How Squid Work

Squid are actually mollusks, although they look much different from their relatives the gastropods ( snails), and bivalves ( clams). Unlike other mollusks, which have a hard outer shell, squid have a soft outer body and an inner shell. Squid are part of the class Cephalopoda (meaning "head-footed"), a group that also includes the octopus, cuttlefish and nautilus. Cephalopods are divided even further into the eight-armed octopods (octopuses) and the 10-armed decapods (cuttlefish and squid).

Public domain image

A variety of cephalopods in the subclass coleoida, which includes squid, octopuses, and cuttlefish, from Ernst Haeckel's "Art Forms of Nature"

Eating Squid

Several animals and birds like to feast on squid, including the sperm whale, the grey-headed albatross, tuna, marlin, shark, seals and penguins. Because several types of fish have such a predilection for squid, they make excellent bait.

Squid are also part of the human diet. We most commonly enjoy them breaded and deep frie­d as calamari, or boiled and stewed as part of various seafood dishes. Although countries around the world eat squid, they are especially popular in regions bordering the Mediterranean Sea, as well as in Japan.

Image used under GNU Free Documentation License

Fried calamari

The squid emerged during a particularly bountiful stage in the ecological timeline -- 500 million years ago during the Cambrian period. So many different animal groups emerged during this period that scientists have termed it the "Cambrian explosion." Originally, thousands of species of cephalopods existed. Today, only four remain -- squid, cuttlefish, octopuses and nautiluses.

­The earliest squid were most likely slow-moving creatures that lived in shallow waters. Squid today are versatile creatures -- they can make their homes in a variety of marine environments, from the deep sea to coastal surface waters.

Squid come in a wide variety of sizes and appearances. They can range from an inch to more than 65 feet in length. Most squid have a long, tube-shaped body with a small head. They have 10 arms (two of which are much longer than the others for grasping prey), which are lined with rows of suckers. Some varieties have claw-like hooks instead of, or in addition to, the suckers. In the center of the arms sits a mouth with a parrot-shaped beak that surrounds a sharp, bony tongue (called the radula). The squid's eyes are large and set into the sides of its head.

Squid are the most intelligent of the invertebrates (animals that lack a backbone), with a brain that is well-developed and larger in proportion to the animal's body than that of most fish and reptiles. They also have a sophisticated nervous system.

The squid's body is enclosed in a soft and muscular cavity called the mantle, which sits behind the head. As water flows through the mantle cavity, it passes over the gills and the squid absorbs oxygen to breathe. Beneath the head is a tube called the funnel. Wastes are excreted through the funnel, as is the squid's defensive ink.

We'll learn more about how squid use their funnels and the daily life of a squid in the next section.

Brian J. Skerry/National Geographic/Getty Images

With angry eye and a cloud of ink, a jumbo squid flees from a diver.

A squid's funnel acts like a jet engine, making them powerful swimmers. It draws water into its mantle cavity by expanding its muscles. The mantle stretches like a rubber band, then contracts and forcibly pushes the water out through the funnel. The squid shoots backward, tail first. When escaping from a predator, a squid can propel itself as quickly as 25 body lengths a second.

With their soft bodies, squid are vulnerable prey. They rely on their speed and agility, as well as their system of camouflage, for defense. Before a squid flees its predator, it releases a cloud of an inky substance called sepia. This temporarily confuses the attacker, allowing the squid to escape.

To blend in with their surroundings, squid have thousands of pigment cells on their arms called chromatophores, which are attached to tiny muscles. Chromatophores expand or contract to change the color or pattern of the squid's skin to match its background (these same cells also help squid attract a mate and communicate with other squid). Squid even can change the texture of their skin to simulate their surroundings by raising little flaps and bumps.

Squid are carnivorous and their favorite foods include small fish, crabs, shrimp and other squid. A squid will stalk its prey by hiding out of sight until the animal is within range, then shoots out its arms to ensnare the food. The squid then pulls the food to its mouth with its arms. It uses its sharp, parrot-like beak to tear off pieces, then the sharp radula on its tongue grinds up the food and pushes it down the squid's throat.

An Abundance of Calamari?

According to many scientists, global warming is having a profound effect -- an often negative one -- on animal species. For squid, though, the phenomenon may actually be more of a blessing than a curse. Research has shown that the squid's digestive juices are more productive in warmer waters, and as a result, squid tend to grow larger and more plentiful as water temperatures rise. As evidence, fishermen off the coasts of New Zealand and Australia say they have been catching greater numbers of squid in recent years. Not all squid seem to benefit from global warming, though. Those that live in the colder depths may not be able to survive in balmier waters, scientists say.

Squid reproduce sexually. A female can produce thousands of eggs, which she stores in her ovary. In male squid, sperm is produced in the testis and stored in a sac. When they mate, the male uses a special arm to transfer packets of his sperm into the female's mantle cavity or around her mouth, where the eggs are waiting. Then the female ejects the gelatinous mass of fertilized eggs from her funnel or mouth and hides them under rocks or in holes. After four to eight weeks, baby squid hatch. At birth, they are smaller versions of their parents. They feed on tiny creatures called plankton while they grow to adulthood.

Many squid live fast and die young -- their entire life cycle takes just one year. After male and female mate, they usually die. Less is known about the life cycles of deep-water squid, though, and they may have considerably longer life spans.

About 300 different species of squid exist. The two main suborders of squid are myopsida and oegopsida . Members of the myopsida suborder live in relatively shallow waters. Their eyes are covered by a transparent membrane, and they have suckers, rather than hooks, on their tentacles. Let's look at some common members of the myopsida suborder:

• California market squid ( Loligo opalescens) - Market squid live in shallow waters close to the shorelines in the eastern Pacific Ocean, from Mexico north to Alaska. They are plentiful in the waters of Monterey Bay, California, where they've been harvested by fishermen since the 1800s.
• Common European squid ( Loligo vulgaris) - European squid can be found in the Mediterranean Sea and eastern Atlantic Ocean. They live in depths of 65 to 850 feet (20 to 250 meters), and are typically small, weighing about 3.3 pounds (1.5 kilograms) and measuring 16 inches (42 centimeters) long.
• Caribbean reef squid ( Sepioteuthis sepioidea) - As their name suggests, these squid live in the Caribbean Sea, as well as off the Florida coast. The torpedo-shaped squid more closely resemble cuttlefish than squid -- they are wider and have larger fins than most other squid varieties.

Image courtesy Hans Hillewaert, used under the Creative Commons Attribution ShareAlike License 2.5

European squid

Members of the oegopsida suborder live out in the ocean and deep sea. They have no cornea over their eyes, and their tentacles are lined with suckers and/or hooks. Here are a few common varieties of the oegopsida suborder:

Image courtesy E.Widder/HBOI/ NOAA Ocean Explorer

A bioluminescent squid, shown here with bioluminescent fish, jellyfish and shrimp, lives in the ocean below 1,640 feet (500 meters).

• Shortfin squid ( Illex illecebrosus) - Shortfins live in the Atlantic Ocean, from Florida to Newfoundland, Canada. These squid have a longer-than-normal migratory period. They travel south to place their eggs in warmer waters.
• Deep-sea luminescent squid ( Taningia danae) - The deep-sea luminescent squid live in depths of up to 3,000 feet in the North Atlantic, and off the coasts of Bermuda, Hawaii, Japan, Australia, and New Zealand. To survive in its pitch-dark surroundings, this type of squid creates its own light -- ( bioluminescence), created by organs called photophores. The Taningia danae gets its name from the Danish research ship, Dana, which in 1931 caught one of these squid off the coast of the Cape Verde Islands.

• Humboldt squid ( Dosidicus gigas) - Humboldts live in the eastern Pacific. These enormous creatures have earned the nickname "red devil" for their red skin and the ferocity of their attacks. They are merciless with their prey, and have even been known to go after sharks. The Humboldt squid grows at an amazing rate -- by adulthood, it can reach 7 to 15 feet in length and can weigh as much as 100 pounds.

Image courtesy NOAA Encyclopedia of the National Marine Sanctuaries

Vampire squid

In the depths of the Atlantic and Pacific Ocean, pairs of glowing red eyes cut through the gloom. They belong to the vampire squid from hell ( Vampyroteuthis infernalis), part of its own squid order -- Vampyromorpha . The vampire squid's sinister name comes from its appearance -- it has red eyes, a black body and webbed arms that resemble Dracula's cape. Despite its intimidating appearance, however, the vampire squid is actually quite docile. It sits motionless in the water until its prey approaches, and then it catches the food in its webbed arms.

Next, we'll look at the giant and colossal squids to learn the truth behind the legends.

For thousands of years, people have told tales of giant, many armed sea monsters. In Homer's "Odyssey," Odysseus had to navigate his boat around a many-headed sea monster called Scylla. Jules Verne later wrote of giant squid attacking the Nautilus submarine in "20,000 Leagues under the Sea." These legends were likely based on real sightings of the giant squid ( Architeuthis), the world's largest invertebrate, and the biggest member of its species.

These enormous animals, which live deep in the Atlantic Ocean, can reach lengths of 60 feet and can weigh nearly 1,000 pounds. They have eyes the size of soccer balls, and 35-foot-long tentacles lined with suckers measuring two inches each in diameter.

Image courtesy NASA/SeaWiFS

A giant squid caught by a deep-water fishing trawler in the waters around New Zealand and Australia in 1999

Very little is known about giant squid, because they are so rarely seen. Until recently, the only time scientists had seen giant squid was when they found them among the stomach contents of sperm whales (their only predators). The sucker-shaped scars on the whales' jaws and lips attested to the battle the whales had to put up in order to capture their prey.

In 2005, a team of Japanese marine biologists was able to capture photographs of the elusive giant squid swimming deep in the Pacific Ocean for the first time. It took three years for the scientists to locate the squid, which they accomplished by following the migratory patterns of sperm whales. They captured the photos while the squid was attacking bait on a line. The squid became caught up in the line, and struggled for more than four hours to free itself. During the struggle, it lost one of its tentacles, which the scientists recovered. It measured 18 feet in length. A year later, the researchers were finally able to actually capture a giant squid.

Image courtesy Associated Press/KOJI SASAHARA/ Tsunemi Kubodera of the National Science Museum of Japan, HO

A giant squid attacks a bait squid as it is pulled up by Tsunemi Kubodera's research team off the Ogasawara Islands, south of Tokyo, on December 4, 2006. About seven meters (24 feet) long, the squid died in the process of being caught.

In recent years, scientists have also learned more about the giant squid's equally intimidating relative, the colossal squid ( mesonychoteuthis hamiltoni). In 2007, a New Zealand boat was on a fishing expedition in Antarctic waters when its lines snagged something much larger than fish. The fisherman struggled for nearly two hours to pull the colossal squid onto the boat. It weighed 990 pounds, and, according to news records of the event, had the squid been cooked, it would have produced calamari "the size of tractor tires." The colossal squid was frozen and taken to New Zealand's national museum for further study.