Most cephalopods—the group in which scientists classify octopuses, squid, cuttlefish and nautiluses—can change color faster than a chameleon. They can also change texture and body shape, and if those camouflage techniques don’t work, they can still “disappear” in a cloud of ink, which they use as a smoke-screen or decoy.
Cephalopods are also fascinating because they have three hearts that pump blue blood; they’re jet powered; and they’re found in all the oceans of the world—from the tropics to the poles, from the intertidal to the abyss.
Cephalopods have inspired legends and stories throughout history and are thought to be the most intelligent of the invertebrates. Some can squeeze through the tiniest of cracks. They have eyes and other senses that rival those of humans.
The class Cephalopoda, which means “head foot”, are mollusks and therefore related to bivalves (scallops, oysters, clams), gastropods (snails and slugs), scaphopoda (tusk shells), and polyplacophorans (chitons). Some mollusks, such as bivalves, don’t even have a head, much less something large enough to be called a brain! Yet, cephalopods have well-developed senses and large brains. Most mollusks are protected by a hard external shell and many of them are not very mobile.
Although the nautilus has an external shell, the trend in cephalopods is to internalize and reduce the shell. The shell in cuttlefish, when present, is internal and is called the cuttlebone, which is sold in many pet shops to supply calcium to birds. Squid also have a reduced internal shell called a pen. Octopuses lack a shell altogether.
Cephalopods are found in all of the world’s oceans, from the warm water of the tropics to the near freezing water at the poles. They are found from the wave swept intertidal region to the dark, cold abyss. All species are marine, and with a few exceptions, they do not tolerate even brackish water.
Cephalopods are an ancient group that appeared some time in the late Cambrian period several million years before the first primitive fish began swimming in the ocean. Scientists believe that the ancestors of modern cephalopods (Subclass Coleoidea: octopus, squid, and cuttlefish) diverged from the primitive externally-shelled Nautiloidea (Nautilus) very early—perhaps in the Ordovician, some 438 million years ago.
How long ago was this? To put this into perspective, this is before the first mammals appeared, before vertebrates invaded land and even before there were fish in the ocean and upright plants on land! Thus, nautilus is very different from modern cephalopods in terms of morphology and life history.
Cephalopods were once one of the dominant life forms in the world’s oceans. Today, there are only about 800 living species of cephalopods. By comparison, there is 30,000 living species of bony fish. However, in terms of productivity, some scientists believe that cephalopods are still giving fish a run for their money.
Many species of cephalopods to grow very fast, reproduce over a short period of time, and then die. With over-fishing and climate change, there may be more biomass of cephalopods now than anytime in recent history.
Cephalopods use their awesome abilities to change their color and appearance primarily for two things: camouflage and communication. The ability of the cephalopods to change color is a trait that has evolved over time due to a greater need to avoid predators and become competitive in an environment shared with vertebrates.
These abilities, and the behaviors associated with them, have become a major contributing factor to the success of the cephalopod family and are great examples of adaptation—physically, through natural selection, and behaviorally.
Camouflage is usually a cephalopod’s primary defense against predators. As cephalopods don’t have the protection of hard shells like many of their mollusk relatives, they make an easy to digest meal for a hungry predator. Therefore, most cephalopods try to avoid being seen to avoid being eaten. As well as predator avoidance, camouflage can also be used when lying in wait for unsuspecting prey to pass. Interestingly, cephalopods have more than one strategy for camouflage, and these will be discussed here.
Resembling the background
Background resemblance is the most well known form of camouflage. This is when the animal changes its color and texture to match as closely as possible that of its background. Cephalopods use their chromatophores to change color to match the brightness of the environment they are attempting to blend into, and some can also change texture using muscles in their skin. Many also use different body postures to help with this. They may hold their arms in certain ways or flatten them on the substrate to become what appears to be simply part of the scenery.
As well as simply trying to blend into a background, some cephalopods will attempt to make themselves appear like a specific object in their environments. This is termed deceptive resemblance.
The Caribbean reef squid, Sepioteuthis sepioidea, is often seen floating vertically at the surface of the water with its arms pointing downward to resemble floating sargassum weed. Some octopus may curl all their arms up into a ball, and add texture to their skin to appear like a rock.
Octopus cyanea has also been seen swimming in a manner that makes it appear like a reef fish by swimming with all its arms together and creating false eyespots.
Disruptive patterning is seen in many creatures as well as cephalopods and serves to break up the outline of the animal to confuse predators. It involves the chromatophores, which are used to create sharply contrasting patterns on the body, often wide stripes or spots. This is best seen in cuttlefish, which employ this technique more readily than other cephalopods.
Countershading is used to help a cephalopod blend in when there is no substrate against which to match itself. For instance, squid that spends much of their time in midwater rather than on or near the bottom can be seen easily by predators from below. Photophores and reflector cells on their underside match the light coming in through the water column, to make the squid almost invisible to animals below it. Countershading also makes rounded surfaces appear flat. So, a squid with a darker top surface and shades gradually decreasing to a pale under-surface will be harder to spot when viewed laterally.
Deimatic behaviour is often used when camouflage fails, and the cephalopod is still threatened. It involves changing rapidly from the colour it was using to blend into its environment, to bold contrasting colours such as white and black. Some species of octopus will change instantly from their mottled appearance to bright white with black around their eyes. Deimatic behaviour usually also involves body postures that make the animal appear bigger than it is. If this doesn’t work and the animal is still threatened, cephalopods will then usually ink and jet away.
Cephalopods use colour change as well as body postures to communicate, both with members of their own species as well as with members of other species. Many cephalopods have courtship displays in which males attempt to attract females by using chromatic displays (displays using colour changes) to show that they are suitable mates.
This is well developed in squid and cuttlefish but is less common in octopus in which complex courtship rituals have not yet been seen. Often during courtship, males will not only have to attempt to attract females but also to fend off other males. As chromatophores are neurally controlled, the animal may be able to produce a pattern on one side of its body to attract a female while producing another pattern on the other side, which it directs at other males.
Fighting between males also exhibits a lot of communication. With squid, time spent in acts of aggression involve mostly displays and very little physical contact. Squid will often show chromatic displays and body postures with increasing intensity until one back down.
In midwater, light organs and photophores are thought to be used for communication. In the same way, as colour is used in shallow water, bioluminescence can be used where there is less light to attract a mate, lure prey and dissuade predators.
Predator avoidance may also involve some forms of communication to the predator. As with deimatic behaviours, showing a predator that it has been spotted and attempting to make itself larger and more frightful than it is will at least often make a predator stop and think, giving vital seconds for escape. On the other hand, if the bluff is successful, the predator may back away, thinking that it is not as easy a target as anticipated.
Cephalopods have often been referred to as the chameleons of the sea. However, their ability to change colour is more impressive than that of the chameleon. Unlike the chameleon, many of the cephalopod’s colour producing cells are controlled neurally, which allows them to change colours very rapidly.
The patterns and colours seen in cephalopods are produced by different layers of cells stacked together, and it is the combination of certain cells operating at once that allows cephalopods to possess such a large array of patterns and colours.
The most well known of these cells is the chromatophore. Chromatophores are groups of cells that include an elastic saccule that holds a pigment, as well as 15-25 muscles attached to this saccule. These cells are located directly under the skin of cephalopods. When the muscles contract, they stretch the saccule allowing the pigment inside to cover a larger surface area. When the muscles relax, the saccule shrinks and hides the pigment.
Unlike in other animals, the chromatophores in cephalopods are neurally controlled, with each chromatophore being attached to a nerve ending. In some squid, each chromatophore muscle is innervated by two to six nerves that directly link to the animal’s brain.
In this way, the animal can increase the size of one saccule while decreasing the size of another one right next to it. This allows the cephalopods to produce complex patterns, such as the zebra stripes seen in aggressive displays by male cuttlefish.
The speed at which this can be controlled allows the animal to manipulate these patterns in a way that makes them appear to move across the body. In some species of cuttlefish, it has been noted that while hunting, the cuttlefish may produce a series of stripes that move down their bodies and arms. Some scientists have suggested that this could be used to mesmerize prey before striking, but the purpose of this behaviour has yet to be proven.
The pigments in chromatophores can be black, brown, red, orange or yellow. They are not responsible for producing the blue and green colours seen in some species. Interestingly, many deepwater forms possess fewer chromatophores as they are less useful in an environment in little or no light.
Iridophores are found in the next layer under the chromatophores. Iridophores are layered stacks of platelets that are chitinous in some species and protein-based in others. They are responsible for producing the metallic looking greens, blues and golds seen in some species, as well as the silver colour around the eyes and ink sac of others. Iridophores work by reflecting light and can be used to conceal organs, as is often the case with the silver colouration around the eyes and ink sacs. Additionally, they assist in concealment and communication.
Previously, it was thought that these colours were permanent and unchanging unlike the colours produced by chromatophores. New studies on some species of squid suggest that the colours may change in response to changing levels of certain hormones. However, these changes are obviously slower than neurally controlled chromatophore changes. Iridophores can be found in cuttlefish, some squid and some species of octopus.
Leucophores are the last layer of cells. These cells are responsible for the white spots occurring on some species of cuttlefish, squid and octopus. Leucophores are flattened, branched cells that are thought to scatter and reflect incoming light. In this way, the colour of the leucophores will reflect the predominant wavelength of light in the environment. In white light, they will be white, while in blue light, they will be blue. It is thought that this adds to the animal’s ability to blend into its environment.
Cephalopods have one final ability to change colour and pattern, the photophores. These produce light by bioluminescence. Photophores are found in most midwater and deep-sea cephalopods and are often absent in shallow-water species.
Bioluminescence is produced by a chemical reaction similar to that of a chemical light stick. Photophores may produce light constantly or flashlight intermittently. The mechanism for this is not yet known, but one theory is that the photophores can be covered up by pigments in the chromatophores when the animal does not wish for them to show.
Some species also have sacs containing resident bacteria that produce bioluminescence such as the tiny squid Euprymna. Midwater squid use photophores to match downwelling light or to attract prey.
It is the use of these cells in a combination that allow cephalopods to produce amazing colours and patterns not seen in any other family of animal. However, not all species of cephalopod possess all the cells described above.
For instance, photophores may be necessary for animals in deep water environments but are often absent in shallow-water forms. Deep-sea species may possess few or even no chromatophores as their colour changes would not be visible in an environment with no light.
Recent research has suggested that there may be some correlation between the number of chromatophores (and hence the complexity of patterns available) and the type and complexity of a cephalopod’s environment. For instance, midwater species may possess fewer chromatophores. While species living in reef type environments may possess more. However, further research still needs to be conducted in this area.
Cephalopods are known to have excellent senses, and of these senses, their vision is perhaps the best studied. At first glance, cephalopod eyes look very similar to those of humans, whales and fishes. With the exception of the externally shelled and primitive nautilus, all cephalopods can perceive focused images, just like we can.
Cephalopods are invertebrates and other than being multicellular animals, they are not even closely related to vertebrates such as whales, humans and fish. Cephalopods, and their eyes, evolved independently. Why would animals so distantly related as a fish and a cephalopod have developed an eye that is so similar?
Given the amazing ability of cephalopods to change colour perhaps the most surprising difference between vertebrate eyes and those of cephalopods is that most cephalopods are completely colour blind. How do we know? We can train octopuses to pick black objects over white objects, white objects over black objects, light grey objects over dark grey objects and vice versa, but we can not train them to differentiate between colourful objects that look the same in grayscale. Also, most cephalopods only have one visual pigment. We have three.
Although many species have not yet been tested, the only cephalopod known so far to have colour vision is the firefly squid, Watasenia scintillans. This species of midwater squid is bioluminescent and has three visual pigments. All other species tested so far only have one visual pigment.
Although most cephalopods can not see in colour, it has been demonstrated that octopuses and cuttlefish can detect differences in polarized light—without wearing polarized sunglasses. Shashar and Hanlon showed that squids (Loligo pealei) and Sepiolids (Euprymna scolopes) can exhibit polarized light patterns on their skin. Therefore, cephalopods can not only see differences in polarized light, they can also create patterns using these differences on their bodies. (See fact file on next page.)
Vision in cephalopod predators
The predators of cephalopods include fish—such as sharks—birds, marine mammals and other cephalopods. All of these predators have single-lens eyes, although often there is some variation between them to make their eyes more suitable to their environment and behaviour.
On land, it is the air-cornea interface of vertebrates that gives most of the ability to focus. However, underwater, there is no such interface, so the lens must be much more powerful than that of terrestrial animals. The eye of the fish has a wide-angle of view to make up for the fact that fish do not have necks and cannot turn their heads. Fish possess both rods and cones. Rods operate in low light intensity whereas cones allow for colour and high light intensity conditions.
Some fish also possess cones for vision in the ultraviolet part of the spectrum. Some fish have the ability to detect polarized light as do some cephalopods. There is a large variation in eye morphology within fish as they inhabit a large number of habitats with varying light regimes, from complex coral reefs to the pitch black of the deep sea.
Marine mammals that feed on cephalopods include dolphins, sea lions, and whales. Dolphins have a few adaptations to their eyes to assist them. For instance, they have muscles that can bend their lenses, so they can focus above the water. They also have a tapetum lucidum for night vision. Dolphins have rods and cones like humans; however, it is still unknown whether they see in colour.
It is thought that their combination of rods and cones allows them to see a large range of light intensities rather than colours. Sea lions do not have colour vision, although it is possible that they can detect light in the blue and green spectrum. They also have a tapetum lucidum for night vision. Sperm whales are known to feed on the infamous Architeuthis or giant squid.
These squid, however, live in the deep oceans where there is not enough light for vision to be effective. Researchers believe that the sperm whale does not have good eyesight, as its eyes are so disproportionately small to its head. It is thought that sperm whales use echolocation to find their prey.
Evolution of cephalopod vision
It is known that nearly all living things including plants show some form of photosensitivity. How did this come to be?
Firstly, most life, with the exception of some deep-sea vent creatures, is affected by light emitted from the sun, whether they require it for survival or are sensitive to it and must hide from it. All such organisms need to possess some sort of organ that allows an organism to know whether it is in high or low light, and possibly from which direction the light is coming.
500 million years
The ability to detect light with an eye has been developing for more than 500 million years and includes a variety of possible forms ranging from simple photoreceptors in single-celled organisms like Euglena to the highly complex vertebrate eye.
The first “eye” seen in single-celled organisms and flatworms were simple photoreceptors that could ascertain only the amount of light in the environment. The more advanced form of this was cup-shaped, which allowed the animal to discern from which direction the light was coming. However, this sort of eye did not allow the organisms to see as we think of it. Thus, the pinhole eye developed.
The pinhole eye is found in the Nautilus and consists of a small opening into a chamber, which allows a very small amount of light through. Light will pass through the pinhole after bouncing off different points of an object, and in this way basic shapes can be interpreted, not in any detail, however. The hole is so tiny only a small amount of light can get in which makes the image faint. If the hole were larger, the image would be distorted. This type of eye is incapable of focusing on objects at different distances. Instead, the size of the image produced will change in relation to the distance away from the object.
The compound eye was the first true image-forming eye, which was thought to have formed sometime during the Cambrian period, about 500 million years ago. The compound eye is common in insects and arthropods and consists of many ommatidia. Each ommatidia consists of a lens, crystalline cells, pigment cells and visual cells. The number of ommatidia will vary between species but may be up to 1000 per eye. Each ommatidia passes information on to the brain. This forms an image that is made of up dots as if looking very close at a digital photo. A higher number of ommatidia mean more dots which make the image clearer. This type of eye is only useful over short distances. However, it is excellent for movement detection.
For an animal to be able to focus on objects at different distances or even to produce a clear image of its surroundings at all, its eyes needed to develop lenses. It is thought that early cup-shaped eyes, like those of flatworms, contained a substance that protected them from seawater. If this substance were to bulge, it would form a pseudo lens that would help to make an image form more precisely, and this may be favoured by the process of natural selection.
Although the compound eye is full of lenses, the only way to make the image sharper with this design was to add more ommatidia. Of course, this means the eye would have to increase in size and can only do this to a point before it is too large for the animal. Thus, more complex lens eyes formed in both vertebrates and in cephalopods. Although both of these designs have many differences, there are also many similarities.
Cephalopod vs. Vertebrate Vision
As already stated, both cephalopods and vertebrates have very complex image-forming eyes with lenses. Both cephalopods and vertebrates have single-lens eyes. They work by allowing light to enter through the pupil and be focused by the lens onto the photoreceptor cells of the retina. However, between the two groups of animals, there are differences in the shape of the pupil, the way the lens changes focus for distance, the type of receptor cells that receive the light as well as some more subtle differences.
In vertebrates the pupil is round, and it changes in diameter depending on the amount of light in the environment. This is important because too much light will distort the image, and too little light will be interpreted as a very faint image. The cephalopod pupil is square and adjusts for the level of light by changing from a square to a narrow rectangle.
The way in which the two groups use the lens to focus differs. Vertebrates use muscles around the eye to change the shape of the lens, while cephalopods are able to manipulate their lens in or out to focus at different distances.
The receptor cells of vertebrate eyes are rods and cones. The cones are used for vision in high light environments, while the rods are used in low light. The time of day the animal needs its vision to be most effective will dictate the ratio of rods to cones. Cephalopods, however, have receptor cells called rhabdomeres similar to those of other molluscs. These contain microvilli, which allow the animal to see polarized and unpolarized light (see the page on polarization vision).
Lastly, the way in which light is directed at the retina differs between the two groups. Cephalopod retinas receive incoming light directly, while vertebrate retinas receive light that is bounced back from the back of the eye.
The evolution of cephalopods is thought to be due to an evolutionary “arms race”. Over the course of cephalopod history, they have moved from the seafloor, lost their shells, developed abilities to change colour, shape and texture as well as the ability to communicate in complex ways. It was their capacity to adapt to changing pressures that ensured their survival as a family. Those that did not adapt mostly became extinct.
The first cephalopods appeared 500 mya before bony fish existed. These first cephalopods had a hard external shell like many other molluscs but were able to leave the ocean bottom and swim to escape predators. When a predator came along, all the cephalopod had to do was let go of the bottom and float away like a hot air balloon.
One of the first advances may have been the creation of multiple chambers connected by a siphuncle; this allowed these early cephalopods to slowly change their buoyancy.
Other early advances were likely to have been the ability to swim slowly to control direction.
Two groups of cephalopods, the Nautiloids and Ammonids (570 mya), depended on their external shell and ability to swim to protect them from predators. Both of these sub-classes of cephalopods do not have many of the traits of their modern relatives, such as the ability to change colour, to produce sharp images with a lens-based eye, or the ability to swim fast.
It is hard to say why the Ammonites and all but six species of Nautilus have become extinct. These cephalopods had a wide variety of external shells, some coiled, some long and straight, some with spines. These shells provided good protection from predators but inhibited the animals’ mobility.
Predation pressure has long been thought to be one of the major forces driving cephalopod evolution. Perhaps as species of bony fish, many of which swim much faster than an externally shelled cephalopod, appeared in the early oceans, armour just wasn’t enough, and of those species that depended on armour, almost all have become extinct.
Modern cephalopods have evolved a different strategy. Instead of a heavy protective external shell, they have reduced and internalized this armour. The loss of the heavy armour frees them from the weight of carrying it around and the energy needed to produce it. Most modern cephalopods are active predators.
Instead of heavy armour, they rely on speed and visual tricks to avoid being eaten. Some scientists have suggested that these adaptations were in response to pressure from predators. Indeed, many of the tricks such as the ability to change colour, shape and texture as well as the ability to produce a visual ink decoy seem to be aimed directly at their predators. ■