Squids and their relatives, the octopus, are said to represent the ultimate in invertebrate evolution. There is little doubt that these cephalopods have highly advanced nervous systems and other evolutionary features that give them great advantage in the sea. Their ability rapidly change colors and body patterns affords them many uses, including camouflage, escape, mating, and possibly, communication. The truth is stranger than fiction, they say, and where squids are concerned, strangeness and mysteries abound.
Probably no creatures in the sea have evoked visions of monsters than the squid. Noted for their slimy writhing skulking fleshy bodies with all-too-human-like eyes, squids and their cousins, the octopus, could be likened to humans without bones, and they remind us, perhaps, somewhere in the deep recesses of our ancient consciousness, of our own primitive and terrifying origins in the sea.
Even today, giant squids as monsters of the deep find themselves scratched on the pages of terror novels and scripted on the silver screen. Consider this account from the desk of Peter Benchley, author of Jaws, the story of another monster of the sea, the Great White Shark. In the opening of his 1991 novel (and subsequent film), the Beast, he writes:
It hovered in the ink dark water, waiting.
It was not a fish, had no air bladder to give it buoyancy, but because of the special chemistry of its flesh, it did not sink into the abyss.
It was not a mammal, did not breathe air, so it felt no impulse to move to the surface.
It was not asleep, for it did not know sleep, sleep was not among its natural rhythms. It rested, nourishing itself with oxygen absorbed from the water pumped through the caverns of its bullet-shaped body.
Its eight sinuous arms floated on the currents; its two long tentacles were coiled tight against its body. When it was threatened or in the frenzy of a kill, the tentacles would spring forward, like tooth-studded whips...
It existed to survive. And to kill.
For, peculiarly, if not uniquely, in the world of living things, it often killed without need, as if Nature, in a fit of perverse malevolence, had programmed it to that end.
While accounts of giant squids attacking ships and boats abound, few (if any) of these have much credibility. Squid, like the octopus, are rather shy creatures and prefer darkness and seclusion. Yet the very nature of their rare and ghastly appearance, and their ability to rapidly change color and form, have placed them prominently among legendary monsters of the deep, including sea serpents, loch ness monsters, and mermaids.
Squids, octopus, cuttlefish, the chambered nautilus, and their prehistoric relatives, the ammonites, belong to the Phylum Mollusca. Because they appear to have large heads in relation to their body, they have the appearance of being all head and all feet. For that reason, they have been put in a class called Cephalopoda, which means "head foot." Cephalopods also include cuttlefish and the chambered nautilus, one of the few cephalopods still to retain an external shell.
While on the outside they may not much appear like a snail, on the inside they are quite similar to other molluscs. First, behind the parrot-like beak of squids and octopus lies a radula. As you recall when we studied gastropods, the radula is a hard, toothlike structure used in various ways for feeding. In periwinkles and limpets, it is used for scraping algae and bacteria off of rocks. In cone shells, it is used to inject a poisonous venom into its prey. In the squids and octopus, the radula is used for stuffing food down its throat.
The second feature of cephalopods that joins them with other molluscans is the presence of a mantle cavity. The mantle cavity is a flesh fold of skin that houses the gills and, in shell-bearing molluscs, secretes the shell in which they live. The mantle cavity of squids and octopus contains their gills.
The third link to molluscs is the presence of a shell. In the squids, the shell, called a pen, is internal and runs along the squid's back to keep the body stiff and streamlined. The chambered nautilus, as mentioned earlier, still retains a shell, as did all ancestral cephalopods. Octopus lack a shell; however, in all cephalopods, the horny beak is considered another remnant of a shell in these organisms.
What distinguishes cephalopods from other molluscs are their brains. Squids and octopus have the largest brains of any invertebrates, placing them at the pinnacle of invertebrate evolution. Their ability to sense and processes environmental information places them on an level equivalent to many vertebrates, including the fishes. The learning abilities of octopus are well documented and their intelligence has been compared to that of canine puppies.
Many unique features distinguish squid from other invertebrates, but one distinctive difference is their ability to swim. In fact, squids (and fishes and marine mammals) belong to a category of organisms known as nekton. Nekton are organisms that have developed powers of motion to propel themselves against the ocean currents. Unlike plankton, nekton are not at the mercy of the currents. Let's take a closer look at squids and compare and contrast their bodies and behavior with other organisms.
Squid Parts and Purposes
At least 500 species of squid are known to exist. They range in size from the tiny pygmy squid, Idiosepius pygmaeus, measuring 1-2 inches in length, to the giant squid, Architeuthis sp., reaching lengths of 60 feet or more. Off the coast of California, the market squid, Loligo opalescens, averages from 8-12 inches in length. In addition to size, squid species differ in body shape and attachments; some have elaborate fins used as "swimming keels;" some have claws and hooks on their suckers; some have bioluminescent photophores covering their bodies. Most squids travel in schools, particularly when mating, and their numbers can be vast. Cousteau describes an almost solid sea of squid they happened upon one night off the coast of California, describing them as "several yards thick, of writhing squirming creatures who darted to and fro by expelling water from their funnels, like a fleet of miniature jets."
All squids have ten arms, making them decapods, in contrast to octopus, who are octopods. In general, eight of these arms are similar in size; two longer arms are called tentacles. In most species, the tentacles are equipped with "clubs," flattened expanded appendages at the end of the tentacles. These clubs often have wicked hooks and claws in the suckers which only appear on the flattened ends.
Like all cephalopods, squids are carnivores, eating numerous species of fish, crustaceans, shellfish, and worms. Squid feed by grabbing their prey with the two tentacles. These appendages, which are usually held close to their bodies in pouchlike sacs, shoot out like harpoons, grab the prey, and pull it into their circle of arms. Many species of squid have poison glands that inject powerful neurotoxins into the prey when it is caught. Once subdued, the horny beak chops the prey up into bite-size pieces, which are rammed down the squid's throat and into its stomach with the file-like radula. Digestion is accomplished by digestive glands. Food passes into the intestine, which loops back in the body and exits as the anus near the funnel. Similar to bivalves, the squid ejects its fecal matter in the excurrent stream of its siphon.
The squid's role as a predator is enormously enhanced by its ability to swim rapidly. Squids have been called "invertebrate athletes" that "inch for inch...compete in swimming power with any other creature that lives in the sea." How do squid swim so fast? Like other cephalopods, squids make use of jet propulsion. The basic swimstroke is described as follows:
Water enters the mantle cavity at the mantle collar by expansion of the circular muscles that line the mantle. By closing the collar, water is forced outwards through the funnel. When the squid needs speed, it rapidly contracts the mantle which forces water out the funnel and rapidly propels the squid in the direction opposite to that in which the funnel is pointing. Because the funnel is flexible, squid can actually move backwards or forwards, depending on which direction the funnel is pointed. Backwards movements are more effective than forwards movements; the torpedo-like shape of the body and the fins act to reduce drag and assist the squid in jetting through the water. The fins may also be used for propulsion in some species, either by undulating them or flapping them.
Another swimming advantage is realized by the location of the squid's gills, called ctenidia. Squids have two gills attached to the inner wall of the mantle. Water coming in through the mantle collar flows over these gills, oxygen is extracted, and the water is pumped out the funnel. If the squid is swimming fast, more water is passed over the gills and more oxygen can be extracted. This arrangement allows squid to achieve high rates of motion, up to 20 knots per hour, for limited periods of time, much like a drag racer or Olympic sprinter!
The oxygen available to squid is further enhanced by the squid's blood, which contains the copper-based hemocyanin, which is less viscous and easier to pump than hemoglobin. The circulatory system of squids has three "hearts" which pump blood through these ctenidia. The right and left branchial hearts pump blood through the gills where it is returned to the ventricular heart. This heart pumps blood forward and backwards through aorta to the rest of the body. Circulation and breathing are vital to the survival of the squid; low oxygen concentrations cause them to quickly lose strength. Aquariums which successfully keep squid must supply well-oxygenated water to keep them alive.
Despite the squid's swimming abilities, its abilities as a high jumper should not be overlooked. Instances where squid have leaped more than 40 feet out of the water are not uncommon. Apparently, when trying to avoid their predators, which can be numerous, entire schools of squid will leap and fall into the sea simultaneously, in one balletic motion. Consider this account by Thor Heyerdahl and his companions aboard the Kon-Tiki in 1947:
One sunny morning we all saw a glittering shoal of something which shot out of the water and flew through the air like large rain drops, while the sea boiled with pursuing dolphins. At first we took it for a shoal of flying fish, for we had already had three different kinds of these on board. But when they came near, and some of them sailed over the raft at a height of four or five feet, one ran straight into Bengt's chest and fell slap on deck. It was a small squid. Our astonishment was great.
The squids ability to avoid predators is probably what has kept it alive over the eons. Squids are prey for nearly all fishes in the ocean, many marine mammals, and even birds. Man, too, is quite fond of squid; thus, squid have evolved an entire arsenal of tools to avoid being eaten.
One tool animals employ to avoid predation is camouflage and squids are masters in this regard. The skin of squids contains dense concentrations of pigment-filled star-like cells called chromatophores. The elastic walls of these cells, controlled by muscles under nervous control, can be contracted and expanded, producing a dazzling array of spots, stripes, ripples, and changing hues of all colors. These color changes surpass the chameleon in terms of speed and versatility. Squids change colors to blend in with their environment, turning light-colored in shallow waters and deeper hues in darker waters. Squids even employ startle responses, rapidly changing their hues to temporarily confuse or startle their predators. For a squid, a few milliseconds delay in a predator's attack can mean the difference between escape or death.
The highly developed sensory system of squids also gives them an advantage over their predators. The squid "brain" consists of two fused nerve centers that are linked down the length of the body by two giant nerve axons. The giant axons are bundles of fused nerve fibers that transmit nerve signals very rapidly, making them ideal for escape response. In fact, so highly developed are these nerves that they provide ideal material for the study of nerve impulses. For more than 60 years, the giant nerve axons of squids have answered many fundamental questions about neurophysiology. Giant nerve axons still serve as an active subject of research, providing a vast store of information and intrigue to biophysicists, biochemists, pharmacologists, and neurophysiologists to this day.
No less significant are the squid's eyes and provide an excellent example of convergent evolution, where similar characteristics are developed in evolutionarily-distinct animals to solve a similar problem. Squids have eyes very much like human eyes, having a retina composed of rods and cones that may enable squids to distinguish fine detail and even color. Squid eyes also have eyelids and a pupil that can expand and contract. In fact, squids can focus each eye separately, providing them with vision that may be "twice as good as humans." The nerve impulses from each eye travel through huge optic nerves that feed into the brain for processing information. These rapid-fire nerve fibers allow them to quickly respond and maneuver in any situation.
An interesting feature of a squid's nervous system is its connection to structures called statocysts. These fluid-filled vesicles contain calcareous particles which allow the animal to orient itself to the gravitational field. Paired statocysts are embedded in the brain of the squid and allow them to remain aware of their orientation and movement in a three-dimensional manner.
One other well-known "escape" response of squids is their ability to eject black ink. Inside the mantle cavity of squids lies an ink sac. When disturbed, they eject a cloud of ink which temporarily confuses the squid's predators. The ink is believed to function like a smoke screen to create a diversion while the squid escapes. One species of squid turns black all over its body, emits a cloud of black ink, then turns white and slips away, all within the space of a second. All that's left for the predator is a cloud of ink. Another species of squid, which spends all of its life in the dark abyss, ejects a cloud of luminescent bacteria instead. In this way, it's predator might even be temporarily blinded, just long enough for the squid to escape.
The squid's highly developed nervous system allows them to compete well with fishes. As expressed by two Canadian scientists in a recent publication, "there are too many things that fish do well that squid either do poorly or not at all...however, there are a few things that squid do much better." Squid devote as much of their body weight to the nervous system as many reptiles, which are vertebrate. It is postulated that their nervous control of their circulatory system allows them to achieve the athletic feats described above. It is also postulated that their highly evolved nervous system allows them to maintain sophisticated patterns of social organization. Their gregarious nature and their use of color implies a kind of language between individuals. They have even been observed using arm signals; an upraised arm appears to mean "go away."
Many species of squids are equipped with photophores, bioluminescent light organs that function as camouflage, luring prey, reproduction, communication, or other yet undiscovered uses. More than fifty kinds of light organs have been described in squid ranging in size from pinpoints of light to discs the size of a quarter. A wide variety of patterns, colors, and shapes of photophores are described, many having the appearance of brilliant gems. Some species have photophores on their eyes and one species, which is translucent, has photophores on its liver! The use of photophores for communication has not been adequately documented, yet the following account of a squid attack on divers filming at night makes one wonder whether schools of squid emit an "attack" signal before they move in on their prey.
In 1991, Howard Hall, a cinematographer, decided to try to capture footage of a Humboldt squid (ranging up to 6 feet in length) following a hooked fish to the surface, a phenomenon often observed by Mexican fisherman. "Not a good idea," said one of the fisherman. Hall hung at about thirty feet, filming while a thresher shark was being reeled in. Suddenly, he noticed "rapid-fire strobes going off about 5 times per second...flashing from bright red to ivory white." Hall noticed a school of 5-foot long squid ascending from the depths. They attacked the shark and then turned their attention to Hall's dive buddy.
Alex was behind me in the darkness. He had no movie lights to ward off the squid. A group ascended from the depths below, frenzied by the smell of blood in the water. Three large squid grabbed Alex at the same time. Suddenly, he felt himself rushing backward and down. A tentacle reached around his neck and ripped off his pre-Columbian gold pendant and chain, tearing the skin on his neck. Another squid ripped his decompression computer off his pressure gauge. Tentacles tore his dive light from his wrist and his collection bag off his waist. Then, as suddenly as they had grabbed him, the squid were gone.
Whether the "red-and-white" flashes were group signals to attack or not is pure speculation on my part. Notwithstanding, it seems to me that the squid attacked the divers with the speed and skill of a well-seasoned team of navy commandos! The suddenness with which they left also attests to some kind of communication between the squids. And somewhere, I imagine, there is a squid with a gold chain around his neck and another with a computer record of all his dives.
Squid Mating Rituals
Finally, we come to the mating activities of these fascinating creatures. Off the coast of southern California in spring (and possibly in fall), thousands of Loligos gather for their annual mating ritual. Sexes are separate in squids. Eggs are produced by ovaries inside the mantle cavity; sperm is produced from a penis also located inside the mantle cavity. Ducts from the gonads lead to the funnel, where the eggs or sperm can be easily manipulated. When it comes time to mate, male squids flash their own unique signal using their chromatophores. If a female finds favor with his "pattern," they will embrace head to head by intertwining their arms. A packet of sperm will emerge from the funnel of the male, which he will grasp with one of his arms and place it in a pocket beneath the mouth of the female, in some species, or cement them inside the mantle, in others. The female will release all of her eggs and hold them in her arms, triggering the sperm packets to open and fertilize the eggs. (How all this occurs is still somewhat of a mystery!)
Once fertilized, the eggs will be released into the water, the squids will separate, and, if capable, swim away. However, the frenzy of copulation and the rigors of mating weaken the squid and those that do not die outright are quickly munched by the many sea lions, sharks, and dolphins that gather in the wings during such "festivals."
Egg masses are released as clusters; in some species, a single female may lay millions of eggs. The jelly-like substance that surrounds the eggs appears to be distasteful to other marine organisms so that they don't get eaten. However, once hatched, the miniature squid are easy prey for any number of animals. In addition, unlike other molluscs, squid eggs hatch miniature adults directly; there is no planktonic larval stage like there is for other molluscs.
Of all the invertebrates in the sea, squids and octopus must certainly be one of the most fascinating. Their long history with man is documented in numerous legends and folktales dating back to the Greeks. The knowledge of nerve conduction gained from our studies of squid giant axons surpasses that of any other organism. Still, there are many things we don't understand about these organisms and our continued exploitation of their populations and environment may prevent our ever knowing all of their habits. Only by continued research and judicious restrictions on squid harvesting can we hope to understand the true mysteries of these amazing creatures.