Squid

A squid (pl. 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. 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. Squid are rapid swimmers, moving by jet propulsion, and largely locate their prey by sight. They are among the most intelligent of invertebrates.

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".

Taxonomy and phylogeny

Squid belong to the class Cephalopoda and are part of the subclass Coleoidea. They are mainly found in the orders Myopsida and Oegopsida, which are part of the superorder Decapodiformes. Other types often called squid, like the bobtail squid and ram's horn squid, are different from true squid. The vampire squid is actually more closely related to octopuses.

Squid evolved from a common ancestor with octopuses in the late Paleozoic period. True squid appeared during the Jurassic period and became more common around 100 million years ago. Over time, squid developed their distinctive body shape and lost their outer shell, keeping only a small internal structure called a gladius for support. Because their bodies break down easily, fossil squid are rare, but recent discoveries have helped scientists learn more about their ancient history.

Description

Squid are soft-bodied animals with long bodies, large eyes, eight arms, and two tentacles. They move through the water using a funnel to push water out and propel themselves forward. Their arms and tentacles have suction cups to grip prey, and some species also have hooks.

Squid can change colors using special cells in their skin called chromatophores, helping them hide from predators or sneak up on their prey. They also release a cloud of black ink to confuse predators and escape. Squid have well-developed nervous systems and excellent eyesight, with eyes similar to those of fish. They are strong swimmers and can even "fly" short distances above water.

Development

Squid eggs are large for a mollusc and contain a lot of yolk to feed the growing embryo. The embryo develops directly without a larval stage, forming a disc of cells on top of the yolk. As it grows, the embryo develops parts like the shell gland, gills, mantle, eyes, arms, and funnel. The arms eventually form a ring around the funnel and mouth, and the yolk is used up as the embryo grows.

Some squid, like the Hawaiian bobtail squid, have a special light organ that hosts glowing bacteria right after hatching. These bacteria help the squid by providing light, and this relationship is important for the squid but not necessary for the bacteria. The bacteria live in special parts of the light organ and glow more in the evening than in the morning.

Behaviour

Squid move in several ways. They can glide slowly by moving their side fins, or they can jet forward by pushing water out of their bodies like a boat’s engine. When scared, they jet very fast, shooting out extra water to zoom backward and sometimes even leap out of the water like flying fish.

Squid are meat-eaters with strong arms and suckers that help them catch prey. They spot food with their eyes or touch, then shoot out their tentacles to grab it. Some squid even use flashing lights on their arms to confuse their prey. Though their mouths are small, they have sharp beaks to chop food into pieces before swallowing.

Squid are clever animals. Groups of Humboldt squid work together to hunt, moving in patterns through the water. During courtship, males and females change colors and patterns to communicate. The male then uses a special arm to place sperm into the female, who later lays eggs that she attaches to surfaces or lets float freely in the water.

Ecology

Squid live fast and have short lives, growing quickly and dying after they reproduce. They eat things like small animals that float in the water and other sea creatures. In places like Antarctica, they mainly eat tiny shrimp-like animals called krill.

Squid come in many sizes, from very small to very large. They are eaten by many different animals, including sharks, seals, whales, and birds. For example, elephant seals often eat a lot of squid, and sperm whales can eat hundreds of squid in just one day.

Parasites

Squid can carry parasites that make them unsafe to eat. One of the most harmful is a parasite called Anisakis, which can cause severe illness in humans if they eat raw or undercooked squid. To stay safe, it's best to cook squid to at least 60 degrees Celsius (140 degrees Fahrenheit) or freeze it at -20 degrees Celsius (-4 degrees Fahrenheit) for at least 24 hours.

Other common squid parasites include dicyemids, which usually don't harm young squid but can cause problems for older ones, and ciliates, which infect squid and octopuses. In 2024, scientists discovered a new parasite called Ikanecator primu that affects squid egg clutches in research tanks.

Human uses

Further information: Molluscs in culture

Giant squid have been seen as sea monsters for a long time. They were written about by ancient writers like Aristotle and Pliny the Elder. Some myths, like the Gorgon from Greek mythology, might have come from people seeing squid or octopus.

Squid is eaten in many places. In Japan, it is eaten in many ways, such as thin strips called ika sōmen, slices called sashimi, and fried pieces called tempura. In English-speaking countries, squid dishes are often called calamari. Squid is a good source of nutrients like zinc and vitamin B₁₂.

Scientists have also studied squid to make new technologies. For example, researchers at Bristol University made a material that changes colour like a squid’s skin for camouflage. The squid’s giant axon also helped invent a circuit called the Schmitt trigger.