Caribbean reef squid
Squids are related to octopuses and cuttlefish and, more distantly, mollusks such as clams and oysters. Some 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 281 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.

Clyde Roper, a Smithsonian zoologist, is regarded as the world’s foremost expert on squids.

Websites and Resources: National Oceanic and Atmospheric Administration noaa.gov/ocean ; Smithsonian Oceans Portal ocean.si.edu/ocean-life-ecosystems ; Ocean World oceanworld.tamu.edu ; Woods Hole Oceanographic Institute whoi.edu ; Cousteau Society cousteau.org ; Montery Bay Aquarium montereybayaquarium.org

Websites and Resources on Fish and Marine Life: MarineBio marinebio.org/oceans/creatures ; Census of Marine Life coml.org/image-gallery ; Marine Life Images marinelifeimages.com/photostore/index ; Marine Species Gallery scuba-equipment-usa.com/marine

Websites and Resources on Coral Reefs: Coral Reef Information System (NOAA) coris.noaa.gov ; International Coral Reef Initiative icriforum.org ; Wikipedia article Wikipedia ; Coral Reef Alliance coral.org ; Global Coral reef Alliance globalcoral.org ; Coral Reef Pictures squidoo.com/coral-reef-pictures ; The Global Coral Reef Monitoring Network; the International Coral Reef Action Network.


Reef cuttlefish
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.

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.

Cephalopod Features

Hooded Cuttlefish
Cephalopods have a bird-like beak; well-developed eyes; and sucker-covered arms, or tentacles, used to catch prey, move about and transfer sperm from the male to female. The appendage at the top of their arms is not their head. It is actually a mantle that stores their organs.

Cephalopods propel themselves by forcing water through a tubular siphon that draws water behind the eyes and shoot it out under the head. They also have the ability to grown back their arms and tentacles and sometimes communicate by changing the shape of their arms. All cephalopods shoot out ink, which is actually a mix of ink and mucus. The ink isn't intended to hide the animal but serve as a decoy that predators attack while the cephalopod escapes.

Many cephalopods change color to blend in with their backgrounds or to express fear, aggression and sexual excitement. Color changes are caused by chromatophores, pigments sacs situated just under the skin that suddenly swell with signals from the brain. The sacs are yellow, red, black and brown. By expanding and contracting them cephalopods can produce a variety of shades and patterns.

The problem with using skin colors to send messages is that predators can also pick up on them and use them to their advantage such as attacking when the cephalopods are distracted by sexual matters. Studies by Lydia Mathgar and Roger Nento of Wood Holes Institute suggest that squids — and likely cuttlefish and octopuses too — get around this problem by using communication channels that they can see but predators can’t. Cephalopods have two distinct layers of skin: an inner layer of iridophone cells that is both iridescent and reflects light and an outer layer made up of pigment organs called chromatophores. They also have a complex visual system tuned to read the skin patterns that predators can’t pick up.

Shells, Mollusks, Octopus, Squids and Cuttlefish

cuttlefish on an
ancient Greek fish plate
Mollusks are creatures with shells. There are four kinds of mollusks in the phylum, Mollusca: 1) gastropods (single shell mollusks); 2) bivalves or Pelecypoda (mollusks with two shells); 3) cephalopods (mollusks such as octopuses and squids that have internal shells); and 4) amphineura (mollusks such as chitons that have a double nerve).

The world’s first shells emerged about 500 million years ago, taking advantage of the plentiful supply of calcium in seawater. Their shells were composed of calcium carbonate (lime), which has been the source of much of the world limestone, chalk and marble. According to a 2003 paper in Science, the use of large amounts of calcium carbonate for shell-building in early years of life on earth altered the chemistry of the atmosphere to make conditions more favorable for creatures living on land.

Animals with shells have been found living in the Mariana Trench, the deepest places in the ocean, 36,201 feet (11,033 meters) below the sea surface, and 15,000 feet above sea level in the Himalayas. Darwin’s discovery that there were fossil of sea shells at 14,000 feet in the Andes helped shape of theory of evolution and understanding of geologic time.

Some of the simplest eyes are found in shelled creatures like: 1) the limpet, which has a primitive eye made up of a layer of transparent cells that can sense light but not images; 2) Beyrich’s slit shell, which has a deeper eyecup that provides more information about the direction of the light source but still generates no image; 3) the chambered nautilus, which has small gap at the top of the eye that serves as a pinhole pupil for a rudimentary retina, which forms a dim image; 4) the murex, which has a fully enclosed eye cavity which acts as a primitive lens. focusing light on a retina for a clearer image: 5) the octopus, which possesses a complex eye with a protected cornea, colored iris and focusing lens. [Source: National Geographic ]

Squid Features

20120518-squid Histioteuthis_bonellii1.jpg
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 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.

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 Movement, Light and Color Changes

Squids cruise at slow speeds by fluttering their fin-like mantels 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.

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.

20120518-squid Loligo_pealeii.jpg
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.”

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."

Squid Behavior

20120518-squid Mastigoteuthis_flammea.jpg
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 as Predators and Prey

20120518-squid Fish4538.jpeg
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.

Squid as Human Food

squid spinning machine
Squids are common foods in many Asian and Mediterranean countries. Their mantels contain virtually no fat. The Japanese sometimes feed squids to their caged birds and have special squid snacks for dogs.

Squid are often caught at night using bright lights to attract them. When flying over the waters around Japan and Korea at night you can see hundreds of lights from squid-fishing boats. On satellite images, these ships show up as glowing borders around Japan and South Korea.

Squids are hard to keep in a tank. They rarely survive for more than a few days. They spend most of the life roaming the seas and they loath being cooped up. They often cannibalize one another and injure or kill themselves by banging into the walls of the tank. Some suffocate in their own ink while others die of shock.

Overfishing and global warming could turn out to be a good thing for squids. Some scientists believed that their populations will dramatically increase as large fish that feed on them are fished out and warming seas allow them to spread and reproduce more quickly.

Jumbo Flying Squids

squid drying
Humboldt, or jumbo, squid are large creatures found in water waters from Chile to the Gulf of California off of Mexico that are known for their aggressive behavior. They can weigh as much as 70 kilograms and reach a length of four meters. They have been observed lunging at fishing boats and are known to practice cannibalism. Mexican fishermen call them “ Daimblos rojos” (“red devils”) and often get squirted in the face by the creatures when they are hauled aboard fishing vessels.

Jumbo squid have sharp beaks and toothy tentacles. They normally feed on small crustaceans fish but have been observed eating a wide variety of creatures, some of them quite large, including birds, rockfish and other squid. They consumer their prey voraciously and quickly, chomping food into bits and gobbling it down like a speed eater. Jumbos can reach speeds of 24 kph and sometimes hunt in group of 1,200 or more.

Since 2002 large numbers of jumbo squid have been turning up north of their usual range, appearing off California and the northwest United States and seen as far north as Alaska. They were first seen off the United States in large numbers in 1997, an El Nino year, but disappeared when the weather and ocean patterns returned to normal. They showed up again in 2002, another El Nino year, but thus time stuck around. Their presence is disconcerting in that they are so aggressive and eat so much. Off California, Oregon and Washington they have been consuming large amounts of Pacific hake, an important commercial fish widely used in fish sticks and imitation crab meat.

The jumbo squid seem to be thriving in part because their natural predators tunas and sharks have been overfished. They can often out-compete these creatures anyway because they can hunt better in low-oxygen environments, where small fish often hide out.

When large numbers of jumbo squid showed up off of San Diego scuba divers there complained about attacked by the aggressive creatures One diver told AP, a large squid grabbed her from behind with its arm, ripped her buoyancy hose away and turned her round so she didn’t which way was up, “I just kicked like crazy,” she said, “the first thing you think is, “Oh my gosh, I don’t know if I’m going to survive this.” If that squid wanted to hurt me, it could have.” Another diver said, “As soon as we were underwater and turned on the video lights, there they were. They would ram into you, they kept hitting the back of my head.” He also said they scanned his wet suit seemingly to check out whether he was edible or not.

Jumbo Flying Squid on the Fly

20120518-squid Taonius_pavo1.jpg
Marc Silver wrote in National Geographic: The millions of Humboldt squid, aka jumbo flying squid, live “fast and furious” lives, says NOA Fisheries oceanographer Ken Baltz. “They hunt and eat and hunt and eat” for a year or two, then expire. Their diet is mainly fish, an occasional floating seabird — and sometimes each other. [Source: Marc Silver, National Geographic, August 18, 2010]

Marc Silver wrote in National Geographic: “Once in a great while they “fly” by ejecting themselves from the water. Given that a squid’s body plus tentacles can run six feet and top the scales at 80 pounds, that’s quite a feat. Flight might be a way to evade predators, although scientists don’t know exactly why squids soar. Nor do they understand why the squid can quickly change from red to pink to maroon: maybe to confuse prey, maybe to signal each other.” [Source: Marc Silver, National Geographic, August 18, 2010]

If salmon are also on the menu, adds William Gilly, a biologist at Hopkins Marine Station of Stanford University, Northwest fisheries will suffer. But he doesn’t buy reports of summer 2009 attacks on San Diego scuba divers. A squid might nudge with a toothed appendage to assess edibility, he says. “They’re smart and curious and really tactile.” Anyone in a wet suit would be deemed unfit for cephalopod consumption.


Reef cuttlefish
Cuttlefish look like broad squids. Cephalopods and mollusks like octopuses and squids, they have a thick, chalky internal shell, called a cuttlebone, that fills with gas, which allows them gracefully rise and fall in the water column. They also have eight sucker-lined arms and two tentacles, usually concealed unless the cuttlefish is feeding. [Source: Natural History, April 2000]

There are a hundred or so different species of cuttlefish. They range in size from the two-inch “ Metasepia pfefferi” to the three-foot-long “Sepia apama” (giant cuttlefish). They are found in temperate and tropical oceans everywhere except the Americas and prefer coastal environments such as reefs, mangroves and areas of sea grass.

Cuttlefish have very sensitive eyes which can change their shape, which helps the cuttlefish focus in on its prey. Their unusual W-shaped pupils can detect polarized light but not color and see forward and backward at the same time. They have 13 to 14 muscles, controlling their eyes compared to two for humans.

Cuttlefish have powerful beak-like mouths strong enough to crush the shells of oysters and exoskeletons of crabs. They can detect motion the same way fish do. They propel themselves backward with surprising speed with a syphon, a funnel-like tube that shoots out water.

Cuttlefish are prized as food in Korea, Japan and other countries. Around 1996, the market for cuttlefish exploded. The impact of cuttlefish fishing on their numbers is not known.

Cuttlefish Colors and Behavior

Flamboyant cuttlefish
Cuttlefish change color to blend in with their backgrounds and to express fear, aggression and sexual excitement. "A single cuttlefish can become speckled, ocellata, stippled, lineated, whorled, black, white, brown, gray, pink, red, iridescent — all in different combinations and all in less than a second." In addition, cuttlefish have types of muscle groups and cells called papillae that allow them to change the texture of their skin to resemble seaweed or even a bumpy rock.

Each species of cuttlefish has its own repertoire of color changes. The color changes are caused by chromatophores, pigments sacs situated just under the skins that suddenly swell with signals from the brain. The sacs are yellow, red, black and brown. By expanding and contracting them cuttlefish can reproduce a variety of shades and patterns. Attracted by bright colors giant cuttlefish sometimes pull right up to divers in pink, yellow and green wetsuits.

Cuttlefish are generally shy and solitary. They communicate by changing colors and changing the shape of their arms in a complex ways. A zebra pattern produced by males, accompanied by complex arm movements, warns other males to stay away. The only time cuttlefish gather in large numbers is when they are young and when they mate. They generally don’t form social structures.

Cuttlefish feed on crabs, shrimp and fish and in turn are fed on by sharks and dolphins. They are especially adept at hunting crabs. When eating a crab, cuttlefish grab it prey with it arms and tears it apart with it beak or punches a hole in the shell and rasps the meat with its tongue.

Giant Cuttlefish in the Mating Season

Giant cuttlefish are found exclusively in the waters off of southern Australia. They reach lengths of four feet and generally live two or three years. Males usually die after mating season. [Source: Fred Baverendam, National Geographic, September 1995]

During the mating season, giant cuttlefish males battle with one another over choice dens. When two males encounter one another they puff up and stretch out their bodies to look as threatening as possible. Most confrontations end with the weaker cuttlefish slinking away after a ritual combat but occasionally fights take place in which losers — and winners — have arms bitten off.

When a females come around the males retreat to their dens and wave their arms and changes colors in an effort to attract the female. Sometimes while the male is driving off a rival male a “transvestite” male, impersonating a female, shows up and tries to a passes his sperm to the female.

After the female chooses a suitor the mating itself doesn't take long. The male uses one of its arms to place a sperm capsule into a pouch beneath the female’s mouth. The capsules break open, spreading sperm into her mantles and fertilizing her eggs. The female then swims into his den as deep as possible to lay her eggs and swims away, abandoning her young and dying soon afterwards.

The eggs, which are lined with gas chambers to create neutral buoyancy, hang like stalactites on the top of the den. After four months young giant cuttlefish break through the translucent shell and immediately have to fend for themselves in a world full of predators. Young cuttlefish have a natural instinct to hide under rock and in kelp beds.


Nautiluses come from a branch of mollusk that first appeared 550 million years ago and became mobile in the open sea by developing gas-filled flotation tanks. More than 3,500 nautilus species have been identified on the fossil record. Most of them lived in shallow seas. The largest one known had a nine-foot shell. Ancient nautiluses had parrotlike beaks surrounded by treacles. They were among the first animals known to seize prey, which appeared to have been shrimp.

Today about a dozen species survive, and they are found mostly in Pacific between Fiji and the Philippines, with a large concentration of them around Palau, where they are often found at depths of around 2,200 feet. The pearly nautilus grows to about 20 centimeters across and has remained virtually unchanged for 180 million years.

Poems and stories have been written about the nautilus. A Jules Verne submarine was named after it and a famous Edward Westin photographs were taken of a split open one. For a while there was even a magazine devoted to them. Nautilus means “sailor” in Greek, so because they were among the first to find empty nautilus shell floating in the currents. Nautilus shells are exquisitely beautiful and admired for their mathematical sophistication as well as aesthetic delicacy. Some species are endangered because there were never many of them to begin with and many have been taken by shell collectors.

Nautilus Characteristics and Behavior

Nautilus cutaway logarithmic spiral
Nautiluses are the only cephalopods that keep their shells outside of their soft bodies rather than in it. Residing inside a brown-patterned, mother-of-pearl-lined curling white shell, they look like a squid coming out of shell. They have 50 to 90 small suckerless tentacles, two long tentacles, primitive grooved eyes that working like pinhole cameras, and a water-spurting funnel. The spiraling gas-filled, airtight chambers in the shells act as buoyancy devices. The large final chamber is where the animal lives.

Nautiluses have different compartments in their shells. As the animal grows new flotation chambers are added so the nautilus can increase its buoyancy as the animal became heavier. As the nautilus grows the animal moves forward in the expanding shell, and natural secretions from a partition, or septum, behind its fleshy body, seal the old chamber. Thus the nautilus creates a series of ever-larger chambers, at an estimated rate of one every few weeks or months. Shells with 38 chambers have been found.

The pearly nautilus contains a tube that runs from the back of the body chamber into the flotation tanks allowing it to adjust its buoyancy by flooding or taking gas out of the tanks. The paper nautilus is really a kind of scallop.

Nautiluses eat fish, dead fish, lobster, crabs, and mollusks. They search for prey with small stalked eyes and tentacles that are sensitive to taste. It grasps prey with its tentacles and kills it with a bite with its parrotlike beak.

Nautilus move around tube shaped water-expelling funnels like those on squids. Some float freely in the sea. Other times these seek refuge in the crevasse of rock or reefs. When the animal dies it sinks while the shell floats.

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

Text Sources: Mostly National Geographic articles. Also the New York Times, Washington Post, Los Angeles Times, Smithsonian magazine, Natural History magazine, Discover magazine, Times of London, The New Yorker, Time, Newsweek, Reuters, AP, AFP, Lonely Planet Guides, Compton’s Encyclopedia and various books and other publications.

Last updated March 2011

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