Squid are cephalopods with elongated bodies, big eyes, eight limbs, and two tentacles that belong to the Decapodiformes superorder. Squid have a distinct head, bilateral symmetry, and a mantle, much like all other cephalopods. They are mostly soft-bodied, like octopuses, but they contain a tiny internal skeleton formed of chitin in the shape of a rod-like gladius or pen.

During the Jurassic period, squid separated from other cephalopods and now serve in a similar capacity to teleost fish as open-water predators of similar size and behavior. In the open water food web, they play an essential role. The eight arms are used to grip and manipulate prey, while the two lengthy tentacles are utilized to grasp it. The beak next chops the meal into bite-sized bits that can be swallowed.

Squid are fast swimmers who use jet propulsion to propel themselves and rely on sight to seek their food. Humboldt squid have been recorded hunting collaboratively in groups, making them one of the most intelligent invertebrates. Sharks, other fish, marine birds, seals, and cetaceans, notably sperm whales, feed on them.

For concealment and signaling, squid may change color. Many species may expel a cloud of ink to distract predators, while some are bioluminescent and use their light for counter-illumination concealment.

Commercial fisheries in Japan, the Mediterranean, the southwestern Atlantic, the eastern Pacific, and elsewhere employ squid for human food. They are commonly referred to as “calamari” and are used in a variety of cuisines across the world. Since classical times, squid have been depicted in literature, particularly in tales of enormous squid and sea monsters.

At the end of the Paleozoic, in the Permian, crown coleoids (the progenitors of octopuses and squid) diverged. During the Jurassic, squid diverged, although many squid families arose during or after the Cretaceous. Both the coleoids and the teleost fish underwent a lot of adaptive radiation at the same time, and the two current groups are quite similar in size, ecology, habitat, anatomy, and behavior. However, some fish migrated into fresh water while the coleoids stayed in the sea.

The nautiloid-like ancestral coleoid had a straight septate shell that got submerged in the mantle and was utilized for buoyancy control. Spirulida (with one live member), cuttlefishes, squids, and octopuses were the four lineages that split off from this.

The body plan of squid has been compacted antero-posteriorly and expanded dorso-ventrally as it has evolved from the ancestral mollusk. What may have been the ancestor’s foot has been transformed into a complicated collection of appendages that surround the mouth. The sensory organs are well-developed, with sophisticated eyes that resemble those of vertebrates.

Squid are mollusks with soft bodies that have evolved to live an active predatory lifestyle. The squid’s head and foot are located at one end of a long body, which is functionally anterior and leads the animal through the water. The mouth is surrounded by eight arms and two different tentacles; each appendage is in the shape of a muscular hydrostat, is flexible and prehensile, and generally has disc-like suckers.

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