Gill
Store Manager Coalville Aquatics
It has taken over an hour on google hunting for this article, i read a few years ago.
The "Ayes" Have it -- What a Fish Sees
by Jenny Kruckenberg
As published in Aqua News September/October 1992
A Publication of the Minnesota Aquarium Society
Here's what the information center had to say ... A fish has sharp sight both through water and air. Yet because a ray of light is bent (or refracted) when it passes from air to water, the fish sees an angler very differently from the way the angler sees the fish.
When a fish looks around or down in its watery world, it has an unrestricted view. However, when it looks upward, the surface of the water is like a ceiling with a circular skylight above the fish's head. The fish's view of the outside world is confined to the cone shaped area below the circular opening. Known as "Snell's Window" (and named after Winfred Snell who described the law of refraction in 1631) this cone follows the fish everywhere it goes and always has an angle of 97°. The size of the skylight itself depends on how deep the fish is. When the fish is 6" from the surface, the window is a foot across. If the fish is 3 feet below the surface, the window is nearly 7 feet across. This is about where I ran out of tape. But it went on with something about the light rays passing quicker through a thin medium (air) and slower into a thick medium (such as water) which caused the rays to be bent at certain refractive angles.
My mind started traveling back to 7th grade science but more so, I couldn't help citing how unique their eyes were. When I go under water, everything is a blur without a face mask. How does a fish see so clearly underwater and furthermore, how does it have the ability to look up, down, forward, beside and even behind itself without changing its body posture much or possessing a neck? (What super big league pitchers they could be!)
I found some anatomical answers in a huge book by N. B. Marshall called The Life of Fishes. He explains that all vertebrate animals have similar eyes. There's an outer fibrous tunic called the sclera then a transparent watery corncea in front of the iris, pupil and crystalline lens. Behind that, the retina which reacts to light (and is like the screen for a slide show). See figure A.
A fish's eye differs from ours in that the lens is perfectly spherical and bulges well through the pupil. This shape and placement makes up for an "absent" cornea. Our lens' are kinda oval in a cross section view and lie just behind the plane of the pupil.
The cornea is said to be optically absent in a fish since the watery comeal tissue has the same refractive index as the water its swimming in (1·33). In fact, the light rays aren't bent and focused until they reach the lens of the fish, which has the highest affective refractive index of all vertebrates (1·65). And then, the whole lens is moved to bring things into focus.
In land dwelling vertebrates, the lens and the cornea refract and focus light onto the retina. Even the shape of the cornea helps to bring things into focus. It's curved. I was still a bit confused until I read the Wilsons' chapter on vision in their book Watching Fishes. We humans have to have the light rays pass through air for our eyes to focus effectively. A face mask traps air. Fish don't need the air.
Of course, all this technical jargon happens just as naturally for a fish as it does for a person, but so much more is actually physically different. As mentioned (common knowledge) fish don't have a neck, but placement (usually on each side of the head) coupled with their large round lens' and the fact that their eyes generally stick out from the surface of their head, gives most fish all-around vision.
In Marshalls' book, he cites a 1962 study by Baylor and Shaw which confirmed that flsh are not short-sited as was once assumed, but on the contrary are hypermetropic (long sited), especially in their lateral field of vision. The retina is most dense with visual cells at the back of the retinal wall (figure B). This means when the fish is looking straight forward, it's seeing most clearly. In its lateral (or side) field of vision, it can detect movement and see if an object is dark or light, but things are nor seen truly in focus. Fortunately the most detailed sight is right where most fish need it - to make prey avoid obstacles and enemies (not to mention looking at us straight on through the glass.)
Like us, fish have two main kinds of visual cells in the retina -- rods and cones. These are connected to the brain first through elaborate nerve cell layers then individual fibers of the optic nerve (figure C). The rods and cones hold pigments that absorb the light of the focused image and are thereby chemically changed to impulses that travel to the brain. It is interesting that the rod's pigments are rose-colored in marine fish and purple in freshwater species.
The rods are more sensitive and allow the fish to see even under dim lighting situations, but apparently its the cones that are more responsible for seeing color and bright light. So fish who are most active during daylight have relatively high numbers of cones, sometimes double cones. The pike (Esox lucius) is a good example of this. It's a dawn to dusk eater and in clear water can see up to 50 feet away, darting 20-30 feet forward to catch its supper. The rods out-number the cones but it (and most other freshwater and coastal fish) are said to have diurnal eyesight because the rods and cones are in balanced mass amounts (close-up figure C). The pike is also a good example of a fish having excellent binocular vision. This means its eyes are not set so far back from its nose that it looses an overlapping of each eyes field of vision.
If we look at the brain of a fish active during daylight hours, we can see a large, highly-developed optic lobe (figure D). In contrast, nocturnal fish have small optic nerves and retinae where the rids far out number the cones. In a few species the cones have been totally dispensed with.
Checking back on Chet Ekstedts article (Aqua News, Mar./Apr. '92) he noted that his wild caught burbot (Lola lota) had large eyes but he considered it to be essentially blind. He said it would hide most of the time under a rock. Apparently preferring to be out of the light. He suspected that it was nocturnal. Later Chet noted the burbot's attributes to be a strong sense of smell a whisker that was sensitive to taste and that it seemed to hear well.
In her book The Basic Marine Aquarium, Carol Bower says "...to interpret an animal's behavior you must understand its sensory capablties - their ability to receive environmental stimuli."
Chet's suspicions about his burbot were right on the money and he made a number of excellent observations that Marshall confirms. He calls the burbot a "voracious" nocturnal eater. Its eyes match its activities with a rod to cone ratio of 200 to 1. Additionally, the optic center of its brain is really small. Conversely, the olfactory centers are about 1½ times greater. Here "intelligence", or this fish's ability to receive environmental stimuli, is measured through its nasal openings!
Numerous catfish, mormyrids, knifefish and eels live in such turbid waters, seeing maybe an inch in front of them throughout their lives, that even if removed to clear water conditions their eyes would still have rather limited visual powers.
Cause the fact is, they don't need to see details. Not only have they compensated with their nasal passages, but often have over developed taste buds in their mouths, lips, barbers and even nerves with taste buds on their fins and body.
Also, though water may transmit light poorly, it is a very good conductor of sound and electrical impulses. (Ouch!) Fish in cloudy or clear water use their lateral-lines to "hear" another fish swimming nearby or to detect the perimeter of an aquarium. Called 'Ferntastisinn' or the "distance-touch" sense by the Germans. Bower notes that each scale of the lateral-line has a pore which senses low frequency vibrations which are caused by disturbances in the water and changes in the watevs flow patterns.
Interestingly, whales and dolphins do not have lateral lines, presumably because they rely so heavily on sonar. What evolutionary marvels. But now, let's consider two almost cryptic groups of fish -- Cave fish and deep sea fish.
Marshall says there are over 40 species of blind cave fish, about one third of which are catfish. Often their eyes have totally regressed and the visual centers of their brains are a mere pinpoint in size. Yet when given a choice between darkness and light they'll actively move out of the light. Apparently the remnants of their eyes in fish like the Mexican cave characin (Anoplichthys jordani) and the blind Congo barb (Caecobarbus geerisi) function just enough to let them know if they're in a light (dangerous?) place or a dark (safe?) one.
These fish share the increased olfactory, acoustic and lateral line but they seem to also be in a never ending pursuit for food. They glide and swim somewhat frantically through the water and if they happen to bump into something edible its gobbled up immediately. One theory speculates they can't wait around to interpret the information sent by their senses. I have two Pictus cats (Pimelodelia picta) who aren't blind but who share this same enthusiasm when searching for and finding food. They are a joy to watch.
I also have two Plecos (Hypostomus punctalus) which I purchased on the same occasion and that were approximately the same size and color. Shortly after their introduction into the tank, one of them had an eye pecked out by another fish (dam Neets!). Anyway, I've watched both gain the same amount of weight and maneuver around the tank in nearly the same manner for over a year now. Mr. One-eye doesn't seem to have any problems relying on its other senses. The only difference I can note is how it always stays just a couple shades darker. (P. S. If I got into color as it relates to the eyes triggering camouflage or fright responses, you could be reading forever! But I will add that in addition to loosing their eyes, many cave fish have lost their pigment cells. According to my resources, color is only seen by turtles, lizards, birds, primates and fish among veaebrate animals. In a cave, it wouldn't matter if you were the President -- you would have no more need for color than these fish.)
In the sea, its another story though. Even between the depths of 150 to 1000 meters down, blue rays from the sun's spectrum travel down. Still, its very dim, essentially "underwater twilight".
Even in this darkness, fish have adapted. They don't have degenerate vestiges of eyes, like that of cave flsh, but rather elaborate, large ones that capture every speck of available light. Their retinas have no cones, just vast numbers of rods. And instead of rose or purple colored pigments, their rods "glow' with golden-colored ones. These fish are said to have the most sensitive eyes of any vertebrate animals. I wondered, if they suddenly came up into the daylight, would it seem like 12 zillion flashbulbs went off all at once or since they lacked cones, if they could process it at all?
Many of you who have read this probably think I'm a little off base for finding fish eyesight interesting, but consider this; a freshwater eel makes a migratory journey to breeding grounds in the Sargasso Sea but before doing so, its eyes become greatly enlarged and the rods acquire the golden, light absorbing pigment typical of deep sea fish. After they've done their thing, its back home and their eyes return to normal -- fascinating.
The "Ayes" Have it -- What a Fish Sees
by Jenny Kruckenberg
As published in Aqua News September/October 1992
A Publication of the Minnesota Aquarium Society
Here's what the information center had to say ... A fish has sharp sight both through water and air. Yet because a ray of light is bent (or refracted) when it passes from air to water, the fish sees an angler very differently from the way the angler sees the fish.
When a fish looks around or down in its watery world, it has an unrestricted view. However, when it looks upward, the surface of the water is like a ceiling with a circular skylight above the fish's head. The fish's view of the outside world is confined to the cone shaped area below the circular opening. Known as "Snell's Window" (and named after Winfred Snell who described the law of refraction in 1631) this cone follows the fish everywhere it goes and always has an angle of 97°. The size of the skylight itself depends on how deep the fish is. When the fish is 6" from the surface, the window is a foot across. If the fish is 3 feet below the surface, the window is nearly 7 feet across. This is about where I ran out of tape. But it went on with something about the light rays passing quicker through a thin medium (air) and slower into a thick medium (such as water) which caused the rays to be bent at certain refractive angles.
My mind started traveling back to 7th grade science but more so, I couldn't help citing how unique their eyes were. When I go under water, everything is a blur without a face mask. How does a fish see so clearly underwater and furthermore, how does it have the ability to look up, down, forward, beside and even behind itself without changing its body posture much or possessing a neck? (What super big league pitchers they could be!)
I found some anatomical answers in a huge book by N. B. Marshall called The Life of Fishes. He explains that all vertebrate animals have similar eyes. There's an outer fibrous tunic called the sclera then a transparent watery corncea in front of the iris, pupil and crystalline lens. Behind that, the retina which reacts to light (and is like the screen for a slide show). See figure A.
A fish's eye differs from ours in that the lens is perfectly spherical and bulges well through the pupil. This shape and placement makes up for an "absent" cornea. Our lens' are kinda oval in a cross section view and lie just behind the plane of the pupil.
The cornea is said to be optically absent in a fish since the watery comeal tissue has the same refractive index as the water its swimming in (1·33). In fact, the light rays aren't bent and focused until they reach the lens of the fish, which has the highest affective refractive index of all vertebrates (1·65). And then, the whole lens is moved to bring things into focus.
In land dwelling vertebrates, the lens and the cornea refract and focus light onto the retina. Even the shape of the cornea helps to bring things into focus. It's curved. I was still a bit confused until I read the Wilsons' chapter on vision in their book Watching Fishes. We humans have to have the light rays pass through air for our eyes to focus effectively. A face mask traps air. Fish don't need the air.
Of course, all this technical jargon happens just as naturally for a fish as it does for a person, but so much more is actually physically different. As mentioned (common knowledge) fish don't have a neck, but placement (usually on each side of the head) coupled with their large round lens' and the fact that their eyes generally stick out from the surface of their head, gives most fish all-around vision.
In Marshalls' book, he cites a 1962 study by Baylor and Shaw which confirmed that flsh are not short-sited as was once assumed, but on the contrary are hypermetropic (long sited), especially in their lateral field of vision. The retina is most dense with visual cells at the back of the retinal wall (figure B). This means when the fish is looking straight forward, it's seeing most clearly. In its lateral (or side) field of vision, it can detect movement and see if an object is dark or light, but things are nor seen truly in focus. Fortunately the most detailed sight is right where most fish need it - to make prey avoid obstacles and enemies (not to mention looking at us straight on through the glass.)
Like us, fish have two main kinds of visual cells in the retina -- rods and cones. These are connected to the brain first through elaborate nerve cell layers then individual fibers of the optic nerve (figure C). The rods and cones hold pigments that absorb the light of the focused image and are thereby chemically changed to impulses that travel to the brain. It is interesting that the rod's pigments are rose-colored in marine fish and purple in freshwater species.
The rods are more sensitive and allow the fish to see even under dim lighting situations, but apparently its the cones that are more responsible for seeing color and bright light. So fish who are most active during daylight have relatively high numbers of cones, sometimes double cones. The pike (Esox lucius) is a good example of this. It's a dawn to dusk eater and in clear water can see up to 50 feet away, darting 20-30 feet forward to catch its supper. The rods out-number the cones but it (and most other freshwater and coastal fish) are said to have diurnal eyesight because the rods and cones are in balanced mass amounts (close-up figure C). The pike is also a good example of a fish having excellent binocular vision. This means its eyes are not set so far back from its nose that it looses an overlapping of each eyes field of vision.
If we look at the brain of a fish active during daylight hours, we can see a large, highly-developed optic lobe (figure D). In contrast, nocturnal fish have small optic nerves and retinae where the rids far out number the cones. In a few species the cones have been totally dispensed with.
Checking back on Chet Ekstedts article (Aqua News, Mar./Apr. '92) he noted that his wild caught burbot (Lola lota) had large eyes but he considered it to be essentially blind. He said it would hide most of the time under a rock. Apparently preferring to be out of the light. He suspected that it was nocturnal. Later Chet noted the burbot's attributes to be a strong sense of smell a whisker that was sensitive to taste and that it seemed to hear well.
In her book The Basic Marine Aquarium, Carol Bower says "...to interpret an animal's behavior you must understand its sensory capablties - their ability to receive environmental stimuli."
Chet's suspicions about his burbot were right on the money and he made a number of excellent observations that Marshall confirms. He calls the burbot a "voracious" nocturnal eater. Its eyes match its activities with a rod to cone ratio of 200 to 1. Additionally, the optic center of its brain is really small. Conversely, the olfactory centers are about 1½ times greater. Here "intelligence", or this fish's ability to receive environmental stimuli, is measured through its nasal openings!
Numerous catfish, mormyrids, knifefish and eels live in such turbid waters, seeing maybe an inch in front of them throughout their lives, that even if removed to clear water conditions their eyes would still have rather limited visual powers.
Cause the fact is, they don't need to see details. Not only have they compensated with their nasal passages, but often have over developed taste buds in their mouths, lips, barbers and even nerves with taste buds on their fins and body.
Also, though water may transmit light poorly, it is a very good conductor of sound and electrical impulses. (Ouch!) Fish in cloudy or clear water use their lateral-lines to "hear" another fish swimming nearby or to detect the perimeter of an aquarium. Called 'Ferntastisinn' or the "distance-touch" sense by the Germans. Bower notes that each scale of the lateral-line has a pore which senses low frequency vibrations which are caused by disturbances in the water and changes in the watevs flow patterns.
Interestingly, whales and dolphins do not have lateral lines, presumably because they rely so heavily on sonar. What evolutionary marvels. But now, let's consider two almost cryptic groups of fish -- Cave fish and deep sea fish.
Marshall says there are over 40 species of blind cave fish, about one third of which are catfish. Often their eyes have totally regressed and the visual centers of their brains are a mere pinpoint in size. Yet when given a choice between darkness and light they'll actively move out of the light. Apparently the remnants of their eyes in fish like the Mexican cave characin (Anoplichthys jordani) and the blind Congo barb (Caecobarbus geerisi) function just enough to let them know if they're in a light (dangerous?) place or a dark (safe?) one.
These fish share the increased olfactory, acoustic and lateral line but they seem to also be in a never ending pursuit for food. They glide and swim somewhat frantically through the water and if they happen to bump into something edible its gobbled up immediately. One theory speculates they can't wait around to interpret the information sent by their senses. I have two Pictus cats (Pimelodelia picta) who aren't blind but who share this same enthusiasm when searching for and finding food. They are a joy to watch.
I also have two Plecos (Hypostomus punctalus) which I purchased on the same occasion and that were approximately the same size and color. Shortly after their introduction into the tank, one of them had an eye pecked out by another fish (dam Neets!). Anyway, I've watched both gain the same amount of weight and maneuver around the tank in nearly the same manner for over a year now. Mr. One-eye doesn't seem to have any problems relying on its other senses. The only difference I can note is how it always stays just a couple shades darker. (P. S. If I got into color as it relates to the eyes triggering camouflage or fright responses, you could be reading forever! But I will add that in addition to loosing their eyes, many cave fish have lost their pigment cells. According to my resources, color is only seen by turtles, lizards, birds, primates and fish among veaebrate animals. In a cave, it wouldn't matter if you were the President -- you would have no more need for color than these fish.)
In the sea, its another story though. Even between the depths of 150 to 1000 meters down, blue rays from the sun's spectrum travel down. Still, its very dim, essentially "underwater twilight".
Even in this darkness, fish have adapted. They don't have degenerate vestiges of eyes, like that of cave flsh, but rather elaborate, large ones that capture every speck of available light. Their retinas have no cones, just vast numbers of rods. And instead of rose or purple colored pigments, their rods "glow' with golden-colored ones. These fish are said to have the most sensitive eyes of any vertebrate animals. I wondered, if they suddenly came up into the daylight, would it seem like 12 zillion flashbulbs went off all at once or since they lacked cones, if they could process it at all?
Many of you who have read this probably think I'm a little off base for finding fish eyesight interesting, but consider this; a freshwater eel makes a migratory journey to breeding grounds in the Sargasso Sea but before doing so, its eyes become greatly enlarged and the rods acquire the golden, light absorbing pigment typical of deep sea fish. After they've done their thing, its back home and their eyes return to normal -- fascinating.
