Squid
A squid is a mollusc with an elongated soft body, large eyes, eight arms, and two tentacles in the orders Myopsida, Oegopsida, and Bathyteuthida. 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.
Squid diverged from other cephalopods during the Jurassic and radiated at the beginning of the Late Cretaceous, and occupy a similar role to teleost fish as open-water predators of similar size and behaviour. They play an important role in the open-water food web. The two long tentacles are used to grab prey and the eight arms to hold and control it. The beak then cuts the food into suitable size chunks for swallowing. Squid are rapid swimmers, moving by jet propulsion, and largely locate their prey by sight. They are among the most intelligent of invertebrates, with groups of Humboldt squid having been observed hunting cooperatively. They are preyed on by sharks, other fish, sea birds, seals and cetaceans, particularly sperm whales.
Squid can change colour for camouflage and signalling. Some species are bioluminescent, using their light for counter-illumination camouflage, while many species can eject a cloud of ink to distract predators.
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 Cephalopoda, subclass Coleoidea. The squid orders Myopsida and Oegopsida are in the superorder Decapodiformes. 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 octopus 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", except Sepiadariidae and Sepiidae that are in the polyphyletic order "Sepiida",
Evolution
coleoids diverged in the late Paleozoic, according to fossils of Syllipsimopodi, an early relative of vampire squids and octopuses. True squid diverged from other cephalopods during the Jurassic, but only emerged to prominence at the boundary between the Early and Late Cretaceous, around 100 million years ago. 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.The ancestral coleoid was probably nautiloid-like with a straight septate shell that became immersed in the mantle and was used for buoyancy control. Four lines diverged from this, Spirulida, the cuttlefishes, the squids and the octopuses. Squid have differentiated from the ancestral mollusc 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 appendages around the mouth. The sense organs are highly developed and include 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.
Due to their soft bodies and tendency to easily decay, fossil remains of true squid are very rare in the Mesozoic fossil record, in contrast to those of the superficially similar belemnites. Even their beaks, the only hard part of them that can sometimes fossilize, tend to be extremely fragile and can be easily destroyed during preparation of the surrounding rock. The lack of squid remains in the Mesozoic fossil record has skewed calibrations over the phylogenetic divergence times of squid, with some studies suggesting that they may have only evolved during the Cenozoic, although more recent discoveries have clarified and refuted this. In 2006 and 2015, the fossil squid genera Yezoteuthis and Haboroteuthis were identified from the Late Cretaceous-aged Yezo Group of Japan based on fossil beaks.
In 2025, multiple fossil squid taxa belonging to both the Oegopsida and Myopsida were described based on fossil beaks from the Yezo Group, with records of both found dating back to the earliest Cenomanian. The authors resolved the fragility of the fossil beaks by scanning them and the surrounding matrix with grinding tomography. The abundance of squid beaks in carbonite concretions, which would have only formed within a few weeks, suggests that squid had likely already formed large populations by the mid-Cretaceous despite their paucity in the fossil record. Their total biomass would have likely already exceeded that of marine fishes and even ammonoids by that point.
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 anterior, leading the animal as it moves through the water. A set of eight arms and two distinctive tentacles surround the mouth; each appendage takes the form of a muscular hydrostat and is flexible and prehensile, usually bearing disc-like suckers.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. 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.
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 internal. 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 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. 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.
Camouflage
Squid make use of different kinds of camouflage, namely active camouflage for background matching 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 in addition distract prey from the squid's approaching tentacles. The skin also contains light reflectors called iridophores and leucophores that, when activated, in milliseconds create changeable skin patterns of polarized light. Such skin camouflage may serve various functions, such as 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 and the midwater squid 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.
Counter-illumination is also used by the Hawaiian bobtail squid, which has symbiotic bacteria that produce light to help the squid avoid nocturnal predators. This light shines through the squid's skin on its underside and is generated by a large and complex two-lobed light organ inside the squid's mantle cavity. From there, it escapes downwards, some of it travelling directly, some coming off a reflector at the top of the organ. Below there is a kind of iris, which has branches of its ink sac, with a lens below that; both the reflector and lens are derived from mesoderm. The squid controls light production by changing the shape of its iris or adjusting the strength of yellow filters on its underside, which presumably change the balance of wavelengths emitted. Light production shows a correlation with intensity of down-welling light, but it is about one third as bright; the squid can track repeated changes in brightness. Because the Hawaiian bobtail squid hides in sand during the day to avoid predators, it does not use counter-illumination during daylight hours.