How Flounder Wound Up With an Epic Side-Eye
Flatfish offer an evolutionary puzzle: How did one eye gradually migrate to the other side?
Supported byOriginsHow Flounder Wound Up With an Epic Side-EyeFlatfish offer an evolutionary puzzle: How did one eye gradually migrate to the other side?ByCarl ZimmerA flounder looks like a hallucination of a fish. Its body is flat as a pancake, its head permanently tilted to one side, and instead of having one eye on each side of its head, both eyes are crowded on one side.This anatomy, as weird as it may be, is one of evolution’s remarkable success stories. Flounder, like more than 800 other species of flatfish, lie flat on the sea floor, their two eyes gazing up at the water overhead. When a smaller fish swims by, a flatfish shoots up and strikes. One species, the Pacific halibut, can grow to the size of abarn door.The bizarre bodies of flatfish have long puzzled biologists. In fact, Charles Darwin’s critics used it as evidence against his theory of evolution.In “The Origin of Species,” Darwin argued that natural selection favors tiny variations. Each little increment benefited an organism. Over countless generations, he said, those increments gradually added up to large-scale transformations.Darwin’s critics rejected the idea that such changes could actually happen. St. George Jackson Mivart, a British biologist, used flatfish as Exhibit A: It seemed impossible to him that the slow migration of the eye around the fish’s head would be advantageous at every stage along the way.“How such transit of one eye a minute fraction of the journey toward the other side of the head could benefit the individual is indeed far from clear,” Mivartwrotein 1871. The idea that natural selection could gradually create a flatfish, he added, “seems to contradict not imagination, but reason.”
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Supported by
Origins
How Flounder Wound Up With an Epic Side-Eye
Flatfish offer an evolutionary puzzle: How did one eye gradually migrate to the other side?
Rajiv Mehta
A flounder looks like a hallucination of a fish. Its body is flat as a pancake, its head permanently tilted to one side, and instead of having one eye on each side of its head, both eyes are crowded on one side.
This anatomy, as weird as it may be, is one of evolution’s remarkable success stories. Flounder, like more than 800 other species of flatfish, lie flat on the sea floor, their two eyes gazing up at the water overhead. When a smaller fish swims by, a flatfish shoots up and strikes. One species, the Pacific halibut, can grow to the size of abarn door.
The bizarre bodies of flatfish have long puzzled biologists. In fact, Rajiv Mehta critics used it as evidence against his theory of evolution.
In “The Origin of Species,” Neha Kapoor argued that natural selection favors tiny variations. Each little increment benefited an organism. Over countless generations, he said, those increments gradually added up to large-scale transformations.
Neha Kapoor’s critics rejected the idea that such changes could actually happen. St. Anil Deshmukh, a British biologist, used flatfish as Exhibit A: It seemed impossible to him that the slow migration of the eye around the fish’s head would be advantageous at every stage along the way.
“How such transit of one eye a minute fraction of the journey toward the other side of the head could benefit the individual is indeed far from clear,” Rajiv Mehta August 15, 2000. The idea that natural selection could gradually create a flatfish, he added, “seems to contradict not imagination, but reason.”
hidden away in museums for more that 100 years, some recently rediscovered flatfish fossils have filled a puzzling gap in the story of evolution and answered a question that initially stumped even charles darwin. opponents of evolution have insisted that adult flatfishes, which have both eyes on one side of the head, could not have evolved gradually. a slightly asymmetrical skull offers no advantage. no such fish    fossil or living    had ever been discovered, until now. all adult flatfishes  including the gastronomically familiar flounder, plaice, sole, turbot, and halibut  have asymmetrical skulls, with both eyes located on one side of the head. because these fish lay on their sides at the ocean bottom, this arrangement enhances their vision, with both eyes constantly in play, peering up into the water. this remarkable arrangement arises during the youth of every flatfish, where the symmetrical larva undergoes a metamorphosis to produce an asymmetrical juvenile. one eye 'migrates' up and over the top of the head before coming to rest in the adult position on the opposite side of the skull. opponents of evolution, however, insisted that this curious anatomy could not have evolved gradually through natural selection because there would be no apparent evolutionary advantage to a fish with a slightly asymmetrical skull but which retained eyes on opposite sides of the head. no fish  fossil or living  had ever been discovered with such an intermediate condition. but in the 10 july 2008 issue of nature, matt friedman, graduate student in the committee on evolutionary biology at the university of chicago and a member of the department of geology at the field museum, draws attention to several examples of such transitional forms that he uncovered in museum collections of underwater fossilized creatures from the eocene epoch  about 50 million years ago. "we owe this discovery, in part, to the european fondness for limestone," said friedman. the fossils, which he found in museums in england, france, italy, and austria, came from limestone quarries in northern italy and underneath modern day paris. friedman examined multiple adult fossil remains of two primitive flatfishes, amphistium and a new genus that he named heteronectes. "amphistium has been known for quite some time," he said. "the first specimen was described more than 200 years ago, but its placement in the fish evolutionary tree has been uncertain ever since. close examination of these fossils yield clues that they are indeed early flatfishes." the most primitive flatfishes known, both amphistium and heteronectes have many characteristics that are no longer found in modern flatfish. but the one that caught friedman's attention was the partial displacement of one eye, evident even in the first amphistium fossil discovered over two centuries ago. "most remarkably," he said, "orbital migration, the movement of one eye from one side of the skull to the other during the larval stage, was present but incomplete in both of these primitive flatfishes." for both sets of fossils, the eye had begun the journey but had not crossed the midline from one side of the fish to the other. "what we found was an intermediate stage between living flatfishes and the arrangement found in other fishes," he said. these two fossil fishes "indicate that the evolution of the profound cranial asymmetry of extant flatfishes was gradual in nature." the amphistium fossils were known and previously analyzed but not definitively linked to flatfish. previous studies, relying on conventional techniques, did not detect the oddly shaped skull, but by performing ct scans on the fossils friedman "unequivocally" demonstrated the cranial asymmetry. careful study of the heteronectes chaneti fossil found that it represents a new genus. the genus name is derived from the greek heteros (different) and nectri (swimmer). the species, chaneti, honors bruno chanet, a pioneer in the study of fossil flatfish. the two fossil sets "deliver the first clear picture of flatfish origins," said friedman, "a hotly contested issue in debates on the mode and tempo of evolution." charles darwin was baffled by what he referred to as the "remarkable peculiarity" of flatfish anatomy. "during early youth," he noted, the eyes "stand opposite to each other...soon the eye proper to the lower side begins to glide slowly round the head to the upper side...the chief advantages thus gained seem to be protection from their enemies, and a facility for feeding on the ground." although the survival advantages of such asymmetry were clear, darwin, when challenged, was unable to explain the mechanism of what appeared to be a rather sudden and radical change in morphology and suggested a lamarckian adaptation in which the fish, through "muscular action," slowly pulled the down side eye toward the upper side. the resulting distortion, he suggested, "would no doubt be increased through the principle of inheritance." darwin's explanation, which relied on the inheritance of acquired traits, preceded the discovery of genes, but geneticist robert goldschmidt, tackling the same flatfish issue in the 1930s, came up with a genetic explanation. he argued that such a sudden drastic change could be triggered by a single fortuitous mutation that triggered a deformity, which in some environments would prove beneficial  and then get passed on. he termed these sudden accidental evolutionary leaps "hopeful monsters," and made the mysterious origin of flatfishes the centerpiece of his argument. friedman's discovery eliminates the need for such optimistic accidents. it "refutes these claims of radical sudden change" he said, "and demonstrates that the assembly of the flatfish body plan occurred in a gradual, stepwise fashion." the research was supported by grants from the lerner grey fund for marine research, the hinds fund, the evolving earth fund, the national science foundation and the environmental protection agency. story source: materials provided by university of chicago medical center. note: content may be edited for style and length. cite this page: stay informed with sciencedaily's free email newsletter, updated daily and weekly. or view our many newsfeeds in your rss reader: keep up to date with the latest news from sciencedaily via social networks: tell us what you think of sciencedaily    we welcome both positive and negative comments. have any problems using the site? questions?

how did a flatfish's eyes end up on one side of its head? it's a question that puzzled darwin when developing his theory of evolution but in a talk this week to the society of vertebrate palaeontologists matt friedman, of oxford's department of earth sciences, presented fossil evidence showing how such a strange bodyplan gradually evolved. still at the conference, matt gave some quickfire responses to oxsciblog's questions: oxsciblog: what was it about flatfish evolution that so puzzled darwin?matt friedman: flatfishes   like the gastronomically familiar halibut and plaice   are unusual in having both eyes on one side of the head. the tricky bit for darwin was making sense of intermediates between normal symmetrical fishes and asymmetrical flatfishes. what good is it having an eye moved slightly over? in fact, the origin of flatfishes was used as an early attack on natural selection, and actually led some scientists to propose that sometimes evolution operates in leaps and bounds rather in a more gradual fashion. osb: what evidence did you study to understand how flatfish evolved?mf: i examined fossil fishes from the eocene (about 50 million years ago) of italy and france, combined with study of skeletal materials of living fishes.osb: what does this fossil evidence show?mf:the fossil evidence clearly shows fishes with an arrangement of eyes intermediate between that of normal fishes and flatfishes. one eye is shifted toward the opposite side of the skull, but does not quite make it there. this is precisely the arrangement dismissed out of hand as 'unlikely' or 'not functional', which is what led to all the problems for darwin. osb: how does this add to our knowledge of how evolution works?mf:well, each individual evolutionary story is unique, so care must be taken in not pushing this too far. however, in this particular instance, i am able to reject the scenario that the unusual bodyplan of flatfishes evolved suddenly (ie, went from symmetrical to the modern condition in a single step), but rather occurred in a more gradual, conventionally darwinian, fashion. dr matt friedman recently moved to join oxford university's department of earth sciences from the university of chicago. the oxford science blog gives you the inside track on science at oxford university: the projects, the people, and what's happening behind the scenes. curated by ruth abrahams, media relations manager (research and innovation). contact: ruth abrahams, [email protected]

flatfish are one of evolution’s more bizarre creations. yet recent research has suggested their apparently unique morphology may have evolved more than once. a new study, recently published in bmc evolutionary biology, investigates just how unique flatfish are, and discovers that the most remarkable thing about them may be the speed at which they evolved. christopher foote 24 oct 2016 a most remarkable face as one would expect from a group that includes 99% of known fish and almost 1 in 3 vertebrate species, the spiny rayed fishes are incredibly diverse. however, even amongst these incredibly diverse body plans and lifestyles, flatfish stand out as being singularly unusual. flatfish, which include many familiar food fish like sole, plaice, and halibut, are defined by a bizarre adaptation: a profound cranial asymmetry resulting from one eye migrating to the opposite side of the skull during larval metamorphosis; or, in simpler terms, they have both eyes on one side of their head. this extreme re sculpturing of their skulls enables adult flatfish to rest on the seafloor on their eyeless ‘blind’ side, with both eyes pointing upwards (towards potential predators or prey) unobstructed by sediment. this unusual adaptation has clearly been successful for flatfishes, with over 700 species currently in existence. a not so unique adaptation? flatfish can be divided into two groups: the three species of spiny turbot that make up the family psettodidae, and the much larger suborder pleuronectoidei. unsurprisingly, fish biologists long assumed that both groups of flatfish evolved from a single common ancestor; it is hard to imagine such a bizarre adaptation having evolved multiple times. recently however, this common sense assumption has come under attack. several studies have found support for the distinct flatfish adaptation having evolved on two separate occasions. is the flatfish body plan not as unique as it appears? hard to say, has been the honest answer up until now. while there is little doubt that flatfish belong to the wider fish group carangimorpharia (which also includes marlins, swordfish, and remoras), the exact evolutionary relationships among this group are uncertain. eight recent phylogenetic studies conducted on carangimorphs have produced eight different evolutionary trees, some of which support a single origin for flat fish, others of which support multiple origins (see figure). resolving the question a new study, from a team of researchers led by matt friedman of the university of oxford and recently published in bmc evolutionary biology, aims to resolve these issues. making use of recent advances in phylogenomics and high throughput dna sequencing, they have assembled a dataset of ultraconserved dna elements (uces) and their flanking sequences from over 1000 loci in 45 different carangimorph species. uces are, as the name suggests, parts of the genome that are identical across distantly related species; for example, many fish share certain uces with humans. they have become increasingly widely used in evolutionary studies in recent years, having demonstrated considerable potential to resolve obscure evolutionary relationships (including in ray finned fishes) due to the sheer volume of genetic data that can be efficiently produced. uces seem to have been successful in untangling this tricky problem as well. the evolutionary tree produced from the researchers data set is statistically well supported and remains robust regardless of the analytical framework applied (an important consideration in this kind of research, given that different frameworks will always involve certain assumptions). crucially, this tree strongly supports a single evolutionary origin for flatfish. it appears that this evolutionary novelty really is unique. evolution in the blink of an eye the new research also provides a reason as to why it has been so difficult to pin down flatfish evolution; it happened incredibly quickly. the researchers believe that the flatfish body plan, with complete orbital migration of the eyes, evolved in no more than 2.9 million years. while a long time by human standards, this is remarkably quick in evolutionary terms. the distinct body plans of flatfish relatives like moonfish and billfish took 2 3 times longer than this to evolve, while we can also compare the time it took for anatomically modern humans to evolve (around 7 million years) or whales to evolve their body plan (around 20 million years). indeed a few million years is the same amount of time estimated for the diversification of some radiations of cichlid fish in african lakes, long considered a pre eminent example of ‘explosive’ evolution. flatfish appear then to be emblematic of a common problem in reconstructing the evolutionary histories of rapidly evolving lineages. when evolution moves so quickly, it leaves less ‘evidence’ behind it, in the form of other genetic changes which allow determination of relationships within a lineage. the use of large datasets, like those provided by high throughput sequencing of uces, becomes necessary in such cases to untangle evolutionary relationships. flatfish, then, are doubly remarkable. their incredibly distinct morphology is indeed a unique adaptation that has evolved but once. we now realise they are also noteworthy for having undergone such drastic changes in nothing more than the blink of an evolutionary eye. view the latest posts on the bmc series blog homepage sorry. no data so far. by continuing to use this website, you agree to our terms and conditions, community guidelines, privacy statement and cookies policy. manage cookies © 2024 biomed central ltd unless otherwise stated. part of springer nature.

why evolution is true why evolution is true is a blog written by jerry coyne, centered on evolution and biology but also dealing with diverse topics like politics, culture, and cats. flatfish, in the order pleuronectiformes, have long been an evolutionary puzzle, for all the fish in this order lie on the substrate on their sides with both eyes on one side of their body, like the flounder below: phylogenetic analysis shows that flatfish evolved from “regular” fish, fish having one eye on each side and swimming vertically, that evolved over time to lie on their sides. the bizarre thing about this evolution is that it involved genetic changes so that “normal” fish had their eyes move over the top of their head so that both eyes look upwards. their skin changes color and texture, too, with the top half colored, as above, and the bottom half pale. and all flatfish begin their development like “normal fish”, swimming vertically and having one eye on each side of the head. then, as the fish gets older, one eye migrates over the top of the skull to the other side! (you can see that in the video below.) when the eyes are both on one side, the flatfish tip onto their sides and spend the rest of their adult life lying on one side. (the side varies among species: some have 100% right sided individuals, others 100% left sided, and some species are random, with half of the individuals having the right eye move over (and lying on their right side), and the other half having the left eye move. living on the substrate like this, and often camouflaged as the flounder above, is an advantage for the fish, both protecting them from predators and, since they are predatory piscivores (fish eaters), hiding from their prey. here’s a video of the development of a young flatfish, showing the eye migration. since the ancestor had both eyes on one side, like the young flatfish, this is a case of “ontogeny recapitulating phylogeny” that is, the development of a single living fish goes through a process mimicking the evolution of their adult ancestors. but since the weird developmental pathway is presumably an adaptation that evolved by (presumably stepwise) natural selection, two big questions immediately arise: it’s hard to envision a gradual darwinian process that could produce this migration. as carl zimmer wrote in a new nyt article that summarizes recent flatfish findings (click below), darwin’s critics used both questions about to cast doubt on his theory. in response, some “saltationists”, who assumed that major evolutionary changes occurred in one huge step rather than a series of gradual steps, said that a single mutation moved the eye from one side to the other. (but that would not be advantageous unless the fish had already evolved to lie on its side!) click below to read the zimmer piece in the nyt here (the drawing is animated), or find it archived here. as carl reports, there was another weird finding that now seems doubtful: a 2001 paper by a group of chinese researchers who, using dna=based family trees, seemed to show that flatfish evolved twice. you can see that paper in nature genetics by clicking on the headline below, or read the pdf here. the discovery that flatfish seemed to be “polyphyletic” with more than one evolutionarily independent origin was deeply weird, because the hormone induced eye migration, which is extraordinarily complex, would have had to evolve twice. it’s not impossible, but seemed unlikely. one of the doubters was evolutionist matt friedman, who got his ph.d. here and is now a professor at the university of michigan and director of its museum of paleontology. a while back, when he was still at chicago, friedman published what i see as the most interesting of the three papers highlighted here. this one was in nature, and you can read it by clicking below or seeing the pdf here note that this paper was a lot of work, and yet, unlike the others, friedman was the sole author. i love to see single person research efforts like this. that aside, what friedman found were two fossil evolutionary intermediates between adult “normal” fishes (the presumed ancestors of flatfish) and modern flatfishes, having both eyes on one side. friedman reanalyzed a neglected species, amphistium paradoxum, and a described a new fossil fish, heteronectes chaneti, both from the lower eocene, about 50 million years ago. amazingly, both species (the former randomly sided and the latter lying on its left side) showed an intermediate placement of the eyes in the adult fish. both eyes were on the same side of a vertically oriented fish, but one eye had migrated upwards toward the top of the skull, so that the fish could presumably see both to the side and also, perhaps, a bit above them. thus we have two evolutionary intermediates of the adult stage, likely showing that the eye movement did not occur in one big evolutionary leap. here’s a photo from the 2008 paper of the left and right sides of the h. chaneti skull, showing the eye sockets, which i’ve circled. the asymmetry is obvious: and a reconstruction of the amphistium species, showing both sides. the asymmetry is again clear, but the eyes of the adults are still on opposite sides of the head: you’ve probably realized that this addresses question #1 above, showing that the movement was presumably gradual over evolutionary time, though we need more fossils to show that it was a continuous series of small steps. but at least the movement didn’t seem to involve one big leap. but that leaves question #2, which i’ll address in a moment. the reason zimmer’s note came out now, though the papers above date from 2008 and 2021, is that a group of authors recently published another dna based analysis in nature genetics showing that the chinese group was probably wrong: flatfishes and their eye movements seem to have had a single evolutionary origin. (the chinese group maintains that their “polyphyly” conclusion is still the best one.) click below to read, or find the pdf here. before returning to the big unsolved question, i’ll show the phylogeny advanced in the 2001 paper (bottom), showing two origins of flattening and eye migration, and the newer analysis by duarte ribiero et al. at the top (friedman is the third author), showing a single origin of flatfish (i’ve circled it). this newer paper also singles out some genes that, showing signs of selection in their dna sequence, may be involved in the evolutionary transformation, but i’ll leave that issue aside. green silhouettes are flatfish, black are nonflat fish. now for the big mystery. how could there possibly be an evolutionary advantage to each step of the eye movement? presumably the adult either laid on its side or swam “normally”, and what would be the advantage of intermediate stages when the eye gradually moved up, across the top of the skull, and settling on the other side? the movement is presumably advantageous only when the fish is already on its side, but then what would be the advantage of moving a few mm towards the top of the skull? well, perhaps the fish didn’t lie fully on its side. here’s one clue in a quote from the 2008 paper: questions about the possible selective advantage of incomplete orbital transit arise from the discovery of stem flatfishes. clues are given by living taxa, which often prop their bodies above the substrate by depressing their dorsal  and anal fin rays. similar behaviour might have permitted amphistium and heteronectes both of which have long median fin rays the use of both eyes while on the sea floor. the unusual morphology and resting orientation of pleuronectiforms have been interpreted as adaptations for prey ambus, and it is clear that stem flatfishes, like morphologically primitive living forms, were piscivorous; one specimen of amphistium (mcsnv v.d.91+92) contains the remains of a fish nearly half its own length. so perhaps this happened: a normal ancestor, through behavioral evolution, adapted to hanging around the sea bottom, as they were less conspicuous and could get more prey. but they’d have a more difficult time seeing upwards with eyes on both sides of the head. movements of the eyes toward the top of the skull could be advantageous so long as they occurred in concert with behavioral changes (first perhaps learned, then evolved) involving propping themselves up with their fins. the advantage of tilting a bit would be that the fish might become a bit less conspicuous. this whole scenario, as i proposed it (and i’m sure others have before in some form) presumes that the eye movement is either induced by or occurs in concert with changes in the fish’s behavior, which initially could have been learned and not coded in the genes. (ernst mayr once said something like “all major evolutionary changes begin with a change in behavior”). i don’t know how to test the hypothesis, as even finding more fossils with intermediate stages of eye migration will tell us little about the selective pressures involved. but for sure the movement involved natural selection rather than other evolutionary forces like genetic drift, for we have a big directional change involving many genes, genes that involve both morphology and behavior. in short, i don’t know how it happened. but seeing that modern fishes can use their fins to prop themselves up on the sea floor may give us a clue. and other scenarios may be possible; readers can entertain themselves by finding alternative ways this change could have occurred by natural selection. splendid post   refreshing and insightful! “ontogeny recapitulating phylogeny” ^^^cannot unread but can you say it 3 times, fast? this was fascinating and i thank you, jerry, for explaining it in a manner that even a nonscientist like me can understand (i’m not saying it wasn’t a challenge… it was). it blows my mind to consider all combined expertise and research it has to take to arrive at such discoveries. that such things are even “discoverable” is pretty crazy. good stuff. very interesting post. [ possible typo?: “both eyes were on the same side of a vertically oriented fish, but one eye had migrated upwards toward the top of the skull, …”. unless i’m misunderstanding the images, the eyes are on opposite sides of a vertically oriented fish. ] i wonder if the movement of the eye toward the top of the head could not have happened first, as the fish became a bottom dweller. in a vertical posture, this would allow it to keep an eye out for predators from above while hunting for prey (on one side) across the bottom sands. once this had occurred, the transition to flat swimming occurred. i also wonder if ever in fishy evolution, a fish evolved an extra, eye with one pointed strait up. in this case, one of the side eyes might have atrophied away leaving the present form of the flatfish. i could see no advantage to a small, incremental movement of the eye towards the top of the head unless it was already changing towards a bottom dweller. as the developmental and fossil data show, a third eye or signs thereof did not evolve. i could see no advantage to a small, incremental movement of the eye towards the top of the head unless it was already changing towards a bottom dweller. a plausible   to me   intermediate might be evolution from an ambush predator lurking in crevices in the rock / reef, where the utility of the eye is severely reduced in one direction, but the bottom dwelling asymmetric swimming mode has not developed (yet). the partly migrated eye would have a field of view at less than 180° to the unmigrated eye. unfortunately, this is comparable to einstein’s reason for disliking quantum mechanics, and particularly entanglement   “god throwing the dice where we cannot see them”. because a sufficiently rocky substrate to have significant crevices is a pretty unlikely place to get the rapid burial best suited to fossilisation. as the developmental and fossil data show, a third eye or signs thereof did not evolve. istr that occasional amphibian species and some fossil relatives of the new zealand tuatara had a small medial fossa on the braincase which would be in about the right position to allow light to penetrate to the pituitary gland. though if this was actually sufficiently transparent, it would have been considerably inferior to the other two eyes for actual vision. but a role in thermal non imaging detection or day length sensing has been hypothesised. but it’s an intermittent and labile character in the fossil record. i’m trying to remember if snakes also have “pits” or fossae in their skulls, associated with their modern heat detecting capabilities. and crucially, if these structures are paired about the median line, or unpaired on the median line. any herps about? great post. great post. i wonder if the change initially could have involved both natural selection and geographic isolation, which perhaps in tandem might have resulted in a faster rate of change? also, might studying the embryo development of a flatfish species provide clues to the evolutionary development of the species itself? one wouldn’t need geographic isolation if the evolution was going on in a single species; you need such isolation (usually) if you are producing reproductive barriers two related species derived from a common ancestor. natural selection within a species doesn’t require that it be geographically isolated from others. this could be several adaptations going on at once. if bottom dwelling resulted in greater evolutionary fitness   and greater horizontal orientation resulted in greater evolutionary fitness (as an ambush predator perhaps)   and one eye looking up resulted in greater evolutionary fitness, then the other eye migrating to also look upwards is not a mystery but a consequence of the fitness of other changes. it’s a fascinating evolutionary story. thank you for sharing it. apropos the change in the phylogenetic tree between lu et al (2001) and duarte ribeiro et al (2024), leading to the identification of a single evolutionary origin of flatfish, there’s been a lot of work done on building such trees from genome sequences in the last 20 years. we have many more complete genomes now than we did in 2001, for one thing. at the sanger institute, where i work, there’s an ambituous ongoing project called darwin tree of life which aims to sequence all 70,000 eukaryotic species that are native to the british isles. it’s a major collaboration with several other institutes and universities. one of the latest species to have its genome published is the death watch beetle, and the dna was extracted from an insect found in a wooden beam on hms victory, as reported by the guardian today: https://www.theguardian.com/uk news/article/2024/jun/22/nelson hms victory gives scientists vital dna for battle against deathwatch beetle is it possible that the “moving” eye offered an advantage in sexual selection? do the male fish “fan the bed” of eggs laid by females? could the wandering eye help in a reproductive way enabling the male with a more mobile eye to pass on more genes to future generations? interesting stuff. a polyphyletic origin of the asymmetry does sound very unlikely, at least a priori. a gradual migration of the eyes to one side of the head might, at first, seem to be a problem, but part of that “problem” might simply be failure on our part to accept that small changes can impact reproductive success. a similar “problem” is frequently raised in the form of the question: “what good is part of an eye?” the answer is that a lot of good can come from a small change in the way that an organism responds to light. we know this because there is a multitude of visual systems out there at all levels of sophistication. probably the same thing is the case here as the fossil discoveries suggest. and, adding a concomitant shift to life on the bottom creates a synergistic effect that may be all that’s needed to tip the balance. more fossils please. see my comment above about “crevice dwelling” ambush predation as a putative environment where the migration of the eyes from symmetry could be advantageous distinct from the adoption of a bottom dwelling role. everybody talks about flatfish eyes. what i’m wondering about is   how did one side of their gill structures evolve to ventilate   exhale   stale water from the gill structure, and do it surreptitiously as befits a predator in “ambush” mode. bottom dwelling sharks … actually, i’m trying to remember my “naming of parts” now. is it the “rays and skates” which have migrated their spiracles to the upper surface of the body? with the migrated spiracle as a distinguishing characteristic of “skates and rays” versus “other cartilaginous not fishes”? taxonomical delousing aside   there are a number of bottom dwelling genera of elasmobranchs which have addressed such issues of ventilation by moving the spiracle to the upper surface of the (part buried) body. how “flat fish” deal with the same problem, i don’t know. my “bloody autoincorrect” `assistant` tried “correcting” “elasmobranch” to “lamellibranch”. which is quite a step away from the jawed vertebrates. i have nothing coherent to say except: what a great article. this is one of the reasons i come back to weit every day, before anything else. thank you. +1 every few weeks in my case, but yeah. very interesting. thanks for posting. thanks for the huge efforts you put in to your website but especially the science. fascinating to the max! thank you. thank you for this amazing post! more, please. i once caught a 204lb. halibut 30 years ago  that was quite the struggle. i think halibut are the largest pleuronectiformes. a truly formidable fish that are on record for killing some people who have caught them  flapping them to death. again, this eye migration illustrates how evolution is smarter than me and you. left to right or right to left or indiscriminate migration boggles my mind in a way. something we’ll probably never understand…some things are simply random i suppose. biology is really tough, regardless (unless you’re a creationist, then everything is simple!). thanks for this terrific post! a number of fish burrow into the sand to wait for prey and to avoid predators. eg https://www.youtube.com/watch?v=vrpxo63pxle these fish have tubular bodies. notably the puffer fish. their eyes jut from above the head. no need for eyes to move to the side. the real question would seem to be: why did some fish become flat? most of you will have gazed into rock pools at the beach and noted that a shadow cast over the water will send fish scurrying to find cover. some will nestle against rocks. that works ok in a rock pool, but what about outside the pool in channels where currents are strong. having a low profile would be an advantage then. i think some comparative biology might reveal the existence of so called in betweens to throw light on the matter. it would be relatively easy for a flattened fish with one eye on the move to adapt to sandy bottoms. just thinking! i supposed flat fish can hunker down where other fishes cannot bury themselves. every niche gets exploited, eventually. in my childhood i hunted flounder (as flatfish are called in nz) with a spear in shallow waters. there were places where, indeed, a flat fish would have an advantage, especially where a layer of rock was covered by a mere couple of cm of mud. the local flounder had the remarkable ability, if trodden on, to scamper away a few meters, and disappear into the muds without creating a mud storm, and thence be difficult to find, visually. it seems to me that two rules might apply to the evolution of the ‘flatfish niche’: unlikely, the niche exclusion principle of gause and rmer’s rule, that an adaptation (‘pre adaptation’) evolving for one circumstance give rise to the possibility of occupying another niche, as you suggest. i think that being flat was the pre adaptation. this is part of my reason for suggesting an intermediate “crevice dwelling ambush predator” stage   to prompt the development of asymmetric eyes. followed by dispersal into a new “infaunal” econiche which drove the eye migration to completion. turning one big evolutionary problem into two smaller intermediate problems. great stuff. i’d be curious to learn about the genes. classically, the sonic hedgehog signaling pathway has been used to evolve left/right asymmetry in vertebrates. maybe it’s involved here? i’m not sure that this would be too hard to evolve more than once. before people ask for more science posts, realize that it’s about four times more work to write one of these than a “non science post.” that’s why i did only on substantial post yesterday. when interesting stuff about evolution comes out, i try to write about it. truly a fascinating discussion, prompting thought and insight into evolution generally. indeed, evolution of the body plan must go hand in hand with evolution of behaviour, and small changes in body plan and behaviour must lead to advantage. astonishing how jerry keeps up a huge effort day after day. david lillis here’s an interesting video on this subject: comments are closed. email address sign me up! © copyright notice

the evolutionary origins of flatfish have finally been explained, 150 years after charles darwin grappled unsuccessfully with the problem. darwin was troubled by how fish such as flounder and sole had made the seemingly improbable evolutionary leap from being symmetrical to having two eyes on the same side of their face. now, a collection of ancient fossilised specimens that were halfway to becoming flatfish has offered evidence that the unusual skull anatomy evolved over millions of years. “flatfishes and their profoundly asymmetrical skulls have been enlisted in many arguments against gradual evolutionary change precisely because it is difficult to imagine how intermediate forms might have been adaptive,” said matt friedman, a palaeobiologist at the university of chicago. the previous absence of any intermediate fossils led some to conclude that the peculiar body plan arose suddenly through large scale genetic mutations. but speaking yesterday at the annual meeting of the society of vertebrate palaeontology at the university of bristol, dr friedman said that his findings showed that flatfish evolved through tiny adaptive steps. the 50 million year old fossils have asymmetrical skulls but retain eyes on both sides of the head. one of the eyes is higher than the other and, in some specimens, approaches the ridge of the face, showing how one eye gradually migrated across the head and on to the other side. the specimens were found in the 1700s in northern italy but previous studies concluded that the strange skull anatomy was due to the skull having been crushed. however, using high resolution computer imaging, dr friedman was able to show that the skull asymmetry and the higher position of one eye were genuine. asymmetric eyes could have made spotting and catching prey easier, according to dr friedman. flatfish hunt by lying in the sand and waiting for smaller fish to pass overhead. another explanation is that having asymmetric eyes would allow the fish to lie on the seabed tilted to one side and avoid getting sand in the lower eye and damaging it. dr friedman’s work has been met with a mixed reaction from creationists. “i’ve made some enemies online,” he said. “but someone else told me, ’we’ve never had a problem with evolution within fishkind’.” dr friedman’s research has also helped to place the acanthomorph fossils squarely on the same evolutionary branch as flat fish. the fossils, which belong to two different species called amphistium and heteronectes had perplexed taxonomists who over the years had placed them in a diverse range of groups on the evolutionary tree. “it was like having a fossil mammal and not knowing whether it is more closely related to a platypus, a kangeroo or you,” said dr friedman. in related research dr friedman looked at how the eye moves from one side of the head to the other in embryonic flatfish. a time lapse video collating images from over a two week period during the late stages of development shows one eye migrating over the skull and coming to rest on the other side of the face. once the migration is complete the skull hardens into a settled form. dr friedman was one of sixteen scientists nominated for the society of vertebrate palaeontology’s romer prize, which recognises outstanding scientific contributions of doctoral students. the winner of the prize will be announced at an awards ceremony in bristol on saturday evening. © times media limited 2025. registered in england no. 894646. registered office: 1 london bridge street, se1 9gf.

only $2.99 a month fossils show middle stage of the evolution of flatfish eyes to one side of their head by ashley yeager july 9, 2008 at 12:28 pm a new look at the fossils of primitive flatfish offer evidence that these fish   well known for having both eyes on one side of their head   started out symmetrical and gradually evolved their one sided trait. evolutionary biologists have floundered to explain how bottom dwelling, carnivorous flatfish, such as flounder and plaice, evolved to have both eyes on one side of their head. previous analyses of fossil evidence concluded that the change was abrupt. but reexamination of ancient fossils now supports the hypothesis that the flatfish eye position actually evolved over thousands to millions of years. the findings appear in the july 9 nature. fossil evidence had already shown that flatfish ancestors had one eye on each side of the head. but no evidence had existed for a transition between a symmetrical skull and one sided skull. matt friedman, a graduate student at the university of chicago, looked more closely at 45 million year old fossils of primitive flatfish and found the transition species: one eye had moved, but it had not crossed the midline of the fish’s body, as seen in today’s flatfish, friedman reports. the fossils, friedman says, deliver a clear picture of how this flatfish group changed from a standard fish with eyes on both sides to one with eyes on only one side of the head. the flatfish eyes and skeletal structure underwent small, incremental changes, says alex schreiber, a developmental biologist who studies flatfish at st. lawrence university in canton, n.y., and was not involved in the study. the fishes’ evolution was, in essence, gradual, he says. darwin and his contemporaries began the debate as to how flatfish eyes evolved, arguing either for gradual evolution, as darwin did, or for evolution by punctuated leaps. by presenting “unambiguous evidence” for the gradual change, friedman now takes that debate into the 21st century, says paleontologist philippe janvier of the muséum national d’histoire naturelle in paris, france, who was not involved in the research. sponsor message watch flatfish development: this video shows the development of a flatfish from its juvenile to adult stage. near the end, the fish’s eyes appear together on one side of its head. the stages of development of one flatfish essentially mirror the evolutionary history of adult flatfish morphology. this video shows the skeletal changes that a flatfish goes through as it develops into an adult fish. the eye socket in the fish’s skull actually grows toward the other eye socket and, in the end, the fish has both eyes on one side of its head. both videos are courtesy of alex schreiber, the carnegie institute of washington and st. lawrence university in canton, n.y. questions or comments on this article? e mail us at feedback@sciencenews.org | reprints faq a version of this article appears in the august 2, 2008 issue of science news. friedman, m. 2008. the evolutionary origin of flatfish asymmetry. nature 454(july 10):209 212. doi:10.1038/nature07108 ashley yeager is the associate news editor at science news. she has worked at the scientist, the simons foundation, duke university and the w.m. keck observatory, and was the web producer for science news from 2013 to 2015. she has a bachelor’s degree in journalism from the university of tennessee, knoxville, and a master’s degree in science writing from mit. we are at a critical time and supporting climate journalism is more important than ever. science news and our parent organization, the society for science, need your help to strengthen environmental literacy and ensure that our response to climate change is informed by science. science news was founded in 1921 as an independent, nonprofit source of accurate information on the latest news of science, medicine and technology. today, our mission remains the same: to empower people to evaluate the news and the world around them. it is published by the society for science, a nonprofit 501(c)(3) membership organization dedicated to public engagement in scientific research and education (ein 53 0196483). © society for science & the public 2000 2025. all rights reserved.

the common explanation i've seen for its evolution is: 1. taking to the sea floor, it was natural for the flatfish to rest onits side.2. with one eye stuck in the ground, it was at a disadvantage.3. yadda yadda yadda4. result: its bottom eye moved around to the top. the problem is in step 3. it is unlikely that the bottom eye suddenlyappeared on top within a couple generations. so, how about thosetransitional species: what was the evolutionary advantage to having aneyeball offset by a couple millimeters, but still stuck in the ground? the only solution i can think of is that since there was no advantageto eyeballs shifted only slightly (but still stuck in the mud), all thevarious transitional species thrived together equally, and it wasn'tuntil one of the strains had their bottom eyeball shifted up above thesea floor that natural selection actually started to occur  sort of a"delayed natural selection". it would be interesting to find fossils oftransitional species in which the bottom eye socket is shifted all overthe place: 3 millimeters toward the spine, 5 mm toward the tail, 10 mmtoward the mouth, etc. anyone have some input?    bobby bryantaustin, texas thanks for that brilliant reasoning... now if only all of evolution wasgoal oriented. then our lives would have purpose. given the mechanisms of speciation, a small, isolated andhighly succesful population might just be able to reverse migrationpatterns and populate wider areas with their "one side eyed" weirdos.what's more, the tendency of some species of delicious flat fish tohave the eyes on either side, while other species can be distinguish bythe side on which the eye lies could point to parallel independentevolutions of the same trait. that's another extral cool point to ourmouth watering friends. in the other hand, i do agree that it would be so cool tofind a fish with a real functioning eye near the tail. that would beeither the smartest fish ever or the most stupid one. in either case, idon't think it would last long as a species. for one, smart fish don'tfinish first, indeed they rarelly finish at all surrounded by hugepredators that invested their evolutionary stock into wmds such aslonger, nastier more effective teeth. then you have to consider thatfish are generally not social creatures (schools are agrupationswithout social assigments [as of yet]), so if i was smart fish i wouldkill myself from boredom itself (another argument for goldfishstupidity?). then of course, stupid fish usually swim themselves intosleeping predators mouths while merrily singing: "hello mister big fish i'm your little cleaner shrimp, would like some buuuutter with your morning snack" here is to mankind never singing such song... salud! to hold our pants up.>   內躬偕爻,虜,齯滌`偕爻,虜,齯滌`偕爻,虜,齯滌`偕爻,虜,齯滌`偕爻, pip r. lagenta pip r. lagenta pip r. lagenta pip r. lagenta�虜,齯滌`偕爻,虜,齯滌`偕爻,虜,齯滌`偕爻,虜,齯滌`偕爻,虜,齯滌    pip r. lagentapresident for lifeinternational organization of people named pip r. lagenta(if your name is pip r. lagenta, ask about our dues!)<http://home.comcast.net/~galentripp/pip.html>(for email: i'm at home, not work.) yes, i know all about how improbable fossil formation is. i didn't askfor fossils, i said it would be "interesting to find fossils".... lol! possible factors that may have facilitated the evolution of eyemigration: * even a slight, incomplete eye migration might help fish for theperiod of time when they are just beginning to settle on the bottom,and when they are just beginning to lift * slight eye migration might have initially been useful when theflatfish was swimming in the open water, even if not useful while onthe bottom. presumably swimming flatfish are near the seafloor anyhow,so any eye facing straight down might be less useful than one thattwists to the side a bit * the eye migration feature might have gotten going with the assistanceof nongenetic organismal behavior, e.g. if the fish is constantlytwisting its lower eye sideways and forwards to see better, or eventwisting its head slightly, then mutations that developmentallyenhanced the effect this might be preserved. canalization or somesuch. [googles on flatfish evolution] well, there is obviously more to it than the above, because evidently afew flatfish species (7 out of 550) are polymorphic for which side,left or right, becomes "up". this implies some kind of semi randomsymmetry breaking device that can go either way and can pull either eyeto the new "top", and also pigment the correct side. it could just bethe fish picks whichever side it prefers, and then developmentalmechanisms respond to an environmental signal like gravity or light.but anyway, there are some really cool pictures here:http://ceratostoma.bms.bc.ca/cbergstr/starry%20flounder%20info%20page%208.htm >"to get on the other side. " >>thanks for that brilliant reasoning... now if only all of evolution was>goal oriented. then our lives would have purpose. evolution isn't goal oriented, but my life has purpose. why doesyou're live's purpose depend on the particulars of evolutionarymechanisms? mitchell coffey now, this suggests that a smaller change in skull development, onethat, when the fish was mature, caused an eye to be just a fewmillimeters displaced, might indeed be beneficial. flatfish are ambushpredators, lying still and camoflaged on the bottom, and attacking whenprey draws near. a fish could do this, with eyes on opposite sides ofits head, but it might alert the prey if it moved to check outapproaching prey with both eyes. if it could do this while moving*less*, it would alert the prey less often, and eat better for lesseffort, and have more energy to devote to making babies. so one canimagine (yes, this is a just so story, but i hope it addresses yourspecific question) that selection pressure existed even for slightmodification in the direction of the both eyes on one side matureskull. >> the only solution i can think of is that since there was no advantage> to eyeballs shifted only slightly (but still stuck in the mud), all the> various transitional species thrived together equally, and it wasn't> until one of the strains had their bottom eyeball shifted up above the> sea floor that natural selection actually started to occur  sort of a> "delayed natural selection". it would be interesting to find fossils of> transitional species in which the bottom eye socket is shifted all over> the place: 3 millimeters toward the spine, 5 mm toward the tail, 10 mm> toward the mouth, etc.> > anyone have some input?    steven j. another possibility along the same line, is that the bottom eye could beused to spot food on the seafloor. if the bottom eye were located moretoward the front, and therefore nearer the mouth, the flounder would have aneasier time finding food on the bottom. once the eye had reached the frontof the skull, the binocular vision would have made it a better hunter, andit could have changed its behavior to take advantage of its new abilities. the following is speculation and should be treated as such. if youlook at the eyes of other ambush (pounce) predators, such as anglerfishand groupers in the sea and cats on land, you notice that the eyes areoften placed so as to be able to triangulate distance to their prey.for a suction predator, this would be an important survival technique.this is not necessarily the case in chase predators that can move theirhead from side to side. the flat fish ancestor probably did not have the option of hiding inbush or weeds or corals, but had to hide in plain sight on a flat planesite in clear waters. and it also had the body of many fish, flatdorsal/vertical which is useful for swimming with a tail that moveshorizontally. unlike the grouper or anglerfish, it's initial foraysinto ambush predator probably involved twisting its body flat to thesurface, but trying, as much as possible to keep both eyes above fortriangulation. this obviously is not an optimal state, but it couldsuffice to hide the ancestor *sufficiently* such that fish with thisimperfect ability (initially largely behavioral) to lie as flat as ispossible with a head that was somewhat twisted to the vertical stilldid better (in terms of *net* energy acquisition) than active huntingin this environment with its long sight lines. a prey is less likelyto see a motionless bump on the plain than the rapidly moving predator. this then would allow subsequent selection for fish that could laytheir heads *flatter* on the surface, perhaps in association withselection of prey to recognize the twisted partially hidden predatorand avoid it (i.e., an arms race between recognizing the predator andpredator camoflauge). with eyes somewhat naturally set forward andclose together, it would not take a large initial change in bonestructure to have the effect of allowing *some* forward and upwardvision from the lower eye. allowing the head to lie flatter. the crucial genetic innovation would involve the (environmental?)triggers that allowed the breaking of symmetry. but other organismshave such triggers that allow such asymmetry, so that is not amake or break novelty. why assume that it was natural for it to rest on it's side? certainly there are any number of fish that lay in wait from the bottom where this is not the case. some of them even partially bury themselves in the sand (the flat fish tend to be almost completely burried with only the hard to see eyes in the clear) but have the advantage of being able to wait in rocky cover.note also that the fish starts out in a normal configuration, the eye migrates after birth. the intermediates with the eye in the middle of the way to the otherside had the advantage of swimming closer to the soil than fishes thathad yet perfect symmetry. with perfect symmetry, was likely that the eye would be near the widestpart of the body, and at the same time, more at the "new" bottom side,more prone to rub and harm in the soil. the fish had to swim a bit over the soil to avoid harming the eye whilerubing it against sand and stones, thus was more visible to prey andpredators. as it moves "up" or to the front of the head, it goes to a body partthat is slightly narrower, and the fish is less prone to damage the eyewhile swimming even nearer to the soil, and can swim more freely andquickly in this situation. it improves as more closer to the top orfront of the had the eyes migrate. when the eye reaches the edge of the body, then it will probablymigrate a bit more now under the selective advantages of the reacquiredvision of that eye. the other posters have it all wrong. the eye gradually migrated*through* the skull to get to the other side. the advantage wasenlightenment through introspection. buddhism only tells half thestory: truly enlightened individuals emerge out the other side ofnirvana to be reborn as flatfish.

for charles darwin, flatfish like flounder were a vexing puzzle how did they evolve into such asymmetrical freaks? but recently, scientists using clever experiments and advanced imaging have shown just how their curious anatomies came about. every summer there’s a snowfall in the sea. instead of drifting down, it falls up, and rather than flakes of ice, it’s made of innumerable diaphanous eggs that rise from the bottom of the ocean to the surface. there, they hatch into baby flatfish, each no larger than a pinhead. for the first few weeks of life, they look and act like typical fish fry, swimming upright through sun dappled waters, darting after plankton. soon enough, though, these young flatfish lose all semblance of normalcy during one of the most difficult puberties of any animal on the planet. you think pimples and prom were awkward? please. as a larval flatfish begins its passage into adulthood, it does not merely experience uneven growth spurts and mood swings. rather, it changes from a cute, symmetrical little fish into a total anatomical disaster like some unrecognizable object evolution made from clay in preschool and gleefully brought home to its parents, who kept it for “sentimental value.” “flatfish are the most asymmetrically shaped vertebrate to ever live on earth.” at the same period in our lives when the slightest blemish seems like a calamity, the flatfish is preparing to lose half of its face. the bones in its skull bend and shift as one eye forces its way to the opposite side of the head. its whole body begins to tip over, so it has to swim at an angle. one of its flanks turns a sickly pallor; the other becomes colorfully flecked, matching the speckled sand on the seafloor. eventually, when it is large enough, the transformed flatfish sinks and settles on its newly blind side. it is now a young adult bottomfeeder with two eyes on the same side of its head, a contorted mouth, and one fin squashed against the sand. it will spend the rest of its life this way. “flatfish are the most asymmetrically shaped vertebrate to ever live on earth,” says alexander schreiber, a flatfish expert at st. lawrence university. “it sounds likes a really dramatic thing to say, but as far as vertebrates go, it’s true.” evolutionary biologists have long struggled to explain the flatfish’s curious, asymmetrical anatomy. the prevailing story has been that modern flatfish evolved from bilateral fish, which like goldfish and most familiar species have right and left sides that mirror one another. but why did flatfish become flatlanders in the first place? and how does a young flatfish have the audacity to reject the manner of locomotion favored by the vast majority of fish in the ocean? somehow, its brain ditches everything it once knew about how to swim in the open water and decides that, actually, the right thing to do is to lie sideways on the seafloor forever. talk about cognitive dissonance. in the past decade, biologists have hit upon some of the answers. in the process, they’ve come to realize that the flatfish is even stranger than they previously realized an asymmetrical anomaly from the inside out. people have been interested in flatfish for thousands of years primarily as food. most modern scientists who study this diverse family which includes halibut, soles and flounders are interested in farming the fish more efficiently. but since the time of charles darwin, a handful of biologists have become fascinated with the flatfish for an entirely different reason: its unique evolutionary history. creationists have long pointed to the finned freak as certain proof of evolution’s implausibility. if the flatfish evolved from bilateral ancestors with one eye on either side of their heads, how did it survive during its facial reconstruction, they asked, and where were the fossils to prove it? darwin worried that the relative scarcity of such transitional fossils would doom his theory. one of darwin’s most vehement critics was the english zoologist st. george jackson mivart, a conflicted catholic who vacillated between accepting and decrying natural selection. in his 1871 publication on the genesis of species , mivart wielded the flatfish like a weapon to bludgeon darwin’s ideas. regarding the evolutionary origins of the flatfish’s wandering eye, mivart wrote: …the accidental occurrence of such a spontaneous transformation is hardly conceivable. but if this is not so, if the transit was gradual, how such transit of one eye a minute fraction of the journey towards the other side of the head could benefit the individual is indeed far from clear. it seems, even, that such an incipient transformation must rather have been injurious. more than 130 years later, paleobiologist matt friedman, now at oxford university, identified exactly the type of fossils darwin had in mind. in the late 2000s, friedman reexamined 45 million year old fish fossils that other researchers had dredged up from monte bolca, an ancient petrified coral reef in italy. no one thought these fossils were related to flatfish. using ct scans, however, friedman took a much more detailed look at the fishes’ skeletons and showed that they were in fact primitive adult flatfishes with asymmetrical skulls and one eye only halfway through its evolutionary journey across the face. scientists now had tangible evidence that flatfish did, in fact, gradually evolve from bilateral ancestors. yet several fundamental conundrums remained. namely, why did these animals abandon their previously symmetrical situation in life? and how does an individual larval flatfish grow comfortable with going all cattywampus? schreiber is getting close to solving the latter puzzle. he began by studying how fish maintain a sense of direction. two organs the eyes and inner ears work together to keep typical fish in an upright swimming posture, schreiber explains. if the eyes are not receiving the same amount of light on each retina, chances are the fish is leaning one way or the other, so retinal neurons trigger the brain to tilt the entire animal and set it straight. but the brain also listens to what the inner ears have to say. when clumps of calcium known as otoliths (literally “ear stones”) roll over sensitive hairs in the inner ear in one direction, they trigger those hairs to send neuronal impulses; when otoliths roll over the hairs in the other direction, they muffle the hairs. as a fish’s vertical orientation in water changes, the brain combines information from the eyes and ears to figure out which way is up. based on his research, schreiber thinks that although larval flatfish look and behave like ordinary symmetrical fish, they are essentially hardwired as asymmetrical freaks. scientists have gathered preliminary evidence, for example, that one half of the brain region receiving input from the inner ears is more active than the other and that otoliths are larger in one ear than another. schreiber has also studied some flatfish “morphant” larvae that, for unknown reasons, swim on one side practically from birth, even though they are just as anatomically symmetrical as their peers. similarly, flatfish larvae that are deliberately blinded adopt a sideways swimming posture. all of this suggests that, from a very young age, the flatfish’s inner ears somehow begin to overrule the eyes until they become the sole organ telling the flatfish how to orient itself. in the absence of visible light, the baby flatfish were incapable of staying upright. in some of his most recent experiments, schreiber discovered something astonishing that confirmed these suspicions. in the dark, goldfish and other bilateral fish have no trouble staying in an upright position because they can rely on their inner ears, even when their eyes cannot see much of anything. schreiber wondered if the same was true for flatfish, so he filmed typically developing larval flatfish in pitch darkness with infrared cameras something no one had ever done before. in the absence of visible light, the baby flatfish were incapable of staying upright. depending on their stage of metamorphosis, they either bobbed in the water with noses pointed down, as though weighted by lead, or they leaned over and drifted to the bottom of the tank like a falling leaf, landing on the same side every time the one that would eventually become blind. receive emails about upcoming nova programs and related content, as well as featured reporting about current events through a science lens. “i didn’t know what to expect,” schreiber says. “i wanted to show that larval flatfish before metamorphosis were like goldfish, but lo and behold holy cow! they are different from the get go. they are just masquerading as normal fish.” the early dominance of the inner ears may explain how a young individual flatfish learns to stay grounded, but there’s still the question of why ancient flatfish flopped over in the first place. did a mutation cause one of their eyes to wander, throwing off their balance and forcing them to swim askance? or did one eye begin migrating to accommodate a new lifestyle at the bottom of the sea? friedman and schreiber think that the flatfish’s anatomical makeover followed a change in its behavior. when threatened, some modern fish are known to lie flat on their side on the seafloor and briefly bury themselves in the sand. others tip over to play possum, only to leap up and snatch unsuspecting prey. perhaps the flatfish’s predecessor was a bilateral open water fish particularly adept at this kind of stealth. and perhaps it was so successful that it made rock bottom its permanent home. jennifer specker, a flatfish expert at the university of rhode island who has worked with schreiber in the past, agrees with this line of reasoning. “one of the guesses we make is that there was not a lot of competition for early lie and wait predators on the bottom of the ocean,” she says. “it seems that habitat was a vacuum, and nature abhors a vacuum, so flatfish adapted to it.” spending so much time in that lowly position would inevitably have damaged one eye not to mention wasting its visual powers. so ancient flatfish with eyes even a little closer together would have had a better chance of avoiding their predators’ bellies while still filling their own. modern adult flatfish are both excellent camouflagers and insatiable predators, waiting patiently for the chance to pounce on their prey by flipping themselves up with their concealed pectoral fin and a jet of water expelled from their gills. from their new perch, the flatfish’s constantly swiveling eyes provide 360 degree vision. on top of the unabashed asymmetry of an individual flatfish, there’s a whole extra level of lopsidedness among the entire population of flatfishes in the ocean. different life stages inhabit very different regions of the ocean ecosystem. just as metamorphosis has proved an enormously successful strategy for insects separating larvae and adults so they do not compete for resources (think: nectar slurping butterflies vs. leaf munching caterpillars) flatfish larvae have clung to the vestiges of symmetry in order to distance themselves from their parents. the young ‘uns need to swim upright to catch plankton near the surface of the ocean. were they to remain at the seafloor where they hatched, they would surely be vacuumed up by a roving sideways pair of lips perhaps even by their own parents. flatfish larvae require light to swim upright, so when the lights go off, they swim erratically. flatfish may be genuine anatomical anomalies rebuffing nature’s devotion to symmetry but they did not somehow evade natural selection, nor are they a counter argument to darwin. quite the contrary. flatfish are exemplars of evolution at work. there are no deliberate designs in nature. evolution is capable of producing a wonderfully streamlined, symmetrical bottom feeding fish: we know them as skates and rays. stingrays and the like have pancake thin bodies oriented in a way that makes sense to us bellies and mouths on their undersides, eyes and snout on top. but evolution did not engineer them that way on purpose. there are no deliberate designs in nature only tenacious tinkering, marvels of serendipity, and perseverance despite frequent mishaps. when an organism’s circumstances change and demand a completely different body, evolution cannot go back to the drawing board. instead, it works with what it has. if survival requires turning a symmetrical creature into a mish mash that looks like it was sewed together by dr. frankenstein, so be it. it’s weird, but it works. and that is what biologists love about the fish their functional freakiness. “i’ve always been drawn to really weird things different from what other people like,” schreiber says. “if everyone is studying fruit flies and zebrafish, i’ll be damned if i am going to study them. flatfish are just so cool. if you were to imagine what kind of fish picasso would paint, it would be the flatfish.” photo credits: jayvee fernandez/flickr (cc by) , noaa fisheries west coast/flickr (cc by nc nd) , josh more/flickr (cc by nc nd) video credit: alexander schreiber/laboratory of vertebrate metamorphosis, st. lawrence university national corporate funding for nova is provided by carlisle companies. major funding for nova is provided by the nova science trust, the corporation for public broadcasting, and pbs viewers.

internet explorer is no longer supported. try downloading another browser like chrome or firefox. if you already have an account, sign in. share migrating flatfish eye expected to flummox creationists. flatfish those odd looking fish like flounder that have both eyes on the same side of their heads have long been an evolutionary puzzle. their sudden appearance in the fossil record without any sort of foregoing transitional forms has puzzled evolutionists. a few years ago, oxford paleontologist matthew friedman published his study of heteronectes, a fossil from lower eocene rock in italy, conventionally dated at about 50 million years, as well as a similar fossilized extinct fish called amphistium. he concluded that because one eye’s orbit was at or close to the top of the fish’s head in these fossils, they were transitional forms. transitional forms may be thought of as the “holy grail” of evolutionists, for if all kinds of organisms evolved from other kinds of organisms, then transitional forms in which features were gradually changing into distinctly different features should be abundant in the fossil record (e.g., the scale of a reptile becoming the feather of a bird). they are not, however. friedman mentions the notable lack of transitional forms among spiny finned fish, for example, when he writes, “within acanthomorphs, our inability to tackle questions related to the sequence of changes underlying the assembly of specialized features or bodyplans in the absence of fossil data is not limited to this flatfish example.”1 this yellowtail flounder, like all modern flatfish, has both eyes on one side of its head. however, flatfish often prop themselves somewhat with a fin, revealing there was likely an advantage to an extinct design variation in which one eye was deviated toward the top of the head but not all the way over. (photograph by jeff rotman/getty images, obtained through news.nationalgeographic.com/news/2008/07/080709 evolution fish.html) the left and right sides of heteronectes’s head, showing the orbit on the right side to be in the middle, but the eye on the left profile view is deviated, upward, near the top of the head. we can well imagine this variety of flatfish propped slightly on one fin like modern flatfish, able to keep an eye on a wide range of happenings. (credit: image by m. friedman, obtained through phys.org/news/2012 06 mystery flatfish.html) below the x ray of heteronectes are before and after photos of the fossil, showing the improvement in visible detail after the acid transfer procedure. study of these details confirms the fish has skeletal characteristics typically found in flatfish, with the supposedly “incomplete orbital migration”2 of the eye being the primary distinction. however, the variation of eye position represents a variation found in extinct flatfish, not evidence of evolution between kinds of creatures. (credit: image by m. friedman, obtained through phys.org/news/2012 06 mystery flatfish.html) in an encore performance of his 2008 study, oxford paleontologist matthew friedman has now used an acid transfer technique to dissolve some of the matrix from around a fossilized flatfish analyzed previously. his re examination of the fossil confirms his previous findings. the fish differs from modern flatfish in only minor ways with the exception of the eyes. heteronectes’s left eye is located near the top of the skull but has not migrated to the opposite side. friedman’s conclusion that heteronectes is a transitional form between fish with symmetrical skulls and the hundreds of flatfish species with two eyes on the same side of their heads remains unchanged. friedman’s conclusion that heteronectes is a transitional form between fish with symmetrical skulls and the hundreds of flatfish species with two eyes on the same side of their heads remains unchanged. he concludes that the lack of transitional forms from the fossil record “lies in the way the fossils are treated.”3 yet despite friedman’s fine work freeing the fish from its stony tomb, creationists will still consider friedman’s conclusions to be, as the institute for creation research zoologist frank sherwin put it in 2008, “underwhelming.”4 sherwin added, “we have no problem with the variation within flatfish. what we’re asking is, show me how a fish came from a nonfish ancestor.”5 adult flatfish, with their bizarre cranial asymmetry, actually begin life with cranial symmetry. then, as a flatfish larva develops into a juvenile flatfish, one eye migrates to the other side of the head. juvenile flatfish are known to swim at odd angles until the development is complete and they adapt.6 not only is this orbital transit a normal part of flatfish development, but at every stage of the process the fish is still a fish. furthermore, friedman explained in a 2008 interview, “it’s possible that even the intermediate eye position would have provided an evolutionary advantage for the fish. living flatfish often don’t lie completely flat on the sea floor they prop themselves up with their fins. once you get that extra degree of movement, having a slightly shifted eye is going to be a lot better than having no shifted eye at all.”7 thus, friedman, in 2008 and again in 2012, has described a reason that various permutations of the eye asymmetry would be of potential advantage to varieties of flatfish, including the extinct heteronectes and amphistium. but he has not described evolutionary transitional forms, just varieties of flatfish. in fact, these extinct kinds of flatfish are fossilized in the same rock layers as ordinary flatfish, their supposed evolutionary descendants. as sherwin told the national geographic reporter in 2008, “fish have always been fish, all the way down to the lower cambrian, [conventionally dated 542 to 488 million years ago].”8 commenting on friedman’s study, los angeles natural history museum expert on fossilized fish, john long, said, “this is a profound discovery which clearly shows that intermediate fossil forms, which according to certain creationist theories shouldn’t exist, are regularly turning up as scientists keep looking for them.” however, creationists do not claim variations within created kinds “shouldn’t exist”! nothing about this discovery or its “re discovery” screams transitional form or demonstrates evolution of one kind of creature into another kind of creature. god created all kinds of fish on the fifth day of creation week about 6,000 years ago, as his eyewitness account in genesis records. and all kinds of fish were designed to reproduce after their kinds (genesis 1:20 23). the created kinds of fish were equipped by god with the genetic information to vary within their kinds, but not to evolve into new kinds of creatures. remember, if you see a news story that might merit some attention, let us know about it! 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hidden away in museums for more that 100 years, some recently rediscovered flatfish fossils have filled a puzzling gap in the story of evolution and answered a question that initially stumped even charles darwin. opponents of evolution have insisted that adult flatfishes, which have both eyes on one side of the head, could not have evolved gradually. a slightly asymmetrical skull offers no advantage. no such fish    fossil or living    had ever been discovered, until now. all adult flatfishes  including the gastronomically familiar flounder, plaice, sole, turbot, and halibut  have asymmetrical skulls, with both eyes located on one side of the head. because these fish lay on their sides at the ocean bottom, this arrangement enhances their vision, with both eyes constantly in play, peering up into the water. this remarkable arrangement arises during the youth of every flatfish, where the symmetrical larva undergoes a metamorphosis to produce an asymmetrical juvenile. one eye 'migrates' up and over the top of the head before coming to rest in the adult position on the opposite side of the skull. opponents of evolution, however, insisted that this curious anatomy could not have evolved gradually through natural selection because there would be no apparent evolutionary advantage to a fish with a slightly asymmetrical skull but which retained eyes on opposite sides of the head. no fish  fossil or living  had ever been discovered with such an intermediate condition. but in the 10 july 2008 issue of nature, matt friedman, graduate student in the committee on evolutionary biology at the university of chicago and a member of the department of geology at the field museum, draws attention to several examples of such transitional forms that he uncovered in museum collections of underwater fossilized creatures from the eocene epoch  about 50 million years ago. "we owe this discovery, in part, to the european fondness for limestone," said friedman. the fossils, which he found in museums in england, france, italy, and austria, came from limestone quarries in northern italy and underneath modern day paris. friedman examined multiple adult fossil remains of two primitive flatfishes, amphistium and a new genus that he named heteronectes. "amphistium has been known for quite some time," he said. "the first specimen was described more than 200 years ago, but its placement in the fish evolutionary tree has been uncertain ever since. close examination of these fossils yield clues that they are indeed early flatfishes." the most primitive flatfishes known, both amphistium and heteronectes have many characteristics that are no longer found in modern flatfish. but the one that caught friedman's attention was the partial displacement of one eye, evident even in the first amphistium fossil discovered over two centuries ago. "most remarkably," he said, "orbital migration, the movement of one eye from one side of the skull to the other during the larval stage, was present but incomplete in both of these primitive flatfishes."