Squid

A squid (pl. squid) is a mollusc with a soft, stretched-out body, big eyes, eight arms, and two tentacles in the orders Myopsida, Oegopsida, and Bathyteuthida. Like all other cephalopods, squid have a clear head, bilateral symmetry, and a mantle. They are mostly soft-bodied, like octopuses, but have a tiny internal skeleton shaped like a rod, called a gladius or pen, made of chitin.

Squid appeared long ago during the Jurassic and became more common at the start of the Late Cretaceous. They are important in the ocean's food web. The two long tentacles help them catch food, while the eight arms hold and guide it. Squid swim quickly using jet propulsion and mostly find food by looking for it. They are some of the smartest invertebrates.

Squid can change colour for camouflage and signalling. Some species are bioluminescent, making light to blend in, while many can shoot out a cloud of ink to confuse predators. Squid are eaten by people in many places, such as Japan, the Mediterranean, the southwestern Atlantic, and the eastern Pacific. They are cooked and enjoyed in kitchens all over the world, often called "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. Over time, squid developed their special 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.

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.

Squid can change colors using special cells in their skin to help 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 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 big for a mollusc and have lots of yolk to feed the growing embryo. The embryo grows 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 different 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 and zoom backward.

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.

Ecology

Squid live fast and have short lives. They grow quickly and die after they have babies. They eat 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. Many animals eat them, including sharks, seals, whales, and birds. For example, elephant seals often eat a lot of squid, and sperm whales can eat many squid in one day.

Parasites

Squid can carry parasites that can make them unsafe to eat. One of the most harmful is a parasite called Anisakis. It can make people very sick if they eat raw or undercooked squid. To stay safe, 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 parasites can affect squid, but they usually do not harm young squid. Scientists found a new parasite in 2024 that affects squid egg clutches in research tanks.

Human uses

Further information: Molluscs in culture

Giant squid have long been thought of as sea monsters. Ancient writers like Aristotle and Pliny the Elder wrote about them. Some old stories, like the Gorgon from Greek mythology, may have started 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 gives us nutrients like zinc and vitamin B₁₂.

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