Quick viewing(Text Mode)

Jet-Powered Masters of Disguise

Jet-Powered Masters of Disguise

It sucks! license . according reproduced are A like 3.0 nported U Jet-powered on W ikimedia B ommons . I mage Masters of t h e C reative C ommons ttribution -Sh A

to U ser Nh obgood Text based on Dr James Wood and Kelsie Jackson’s Lesson Plan. Disguise Adaption edited by Peter Symes

Most cephalopods—the group in which scientists classify , squid, and —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 fascinat- less something large enough to to the dark, cold abyss. All spe- ing because they have three be called a brain! Yet, cepha- cies are marine, and with a few Cephalopods hearts that pump blue ; lopods have well-developed exceptions, they do not tolerate they’re jet powered; and they’re senses and large brains. Most even brackish water. use color found in all the oceans of the mollusks are protected by a hard world—from the tropics to the external shell and many of them Ancient change to poles, from the intertidal to the are not very mobile. Although Cephalopods are an ancient interact with abyss. the has an external group that appeared some time Cephalopods have inspired shell, the trend in cephalopods in the late period sev- their own legends and stories throughout is to internalize and reduce the eral million years before the first history and are thought to be shell. The shell in cuttlefish, when primitive began swimming in species, with the most intelligent of the inver- present, is internal and is called the ocean. Scientists believe that other species, tebrates. Some can squeeze the , which is sold in through the tiniest of cracks. many pet shops to supply and with They have eyes and other senses calcium to birds. Squid that rival those of humans. also have a reduced their envi- internal shell called a Brainy pen. Octopuses lack a ronments. The class Cephalopoda, which shell altogether. means “head foot”, are mol- Cephalopods are lusks and therefore related found in all of the to bivalves (scallops, oysters, world’s oceans, from clams), gastropods ( and the warm water of the Cut– slugs), scaphopoda (tusk shells), tropics to the near freez- away and polyplacophorans (). ing water at the poles. show- Some mollusks, such as bivalves, They are found from the ing the don’t even have a head, much wave swept intertidal region chambers in a nautilus shell

64 X-RAY MAG : 38 : 2010 EDITORIAL FEATURES TRAVEL NEWS EQUIPMENT BOOKS SCIENCE & ECOLOGY EDUCATION PROFILES PORTFOLIO CLASSIFIED feature

Background resemblance is when the ani- mal changes its color and texture to match ecology as closely as possible that of its background. J ames W ood Squids the ancestors of modern cepha- reproduce over a short period color is a trait that has evolved appear like a spe- lopods (Subclass Coleoidea: of time, and then die. With over- over time due to a greater need cific object in their , squid, and cuttlefish) fishing and climate change, to avoid predators and become environments. This is diverged from the primitive there may be more biomass of competitive in an environment termed deceptive externally-shelled Nautiloidea cephalopods now than anytime shared with vertebrates. resemblance. (Nautilus) very early—perhaps in in recent history. These abilities, and the behav- The Caribbean reef the , some 438 million iors associated with them, have squid, Sepioteuthis years ago. Color change become a major contributing sepioidea, is often how long ago was this? To Cephalopods use their awe- factor to the success of the seen floating verti- put this into perspective, this some abilities to change their cephalopod family and are cally at the surface is before the first mammals color and appearance primarily great examples of — of the water with its appeared, before vertebrates for two things: camouflage and physically, through natural selec- arms pointing down- invaded land and even before communication. The ability of tion, and behaviorally. ward to resemble there were fish in the ocean and the cephalopods to change floating sargassum upright plants on land! Thus, nau- weed. Some octopus tilus is very different from modern may curl all their arms cephalopods in terms of mor- up into a ball, and phology and history. add texture to their Cephalopods were once one skin to appear like a of the dominant life forms in the rock. world’s oceans. Today, there are more than one strat- Octopus cyanea has also only about 800 living species of egy for camouflage, been seen swimming in a man- cephalopods. By comparison, and these will be dis- ner that makes it appear there is 30,000 living species of cussed here. like a reef fish by swim- bony fish. However, in terms ming with all its arms of productivity, some scientists Resembling together and believe that cephalopods are the background creating still giving fish a run for their Background resem- Deceptive false eye money. blance is the most resemblance spots. Many species of cephalopods well known form of As well as simply try- to grow very fast, camouflage. This ing to blend into a back- is when the ground, some cephalopods will

changes its color and attempt to make themselves Jan Derk texture to match as

J on G ross closely as possible that of its background. The nautilus is similar in gener- Camouflage Cephalopods use their chro- al form to other cephalopods, Camouflage is usually a cepha- matophores to change color to with a prominent head and lopod’s primary defense against match the brightness of the envi- tentacles. Nautiluses typi- predators. As cephalopods don’t ronment they are attempting to cally have more tentacles have the protection of hard blend into, and some can also than other cephalo- shells like many of their mollusk change texture using muscles in pods, up to ninety. These tentacles relatives, they make an easy to their skin. Many also use differ- are arranged digest meal for a hungry preda- ent body postures to help with into two circles tor. Therefore, most cephalopods this. They may hold their arms in and, unlike the try to avoid being seen to avoid certain ways or flatten them on tentacles of other being eaten. As well as preda- the substrate to become what cephalopods, they tor avoidance, camouflage can appears to be simply part of the have no suckers, also be used when lying in wait scenery. are undifferentiat- for unsuspecting prey to pass. ed and retractable. Interestingly, cephalopods have J ames W ood Deceptive resemblance 65 X-RAY MAG : 38 : 2010 EDITORIAL FEATURES TRAVEL NEWS EQUIPMENT BOOKS SCIENCE & ECOLOGY EDUCATION PROFILES PORTFOLIO CLASSIFIED Disruptive patterning being displayed by the cuttlefish, It is hard to know for sure the Sepia pharaonis. The large first benefit that the adapta- ecology white band helps to break tion of changing appearance up the outline of the cuttle- had for cephalopods. The fos- fish, making it harder to dis- sil record is spotty and does tinguish it from a complex environment when viewed not provide many clues as to from above by predators behavioral .

However, most scientists One side of the body produces believe the initial benefit of a pattern to attract a female the adaptation of changing while producing another pat- tern on the other side, which appearance was crypsis, the is directed at other males ability to blend in with the envi- J ames W ood

J ames W ood A California two-spot octopus, Octopus bimaculatus, displays its abili- ties to camouflage in different environments. This type of camouflage is Disruptive patterning color it was using to blend into its known as background resemblance, as both octopuses have adopted Disruptive patterning is seen in many environment, to bold contrasting colors, textures and postures to attempt to blend into the background creatures as well as cephalopods colors such as white and black. and serves to break up the outline Some species of octopus will of the animal to confuse preda- change instantly from their mottled tors. It involves the , appearance to bright white with which are used to create sharply black around their eyes. Deimatic contrasting patterns on the body, behavior usually also involves often wide stripes or spots. This is best body postures that make the ani- seen in cuttlefish, which employ this mal appear bigger than it is. If this technique more readily than other doesn’t work and the animal is still cephalopods. threatened, cephalopods will then usually ink and jet away. Countershading Countershading is used to help a Communication cephalopod blend in when there is Cephalopods use color change as no substrate against which to match well as body postures to commu- itself. For instance, squid that spend nicate, both with members of their J ames W ood much of their time in midwater own species as well as with mem- J ames W ood rather than on or near the bottom bers of other species. Many cepha- Octopuses are, of course, not the only cephalopods to dis- can be seen easily by predators lopods have courtship displays in Fighting between males also ronment. This allowed cepha- play background resemblance. A cuttlefish, Sepia pharaonis, is from below. and reflec- which males attempt to attract exhibits a lot of communication. With lopods to be camouflaged so attempting to match the color and texture of the sand in its labo- tor cells on their underside match females by using chromatic displays squid, time spent in acts of aggres- they could more easily catch ratory tank. Notice the white tipped papillae closest to the sand the light coming in through the (displays using color changes) to sion involve mostly displays and very their prey. Perhaps even more that give the appearance of small pebbles that can be seen in other areas of the tank. This ability not only protects the animal water column, to make the squid show that they are suitable mates. little physical contact. Squid will importantly, camouflage was from predators, it also assists with hunting as prey can come quite almost invisible to below it. This is well developed in squid and often show chromatic displays and the first line of defense against close without realizing there is a hungry cephalopod nearby Countershading also makes rounded cuttlefish but is less common in octo- body postures with increasing inten- predators. surfaces appear flat. So, a squid with pus in which complex courtship ritu- sity until one backs down. Much of cephalopod evo- a darker top surface and shades als have not yet been seen. Often In midwater, light organs and pho- lution is thought to be driven by predator two groups relied on their external shell to gradually decreasing to a pale during courtship, males will not only tophores are thought to be used for avoidance. The earliest of cephalopods are protect them from predators. The ammo- under-surface will be harder to spot have to attempt to attract females, communication. In the same way as thought to have used the ability to leave nites are extinct, and there are now only six when viewed laterally. but also to fend off other males. As color is used in shallow water, bio- the bottom and swim up into the water col- species of nautilus in existence. All the rest chromatophores are neurally con- luminescence can be used where umn as a way to escape predators. of the modern cephalopods, the coleoid Deimatic behavior trolled, the animal may be able to there is less light to attract a mate, As both predators and their prey evolved, cephalopods, have reduced and internal- Deimatic behavior is often used produce a pattern on one side of its lure prey and dissuade predators. two major groups of cephalopods—the ized shells and have the ability to change when camouflage fails, and the body to attract a female while pro- Predator avoidance may also ammonites and nautilus—became some of color, texture and shape to camouflage cephalopod is still threatened. It ducing another pattern on the other involve some forms of communica- the most common marine animals. These and avoid detection from predators. ■ involves changing rapidly from the side, which it directs at other males. tion to the predator. As with deimat-

66 X-RAY MAG : 38 : 2010 EDITORIAL FEATURES TRAVEL NEWS EQUIPMENT BOOKS SCIENCE & ECOLOGY EDUCATION PROFILES PORTFOLIO CLASSIFIED Skin pattern created by cromatophores in

this Atlantic Bobtail A ndy M urc h ecology squid Sepiola atlantica shrinks and hides the pig- ment. Unlike in other animals, the chro- matophores in cephalopods are neurally controlled, with each chro- matophore being attached to a nerve ending. In some squid, each muscle is responsible for innervated by two to six nerves producing the that directly link to the animal’s blue and green colors seen in brain. some species. Interestingly, many according

license . In this way, the animal can deep water forms possess fewer increase the size of one sac- chromatophores as they are less cule while decreasing the size of useful in an environment in little reproduced another one right next to it. This or no light. allows the cephalopods to pro- duce complex patterns, such as Iridophores are A like 3.0 nported U the zebra stripes seen in aggres- Iridophores are found in the sive displays by male cuttlefish. next layer under the chromat- The speed at which this can opphores. Iridophores are lay- be controlled allows the animal ered stacks of platelets that are to manipulate these patterns in chitinous in some species and on W ikimedia B ommons . mage I a way that makes them appear protein based in others. They are to move across the body. In responsible for producing the

Hillewaert some species of cuttlefish, it has metallic looking greens, blues been noted that while hunt- and golds seen in some species, Hans

t h e C reative C ommons ttribution -Sh A ing, the cuttlefish may produce as well as the silver color around U ser to a series of stripes the eyes and of others. ic behaviors, showing a predator many of the cephalopod’s color Chromatophores are that move down Iridophores work by reflect- that it has been spotted and producing cells are controlled groups of cells that their bodies and ing light and can be used to attempting to make itself larger neurally, which allows them to include an elastic sac- arms. Some sci- conceal organs, as is often the and more frightful than it is will at change colors very rapidly. cule that holds a pig- entists have sug- case with the silver coloration least often make a predator stop The patterns and colors ment, as well as 15-25 gested that this around the eyes and ink sacs. and think, giving vital seconds seen in cephalopods are pro- muscles attached to could be used to Additonally, they assist in con- for escape. On the other hand, if duced by different layers of this saccule. These mesmerize prey cealment and communication. the bluff is successful, the preda- cells stacked together, and it is cells are located before striking, but Previously, it was thought that tor may back away, thinking the combination of certain cells directly under the the purpose of this these colors were permanent that it is not as easy a target as operating at once that allows skin of cephalopods. behavior has yet and unchanging unlike the anticipated. cephalopods to possess such When the muscles to be proven. colors produced by chromato- Cephalopods have often a large array of patterns and contract, they stretch The pigments phores. New studies on some been referred to as the chame- colors. the saccule allowing in chromato- species of squid suggest that the leons of the sea. However, their the pigment inside to phores can be colors may change in response ability to change color is more Chromatophores cover a larger surface black, brown, red, to changing levels of certain hor- impressive than that of the cha- The most well known of these area. When the mus- orange or yel- mones. However, these changes meleon. Unlike the chameleon, cells is the chromatophore. cles relax, the saccule low. They are not are obviously slower than neural- Closeup of cromatophores 67 X-RAY MAG : 38 : 2010 EDITORIAL FEATURES TRAVEL NEWS EQUIPMENT BOOKS SCIENCE & ECOLOGY EDUCATION PROFILES PORTFOLIO CLASSIFIED Chromatophores 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 sac- Squids ecology cule shrinks and hides the pigment. ly controlled chromatophore changes. chemical light stick. Photophores may Iridophores can be found in cuttlefish, produce light constantly or flash light some squid and some species of octo- intermittently. The mechanism for this is pus. not yet known, but one theory is that the photophores can be covered up by Leucophores pigments in the chromatophores when Leucophores are the last layer of cells. the animal does not wish for them to These cells are responsible for the white show. spots occurring on some species of cut- Some species also have sacs con- tlefish, squid and octopus. Leucophores taining resident bacteria that produce are flattened, branched cells that are bioluminescence such as the tiny squid thought to scatter and reflect incoming Euprymna. Midwater squid use photo- light. In this way, the color of the leu- phores to match downwelling light or to cophores will reflect the predominant attract prey. wavelength of light in the environment. It is the use of these cells in combina- In white light, they will be white, while in tion that allow cephalopods to produce blue light, they will be blue. It is thought amazing colors and patterns not seen that this adds to the animal’s ability to in any other family of animal. However, blend into its environment. not all species of cephalopod pos- sess all the cells described above. For Photophores instance, photophores may be neces- Cephalopods have one final ability to sary for animals in deep water environ- change color and pattern, the photo- ments but are often absent in shallow phores. These produce light by biolu- water forms. Deep sea species may minescence. Photophores are found in possess few or even no chromatophores most midwater and deep sea cepha- as their color changes would not be vis- lopods and are often absent in shallow ible in an environment with no light. water species. Recent research has suggested Bioluminescence is produced by a that there may be some correlation chemical reaction similar to that of a between the amount of chromato- J on G ross phores (and hence the complexity of pods can perceive focused images, just objects, light grey objects over dark grey patterns available) and the type and like we can. objects and vice versa, but we can not complexity of a cephalopod’s environ- Cephalopods are invertebrates and train them to differentiate between color- ment. For instance, midwater species other than being multicellular animals, ful objects that look the same in gray- may possess fewer chromatophores. they are not even closely related to ver- scale. Also, most cephalopods only have While species living in reef type envi- tebrates such as whales, humans and fish. one visual pigment. We have three. ronments may possess more. However, Cephalopods, and their eyes, evolved Although many species have not yet further research still needs to be con- independently. Why would animals so dis- been tested, the only cephalopod known ducted in this area. tantly related as a fish and a cephalopod so far to have color vision is the firefly have developed an eye that is so similar? squid, Watasenia scintillans. This species Cephalopod vision of midwater squid is bioluminescent and Cephalopods are known to have excel- Colorblind has three visual pigments. All other spe- lent senses, and of these senses, their Given the amazing ability of cephalo- cies tested so far only have one visual vision is perhaps the best studied. At first pods to change color perhaps the most pigment. glance, cephalopod eyes look very simi- surprising difference between vertebrate lar to those of humans, whales and . eyes and those of cephalopods is that Polarized light With the exception of the externally most cephalopods are completely color Although most cephalopods can not see shelled and primitive nautilus, all cephalo- blind. How do we know? We can train in color, it has been demonstrated that octopuses to pick black objects over octopuses and cuttlefish can detect dif- Octopus burryi showing white white objects, white objects over black ferences in polarized light—without wear-

J ames W ood spots due to leucophores 68 X-RAY MAG : 38 : 2010 EDITORIAL FEATURES TRAVEL NEWS EQUIPMENT BOOKS SCIENCE & ECOLOGY EDUCATION PROFILES PORTFOLIO CLASSIFIED Peter Symes ecology Squids of Given the amazing ability of cephalopods to change cephalopod vision color perhaps the most surprising difference be- It is known that nearly all living tween vertebrate eyes and those of cepha- things including plants show the water. They also some form of photosen- lopods given is that most cephalo- have a tapetum lu- sitivity. How did this pods are completely color cidum for night vision. come to be? Dolphins have rods and Firstly, most life, blind. cones like humans; however, it is with the excep- still unknown whether they see in tion of some deep sea vent crea- color. It is thought that their com- tures, is affected by light emitted bination of rods and cones allows from the sun, whether they require Glossary: ever, them to see a large range of light it for survival or are sensitive to The tapetum underwa- intensities rather than colors. Sea it and must hide from it. All such lucidum (Latin ter, there is lions do not have color vision, organisms need to possess some for “bright no such inter- although it is possible that they sort of organ that allows an organ- tapestry”; face, so the lens can detect light in the blue and ism to know whether it is in high or plural tapeta must be much more green spectrum. They also have a low light, and possibly from which lucida) is a powerful than that of tapetum lucidum for night vision. direction the light is coming. layer of tissue terrestrial animals. The Sperm whales are known to feed in the eye of eye of the fish has a on the infamous Architeuthis, or 500 million years many verte- wide angle of view to giant squid. The ability to detect light with brate animals, make up for the fact These squid, however, live in the and eye has been developing for that lies imme- that fish do not have deep oceans where there is not more than 500 million years and diately behind or necks and cannot enough light for vision to be effec- includes a variety of possible forms sometimes within the turn their heads. Fish tive. Researchers believe that the ranging from simple photorecep- retina. It reflects visible possess both rods and sperm whale does not have good tors in single celled organisms like light back through the cones. Rods operate in eyesight, as its eyes are so dispro- Euglena to the highly complex retina, increasing the light low light intensity whereas portionately small to its head. It vertebrate eye. available to the photorecep- cones allow for color and is thought that sperm whales use The first “eye” seen in single- tors. This improves vision in high light intensity conditions. echolocation to find their prey. celled organisms and flatworms low-light conditions, but can Some fish also possess cones were simple photoreceptors that cause the perceived for vision in the ultraviolet part of image to be blurry

W ood J ames the spectrum. Some fish have the from the interference ability to detect polarized light as of the reflected light. Vision in do some cephalopods. There is The tapetum lucidum cephalopod predators large variation in eye morphology contributes to the The predators of cephalopods within fish as they inhabit a large superior night vision of ing polarized sunglasses. Shashar include fish—such as sharks—birds, number of habitats with varying some animals. Many and Hanlon showed that squids marine mammals and other ce- light regimes, from complex coral of these animals are (Loligo pealei) and Sepiolids phalopods. All of these predators reefs to the pitch black of the nocturnal, especially (Euprymna scolopes) can exhibit have single lens eyes, although deep sea. carnivores that hunt polarized light patterns on their often there is some variation be- their prey at night, skin. Therefore, cephalopods can tween them to make their eyes Marine mammals that feed on ce- while others are deep not only see differences in polar- more suitable to their environment phalopods include dolphins, sea sea animals. Although ized light, they can also create and behavior. lions, and whales. Dolphins have strepsirrhine primates patterns using these differences a few adaptations to their eyes have a tapetum luci- on their bodies. (See fact file on Fish On land, it is the air-cornea to assist them. For instance, they dum, humans and next page.) interface of vertebrates that gives have muscles that can bend their other haplorhine most of the ability to focus. How- lenses, so they can focus above primates do not. ■ J ames W ood

69 X-RAY MAG : 38 : 2010 EDITORIAL FEATURES TRAVEL NEWS EQUIPMENT BOOKS SCIENCE & ECOLOGY EDUCATION PROFILES PORTFOLIO CLASSIFIED Peter Symes

and in many directions at once How does it happen? have photoreceptor cells that polarized light in Seeing while still traveling in the same Polarization can happen in a contain microvilli. The microvilli of a way that they general direction. Looking head number of ways. Firstly, when each receptor cell are lined up can intensify or shut Polarized Light on at a light wave, the assump- light hits an object, it can parallel to each other. Microvilli off. This could be a tion is that the wave is a straight become polarized if it is reflect- contain the visual pigment rho- form of communica- It has been shown through sci- vertical line as it moves toward ed, refracted or scattered of dopsin, which is also orientated tion between members entific experiments that squid, the viewer. But, in actual fact, off certain surfaces. Light may parallel in the microvilli. Receptor of a species not visible octopus and cuttlefish are the wave moves vertically, hori- reflect off a non-metallic object cells are aligned at right angles to some other ani- able to detect polarized light zontally, and diagonally all at the or substance (like water) and to each other, and hence the mals, especially as well as create signals using same time. This is how unpolar- become polarized. Polarized light microvilli of one receptor cell will predators such polarized light on their skin. ized light from the sun behaves, that has experienced reflection be at right angles to that of the as sharks, seals it is disorganized. Polarized light, will travel parallel to the surface next receptor cell. The rhodopsin and cetaceans. What the difference? on the other hand, only vibrates of the object, which in the case assist in seeing the polarized What is polarized light and how on one plane. The wave of of bodies of water, creates glare. light. Because the microvilli are Invisible communications is it different from unpolarized polarized light, traveling toward arranged at right angles to one It is thought that these creatures The reflec- light? Light is a form of electro- the viewer appears as only The amount of polarization will another, the animal is able to dis- do not possess the ability to see magnetic radiation that travels one vertical or horizontal line tinguish between different planes polarized light, and thus cepha- tion depend on the angle of the created as a wave. The wave that the light is traveling on. lopods may have an advantage incoming light. When light under- by silvery doesn’t just vibrate goes refraction (i.e. when it over them in being able to com- fish scales is on one plane; passes from air to water and Reflections municate with one another with- often polarized instead, it vibrates gets bent), it may become Cephalopods can use their out attracting the attention of on many polarized, although this time ability to see polarized light in predators. It is also thought that planes the polarized wave will usually many ways. Firstly, it is thought cephalopods and other marine travel perpendicular to the that they can see though the animals that can detect differ- Unpolarized light is disor- surface of the substance reflection created by sil- ences in polar- it has passed through. very fish scales to better Cuttlefish ized light may use ganized. Polarized light, Light may also identify prey and predators their abilities to on the other hand, only become par- Often this reflection is polar- reflect polar- detect polariza- tially polar- ized. Just as humans put on ized light in a tion to assist them vibrates on one plane. ized by polarized sunglasses to see in navigation. ■ scat- through the glare created way that they ter- by polarized reflection off can intensify the surface of the ocean, the cephalopod can cut out or shut off the glare of polarized light produced

W ood J ames by reflection off fish scales to better ing, distinguish prey. as Translucent prey light may also be more waves visible for the same rea- bounce son, as light reflecting off the off particles while pass- tissues of the prey may be ing through a substance. polarized, and while it may Stress in a pair Author Spigget via Wikimedia not produce glare, it would Commons. This file is licensed under make the prey animal more vis- of plastic glasses the Creative Commons Attribution- They can but we cannot can be seen Share Alike 3.0 Unported license. ible to animals that can see this So why can cephalopods, and by placing it the majority of mobile marine reflection such as cephalopods. between two animals, see polarized light and polarizing filters humans cannot? Cephalopods Manipulating polarisations have different photoreceptor It has been shown that the iri- cells from humans. Cephalopods dophores on cuttlefish reflect

70 X-RAY MAG : 38 : 2010 EDITORIAL FEATURES TRAVEL NEWS EQUIPMENT BOOKS SCIENCE & ECOLOGY EDUCATION PROFILES PORTFOLIO CLASSIFIED W ood J ames

The ability to A ndy M urc h ecology detect light with could ascertain only the amount of light in the environment. The more ad- and eye has been vanced form of this was cup-shaped, which allowed the animal to discern developing for from which direction the light was more than 500 coming. However, this sort of eye did not allow the organisms to see as we million years and think of it. Thus, the pinhole eye devel- includes a variety oped. of possible forms Pinhole eye The pinhole eye is found in the Nautilus ranging from sim- and consists of a small opening into a chamber, which allows a very small ple photoreceptors amount of light through. Light will pass in single celled through the pinhole after bouncing off different points of an object, and in this organisms to the way basic shapes can be interpreted, highly complex not in any detail however. The hole is so tiny only a small amount of light can vertebrate eye. get in which makes the image faint. If The pinhole eye of a Nautilus is incapable of focusing on objects at different distances. the hole were larger, the image would Instead, the size of the image produced will change in relation to the distance away be distorted. This type of eye is incapa- from the object. ble of focusing on objects at different distances. Instead, the size of the im- worms, contained a substance that and vertebrates have single lens eyes. age produced will change in relation protected them from seawater. If this They work by allowing light to enter to the distance away from the object. substance were to bulge, it would form through the pupil and be focused by The compound eye was the first a pseudo lens that would help to make the lens onto the photoreceptor cells of true image-forming eye, which was an image form more precisely, and the retina. However, between the two thought to have formed some time this may be favored by the process of groups of animals, there are differences during the Cambrian period, about 500 . Although the com- in the shape of the pupil, the way the million years ago. The compound eye pound eye is full of lenses, the only way lens changes focus for distance, the is common in insects and to make the image sharper with this type of receptor cells that receive the and consists of many ommatidia. Each design was to add more ommatidia. light as well as some more subtle differ- ommatidia consists of a lens, crystal- Of course, this means the eye would ences. line cells, pigment cells and visual cells. have to increase in size James Wood The number of ommatidia will vary and can only do this to a between species but may be up to point before it is too large 1000 per eye. Each ommatidia passes for the animal. Thus, more information on to the brain. This forms complex lens eyes formed an image that is made of up dots, as in both vertebrates and in if looking very close at a digital photo. cephalopods. Although A higher number of ommatidia mean both of these designs have more dots which make the image many differences, there clearer. This type of eye is only useful are also many similarities. over short distances. However, it is ex- cellent for movement detection. Cephalopod vs. For an animal to be able to focus on Vertebrate Vision objects at different distances or even As already stated, both to produce a clear image of its sur- cephalopods and verte- roundings at all, its eyes needed to brates have very complex develop lenses. It is thought that early image-forming eyes with cup-shaped eyes, like those of flat- lenses. Both cephalopods

71 X-RAY MAG : 38 : 2010 EDITORIAL FEATURES TRAVEL NEWS EQUIPMENT BOOKS SCIENCE & ECOLOGY EDUCATION PROFILES PORTFOLIO CLASSIFIED feature Ecology

Squid hun- dreds of millions Squids ecology years old In vertebrates the pupil is round, contain microvilli, which allow pods are active predators. Instead of and it changes in diameter de- the animal to see polarized and heavy armor, they rely on speed and pending on the amount of light in unpolarized light (see page on visual tricks to avoid being eaten. the environment. This is important polarization vision). Some scientists have suggested that because too much light will dis- Lastly, the way in which light these adaptations were in response tort the image, and too little light is directed at the retina differs to pressure from predators. Indeed, will be interpreted as a very faint between the two groups. Cepha- many of the tricks such as the ability image. The cephalopod pupil is lopod retinas receive incoming to change color, shape and texture square and adjusts for the level of light directly, while vertebrate reti- as well as the ability to produce a light by changing from a square nas receive light that is bounced visual ink decoy seem to be aimed to a narrow rectangle. back from the back of the eye. directly at their predators. ■ The way in which the two groups use the lens to focus dif- Evolution fers. Vertebrates use muscles The around the eye to change the is thought to be due to an evo- shape of the lens, while cephalo- lutionary “arms race”. Over the When a predator came along, all pods are able to manipulate their course of cephalopod history, they the cephalopod had to do was lens in or out to focus at different have moved from the sea floor, let go of the bottom and float distances. lost their shells, developed abilities away like a hot air balloon. One The receptor cells of vertebrate to change color, shape and tex- of the first advances may have eyes are rods and cones. The ture as well as the ability to com- been the creation of multiple cones are used for vision in high municate in complex ways. It was chambers connected by a sip- light environments, while the rods their capacity to adapt to chang- huncle; this allowed these early are used in low light. The time of ing pressures that ensured their cephalopods to slowly change day the animal needs its vision to survival as a family. Those that did their buoyancy. be most effective will dictate the not adapt mostly became extinct. Other early advances were ratio of rods to cones. Cephalo- The first cephalopods appeared likely to have been the ability to pods, however, have receptor 500 mya, before bony fish existed. swim slowly to control direction. cells called rhabdomeres similar These first cephalopods had a Two groups of cephalopods, the

to those of other mollusks. These hard external shell like many other and Ammonids (570 J ames W ood mollusks but mya), depended on their external were able shell and ability to swim to pro- lens-based eye, or the ability to the major forces driving cephalo- Glossary: to leave the tect them from predators. Both of swim fast. pod evolution. Perhaps as species The is a strand of ocean bot- these sub-class- It is hard to say why the Am- of bony fish, many of which swim tissue passing longitudinally tom and swim es of monites and all but six species much faster than an externally through the shell of a ce- to escape of Nautilus have become shelled cephalopod, appeared phalopod mollusk. Only ce- predators. extinct. These cephalo- in the early oceans, armor just phalopods with chambered pods had a wide variety wasn’t enough, and of those spe- shells have , such of external shells, some cies that depended on armor, as the extinct ammonites coiled, some long and almost all have become extinct. and , and the straight, some with living nautiluses, cuttlefish, spines. These shells pro- Differently strategy and . In the case of vided good protection Modern cephalopods have the cuttlefish, the siphuncle from predators evolved a different strategy. In- is indistinct and connects all but inhibited stead of a heavy protective exter- the small chambers of that Ammonoids ce- the animals’ nal shell, they have reduced and animal’s highly modified first appeared phalopods do mobility. internalized this armor. The loss of shell; in the other cepha- in the Late t h e license . Silurian to Early not have many of the traits of to pres- the heavy armor frees them from lopods it is thread-like and Hillewaert Devonian their modern relatives, such as sure has long the weight of carrying it around passes through small open- reproduced

(circa 400 mil- the ability to change color, to Hans been thought and the energy needed to pro- ings in the walls dividing the

lion years ago) produce sharp images with a U ser on W ikimedia ommons . B I mage according C reative C ommons A ttribution -Sh are A like 3.0 U nported to be one of duce it. Most modern cephalo- chambers. ■

72 X-RAY MAG : 38 : 2010 EDITORIAL FEATURES TRAVEL NEWS EQUIPMENT BOOKS SCIENCE & ECOLOGY EDUCATION PROFILES PORTFOLIO CLASSIFIED

Top View