered the architeuthid, chiroteuthid, and enoploteuthid family groups as monophyletic, found the family Lycoteuthidae to be paraphyletic (containing Pyroteuthidae), and elevated its two subfamilies to family level, raising the number of oegopsid families from 24 to 25.¹⁰

Evolution

The FAO catalog traces the origin of the true squids to the early Mesozoic (Permian/Triassic), with steady proliferation from the Jurassic through the Recent.³ Fossil records of recent squids are sparse, which long limited knowledge of their detailed evolutionary history.¹²

Genome-scale work has clarified the timeline. A 2026 phylogenomic analysis with chromosome-scale genomes for the ram's horn squid Spirula spirula, the pygmy squid Idiosepius pygmaeus, and the Japanese flying squid Todarodes pacificus estimated a rapid origin of all crown-group decapodiform orders at about 101 Ma, close to the Early–Late Cretaceous boundary. The study interprets the cuttlebone of sepiids, the internal shell of spirulids, and the gladius of squids as convergent elaborations of internal shell structures from a simpler ancestral state, and describes a "long fuse" pattern: lineages diverged in the open ocean in the mid-Cretaceous but radiated into coastal environments only later, after the Cretaceous–Palaeogene extinction.⁶

Behavior and ecology

Squids swim by flapping their fins or by jet propulsion using the siphon, and often occur in schools.¹¹ Jet propulsion works because juvenile squid hatch already swimming in an inertial flow regime, where reciprocal paddling motions would otherwise be ineffective at small scales.¹⁵

Color change is central to squid behavior. The skin contains chromatophore organs, pigment cells under direct neural control; the primary function of cephalopod chromatophores is camouflage.¹⁶ When threatened, a squid can change color and expel a blob of purple ink that acts as a decoy while the animal escapes.¹¹

Oceanic squids occupy multiple trophic levels in mesopelagic food webs, preying on mesopelagic organisms and in turn eaten by sharks and toothed whales, and they can account for huge biomasses.¹⁰ Most species occur in the water column, though some are demersal or epibenthic at some point in their life cycle.³

Squid fisheries

Squid form the basis of major fisheries production worldwide.³ Increasingly heavy demand from Japan led to a commercial cephalopod fishery starting in the 1960s that rapidly expanded to global scope; reported cephalopod catch reached around 3.3 million tonnes in 2001, about 3.6% of the world total marine catch that year. The increase was driven largely by exploitation of southwest Atlantic squid, principally Illex argentinus, along with Todarodes pacificus in the northwest Pacific and Dosidicus gigas in the eastern Pacific.⁹ A handful of commercial flying squid species alone account for half of total world cephalopod fishery landings.¹⁰

Management challenges. Squids are short-lived, semelparous (individuals die after reproducing) ecological opportunists, and their populations fluctuate strongly; in 2002, 75% of the 2.18 million tonnes of reported world squid catch was identified to species, reflecting limited stock-structure knowledge for many fisheries.¹⁷ The neritic family Loliginidae is an important item in many industrial and small-scale fisheries; FAO's fishery yearbook reported about 120,000 metric tons for the genus Loligo in 1981, roughly 9% of the world cephalopod catch, plus another 480,000 metric tons of unidentified loliginids and ommastrephids.³

References

Topic: Encyclopedia › Life and health › Animals › Invertebrates › Molluscs › Cephalopods › Squids › Squid taxonomy & families

Initially written Sep 17, 2026 · Reviewed: Sep 30, 2026; Oct 21, 2026 · Edited: Sep 30, 2026; Oct 21, 2026 · Last review: Oct 21, 2026

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