Squid: Characteristics, History, Reproduction

Home | Category: Cephalopods (Octopus and Squid)


umbrella squid (Histioteuthis bonellii)

Squids have eight arms, two long tentacles, and a beak for mouth. They are related to octopuses and cuttlefish and, more distantly, mollusks such as clams and oysters. Some squids have transparent bodies. Some produce their own light. Others can change colors to express their emotions. Some scientists estimate the biomass of all the worlds squids far exceeds that of human beings. [Source: Roger Hanlon, National Geographic, August 2004, Richard Conniff, Smithsonian; Gilbert L. Voss, National Geographic, March 1967]

There are around 300 known species of squid. They can be found in all the world’s oceans and range in size from dime-size midgets to rare 20-meter-long giant squids. At depths of between one mile and two miles there is large variety of squid, including translucent ones, polka-dotted ones blue eyed ones and aggressive large ones.

Squids have an ancestral shell called a quill or pen. It resembles a leaf made of powdery chalk and is often found washed up in seashores. Squids and cuttlefish are oriented so their body is effectively sideways they are in the open sea to give them a more streamlined shape. Many squids have torpedo-shaped bodies.

Websites and Resources: Animal Diversity Web (ADW) animaldiversity.org; National Oceanic and Atmospheric Administration (NOAA) noaa.gov; Fishbase fishbase.se ; Encyclopedia of Life eol.org ; Smithsonian Oceans Portal ocean.si.edu/ocean-life-ecosystems Clyde Roper, a Smithsonian zoologist, is regarded as the world’s foremost expert on squids.


Octopuses, squid and cuttlefish are cephalopods, a class of mollusks whose name means "head-footed." There are two subclasses of Cephalopoda: 1) chambered nautiluses, with external shells and anatomy that has remained virtually unchanged for 450 million years; and 2) coleoidea, which includes octopuses, squids and cuttlefish. The latter are soft, fleshy mollusks with their shells inside their bodies instead of outside as is the case with most mollusks.

Caribbean reef squid
Cephalopods are common food sources in many counties, particularly in Asia. They reproduce quickly which means that even though two million metric tons of them are caught every year, they are not in danger of being overfished. In the past they were often caught with drift nets, which are now banned not because they caught too many squid but because they caught other animals like dolphins and sharks.

Cephalopods are regarded as more developed and sophisticated than mollusks like snails, clams and oysters. In fact they are considered the most advanced and developed invertebrates (animals without backbones). They have the largest brains and nervous systems of any invertebrate and their brains are much bigger in relationship to their bodies than those of fish. Most cephalopods grow quickly, mate once and die. Most live no more than 18 months.

Squid Evolution and History

Crown coleoids (the common ancestor of octopuses and squid) diverged in the late Paleozoic (Mississippian, 359 and 299 million years ago) based on fossils of Syllipsimopodi, an early relative of vampire squids and octopuses. True squids diverged during the Jurassic Period (201 to 145 million years ago), but many squid families appeared in or after the Cretaceous Period (145 to 66 million years ago).

The ancestral coleoid was probably a nautilus-like creature with a strait septate shell that became immersed in the mantle and was used for buoyancy control. Four lines diverged from this, Spirulida (with one living member), the cuttlefishes, the squids and the octopuses. Squid have evolved to be quite different creatures from the ancestral molluscs they developed from. Their body plan has been condensed front-to-back and extended top-to-bottom. The mollusc foot is modified into a complex set of appendages around the mouth.

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.

Jurassic-Era Squid Killed by a Shark While Eating a Crustacean

Jurassic-era squid fossil: Rhomboteuthis_lehmani

During the early Jurassic period a squid-like creature was eating a crustacean, when it suddenly it grabbed by another marine beast, possibly a shark, that chomped into its soft tissue and killed it, a study published in 2021 found. According to Live Science: “The shark swam away, but the crustacean and the squid-like animal — a 10-armed and two-finned creature called a belemnite — sank to the bottom of the sea, where they fossilized together over the subsequent eras in what is now Germany. The resulting 180 million-year-old fossil is "unique," one of about "10 specimens of belemnites with [well-preserved] soft tissues worldwide," study lead researcher Christian Klug, curator of the University of Zurich's Palaeontological Museum and a professor at its Palaeontological Institute, told Live Science. [Source: Laura Geggel, Live Science, May 8, 2021]

The specimen also shows how predators sometimes become prey themselves. "Predators tend to be happy when they are eating, forgetting to pay good attention to their surroundings and potential danger," Klug said. "That might explain why the belemnite got caught, but there is no proof for that." The fossil also inspired a new term: pabulite, which mean "food stone" and refers to meal "leftovers" that never enter the predator's digestive system and later fossilize. A pabulite can "provide evidence for incomplete predation," which is likely what happened here, the researchers wrote in the study. In fact, it's possible that the shark purposefully targeted the belemnite's squishy parts, rather than its pointy hard tip, known as the rostrum. Vertebrate predators likely learned to avoid the hard-to-digest rostra, and as a result may have "bit off the soft parts, which were poorly protected," the researchers wrote in the study.

The belemnite, they discovered, was the well-known species Passaloteuthis laevigata, whose fossilized remains have been found in Europe and Morocco in rocks dating to the Toarcian age (183 million to 174 million years ago). P. laevigata was a small creature, with a nearly 4-inch-long (9.3 centimeters) bullet-shaped rostrum; each of its 10 arms were up to 3.5 inches (9 cm) long and carried double rows of arm-hooks. These hooks, 400 in all, would have helped P. laevigata grip slippery prey, Klug said. "In this individual, two arms were modified, bearing large hooks," Klug noted, "We guess that these were used for mating and possibly only males had them, while in females, all 10 arms were similar, but we have no proof for that yet."

Belemnites are now extinct, but fossils reveal that they had an internal shell surrounded by muscles and skin, Klug said. These strong horizontal swimmers actively preyed on sealife, including fish and crustaceans, and in turn were eaten by sharks and dolphin-like predators known as ichthyosaurs, he said.

Jurassic-era squid fossil: Plesioteuthis prisca

So, it's no surprise that this belemnite was chomping on a crustacean from the genus Proeryon, which had a broad and flat lobster-like body and long, slender claws, Klug said. However, the Proeryon was poorly preserved, so "we think that these are remains of an old skin (a molt)," he wrote. "Crayfish remove much of the calcium from the shell before they molt, because they later put it into the new skin."

Cephalopods"do love to eat this old skin," Klug added. "Much of it is lying really between the arms of the belemnite, quite close to its mouth, so it is likely that the belemnite was actually feeding on it." Although parts of the belemnite are well preserved, including its rostrum and arms, much of its body is missing. This is why "we must conclude that a larger predator ate most of the belemnite," Klug said.

A prime candidate for the belemnite's "killer" is the early Jurassic shark Hybodus hauffianus. A previously described H. hauffianus fossil was stuffed with belemnite remains, including dozens of rostra. That particular H. hauffianus "possibly ran into a swarm of belemnites and got too enthusiastic about it: It ate about 200 of them but forgot to bite off the rostra, thereby clogging its stomach, which eventually killed it," Klug said.

Squid Characteristics

As we said before squids have eight arms, two long feeding tentacles, and a parrot-like beak for mouth, with arms arranged around it like a flower. The sucker-covered tentacles are used to capture the prey. The arms grasp the prey firmly and shove it down their mouth where a tongue lined with raked-back teeth push it down into the the gullet. Their esophagus passes through the middle of their brain. If they take too big of a bite of food their brains spill into their mouth. Squid also have three hearts — a central one and two more that pump blood through the gills.

Squids are cold blooded (ectothermic, use heat from the environment and adapt their behavior to regulate body temperature) and heterothermic (have a body temperature that fluctuates with the surrounding environment).

All squids shoot out inks. Sometimes it glows. The ink is not used like a smoke screen but rather as a means of creating a cigar shaped cloud or phantom squid which predators will pursue instead of the real animal. Artist have used the ink of squids for 2000 years; it was the forerunner to India ink. The ink stain is so long lasting that writing ink has been prepared from the ink sacs of million year of fossil squids. Stains on clothing are nearly impossible to wash out.

Squid Brain and Senses

Squids have eyes very similar to human eyes, except some species have one eye that is significantly larger than the other, and each of which seems to be adapted to different frequencies of light. Most kinds of squid can see polarized light which humans can not see. Their retinas have a finer structure which means they probably can distinguish finer detail than people can.

Squids have the largest brains of any invertebrates and these brains are quite sophisticated. They also have the largest nerves of any animal, 100 times thicker than a human nerve. These are popular with researching neurologists because the large size means they react very quickly and they are easy to work. Much of what scientists know about nerves has been derived from experiments with squid.

Many squid cannot hear sound, but they can detect the movement of sound waves via organs called statocysts (similar to the human cochlea). Squid statocysts likely respond to low frequency sounds less than 500 Hz, similar to pelagic fish. Squid are essentially deaf to high frequencies and largely rely on visual detection mechanisms to avoid predation.

Squid Movement

Squids use jet propulsion and fin motion to get around. Their fins are located on their mantle. They cruise at slow speeds by fluttering their fins and contracting and pulsing their bodies. They move rapidly by drawing water into their body cavity and squirting it out of funnel-shaped nozzles. The nozzles move and all the squid has to do to move forward and backward is move the position of the nozzles. Although squids usually travel backwards, they actually travel in any direction they want to by adjusting their water-injecting nozzles. Some species of squid have been observed shooting themselves forty feet out of the water and darting a hundred feet to escape a predator. Other species can jet along at speed of 20 mph.

Squid fins flap each time that a jet is released, amplifying the thrust; they are then extended between jets. One reason for this is presumably to avoid sinking. Oxygenated water is taken into the mantle cavity to the gills and through muscular contraction of this cavity, the spent water is expelled through the hyponome, created by a fold in the mantle. [Source: Wikipedia]

The size difference between the front and rear ends of the hyponome (the siphon- and funnel-like cephalopod swimming.organ) controls the speed of the jet the squid can produce. The Japanese flying squid has been observed spreading tentacles in a flat fan shape with a mucus film between the individual tentacles. The Caribbean reef squid has been documented putting the tentacles in a circular arrangement.

Squid Behavior

Squids have a curious nature. They are attracted by light, which is how fishermen usually catch them. Squids form schools but generally don’t have individual relationships. Some squids rest on the bottom of the sea on "their elbows" so mud doesn't get in their propulsion system. Other use sticky tentacles to draw in thick concentrations of plankton.

When threatened some squids retreat into a jackknife position to reduce their size in the eyes of predators. Others roll their arms around their the head, exposing rows of suckers lined with sharp teeth. Their long tentacles are used as lures and fishing lines.

When attacked most squids release black ink or a cloud of luminous mucus that forms a veil that enables it to escape. It was long thought the ink or mucus served solely as a decoy or camouflage now it is believed that it may also contain chemicals that stun prey or disable predators which explains why deep sea squid eject blank ink in places where there is no light.

Squids, Light and Color Changes

Squids have quick reactions and the ability to change their appearance quickly by altering their skin color patterns with what is regarded as the most versatile skin in nature. Squid skin is filled with chromatophores, pigment-filled sacs attached to small muscles. They vary their skin color by expanding and retracting chromatophores. They use this ability to avoid predators, catch prey, seek mates and express their intention to rivals. During the mating season males of some squid species can have one set of patterns on one side of the body to attract females and another set on their other side to deter rivals and predators. Caribbean reef squid produce 35 known patterns. A zebra pattern is a warning to male rivals to keep their distance.

David Grann wrote in The New Yorker: “The squid seems to be using light patterns, color and postures as a means of communication. They didn’t just turn red or pink or yellow, ripples of color would wash across their bodies. And they would contort their arms in elaborate arrangements — sometimes balling them together, or holding them above their heads, like flamenco dancers...They use these movements and color change to warn other squids of predators, to perform mating rituals, to attract prey, and to conceal themselves.”

Controllable chromatophores of different colors in the skin allow squids to change their coloration and patterns quickly, usually for camouflage or signalling.

Many species of squid can produce their own light with freckle-like bioluminescent photospores that include focusing mechanisms and color filters able to produce white, blue, yellow and pink light. Some scientist believe the light is used to attract plankton. Some species flash a light to attract prey.

Diver have a hard time seeing squids around reefs because they are so well camouflaged. Jane Blanksteen, a recreational diver, wrote in the New York Times, "On average, the groups I observed numbered about a dozen. Their colors changed constantly. These big-eyed, quirky-looking creatures float upwards in unison and turn clear and watery-looking so that they are barely discernable. Then they drop in unison and turn mottled and striped browns to blend in with the soft corals that camouflaged them. They would suddenly point all to the left and then to the right as if in some exquisite choreographed aquatic ballet."

Squids as Predators and Prey

Squids are also commonly eat by tuna, sharks, whales, dolphins and other large fish. Squid themselves feed on fish, shrimp and crabs, and they especially love gobbling up baby mackerel with their bird-like beaks. When a squid eats a fish it strips the flesh and leaves the tail and skeleton intact. Beaks in the stomachs of squids are proof that squids also eat each other.

Squid are adept hunters. They can unfurl their feeding tentacles in about 20 milliseconds — a speed nearly invisible to the naked human eye — to snatch a shrimp and small fish. They then push the prey towards their arms that grasp it more firmly and maneuver it into their beak which tears it into small pieces.

Squid routinely eat fish bigger than themselves. Describing such an event in a tank David Grann wrote in The New Yorker: “Though the fish were bigger than the squid, the squid shot toward them, their arms curved over their heads, hiding their tentacles...Then the squid’s arms sprang open, and their tentacles exploded outwardly, lashing their prey. The fish squirmed to break free, but the squid engulfed them in a web of arms. They drew their frantic prey into their necks, and the squid’s stomach turned bright red as they filled with the blood of the fish.”

Squid Reproduction

Squid egg cases
Squid live fast and die young, Most live for only around a year or so and thus need to become sexually mature not long after they are born and reproduce quickly. Squids mate tentacle to tentacle. The male transfers a sperm packet from his body to a sac near the female's mouth.

Studies have shown that squid grow quicker and reproduce quicker in warmer water. Many reproduce not long before they die. Some species mate, lay tens of thousand of eggs and die within a couple of weeks. The sheer number of eggs laid means that there is a good chance that some will survive. Other species lay fewer eggs but take measures to ensure their survival. Caribbean reef squid lay three or four eggs per sac and hide them among stinging fire coral. Within three weeks the embryos have functioning eyes. They hatch about five days after that.

Sperm is often found all over captured squid, both males and females. Some scientists say this is evidence of homosexuality among squids. Others say it simple means that squid ejaculate in response to trauma.

NASA Sends Squid into Space

In June 2021, dozens of baby squid from Hawaii were launched into space by NASA for research. Associated Press reported: “The baby Hawaiian bobtail squid were raised at the University of Hawaii's Kewalo Marine Laboratory and were blasted into space on a SpaceX resupply mission to the International Space Station. “Researcher Jamie Foster, who completed her doctorate at the University of Hawaii, is studying how spaceflight affects the squid in hopes of bolstering human health during long space missions. [Source: Associated Press, June 22, 2021]

“The squid have a symbiotic relationship with natural bacteria that help regulate their bioluminescence. When astronauts are in low gravity their body's relationship with microbes changes, said University of Hawaii professor Margaret McFall-Ngai, who Foster studied under in the 1990s. “We have found that the symbiosis of humans with their microbes is perturbed in microgravity, and Jamie has shown that is true in squid,” said McFall-Ngai. “And, because it’s a simple system, she can get to the bottom of what’s going wrong.”

“Foster is now a Florida professor and principal investigator for a NASA program that researches how microgravity affects the interactions between animals and microbes. “As astronauts spend more and more time in space, their immune systems become what’s called dysregulated. It doesn’t function as well," Foster said. “Their immune systems don’t recognize bacteria as easily. They sometimes get sick.” Foster said understanding what happens to the squid in space could help solve health problems that astronauts face. “There are aspects of the immune system that just don’t work properly under long-duration spaceflights," she said. "If humans want to spend time on the moon or Mars, we have to solve health problems to get them there safely.” The Kewalo Marine Laboratory breeds the squid for research projects around the world. The tiny animals are plentiful in Hawaiian waters and are about 3 inches (7.6 centimeters) long as adults. The squid came back to Earth after weeks after the launch.

Image Sources: National Oceanic and Atmospheric Administration (NOAA); Wikimedia Commons; spinning and drying squid by Ray Kinnane

Text Sources: Animal Diversity Web (ADW) animaldiversity.org; National Oceanic and Atmospheric Administration (NOAA) noaa.gov; Wikipedia, National Geographic, Live Science, BBC, Smithsonian, New York Times, Washington Post, Los Angeles Times, The New Yorker, Reuters, Associated Press, Natural History magazine, Discover magazine,Lonely Planet Guides and various books and other publications.

Last updated April 2023

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