An In-Depth Look at Carp Vision – Tom Dolman

Carp Vision - Tom Dolman

Carp Vision – Tom Dolman

Part 1: Light, Sight and Anatomy
What can carp see? A question we may never truly have a definitive answer to but by gaining an understanding of how sight works, examining the physical characteristics of the fish and their natural environment we can get a pretty good idea of their capabilities. By understanding the capabilities of our prey we give ourselves a better chance of success during the hunt. Whether it’s by using visual tricks to attract or deceive them, understanding what they can see will help us achieve our goals.

In Part 1 we will need to establish an understanding of the basics of light, sight and anatomy. These subjects are admittedly rather dense and can make for heavy reading but I’ve tried to keep it as light as possible without skimping on useful information. I can’t say that this chapter will hold many immediately useful tips but hopefully it will provide you with the basic knowledge to make your own, informed decisions.

Being the first chapter it obviously will not contain all the parts of the puzzle but it will build the framework upon which a greater understanding can be constructed. Before we begin taking a look at the specifics of how a carps eyes work we first need to get an understanding of the basic principals of light and how it allows them, us and other creatures to view the world around us.

Basics of Light:
Simply stated, light is energy transfer over distance. Light is more general term for electromagnetic radiation but most often when we say “light” we are referring to “visible light” which is the range of radiation visible to our eyes. Visible light forms only tiny portion of a huge range of light types that make up the electromagnetic spectrum.

The often difficult thing to comprehend with light is that it has a dual nature, it can be expressed as both a particle and a wave simultaneously.

When describing light it can be convenient to express light as a particle known as a photon. A photon is a little packet of energy which travels away from its source in a straight line at a constant speed (ie. the speed of light in a vacuum = 299,792 km/s). However, not all photons are created equal and some will contain more energy than others, for instance a photon of X-Ray light will contain a lot more energy compared to a visible or infrared photon.

The other way of representing light is as a wave, this is somewhat more difficult for most people to understand but perhaps comparing them with sound waves is the easiest way to grasp the idea. When you play a high note and a low note on the piano they both produce sound waves but the different sounds we hear are a result of different frequencies of sound waves produced by a vibrating wire. When the wire vibrates faster we hear a high note whereas the a slower vibration produces a lower note. If we now shift our focus to the sound waves themselves instead of the vibrating string, we would find that the higher pitched notes have shorter wavelengths, or distances between each successive wave.

In the same sense blue light and red light are both light but blue light has a higher frequency of vibration/shorter wavelength compared to red light. Because they are both travelling at the same speed the faster vibrating photon carries more energy and therefore blue is a more energetic form of light than red.

The amplitude of the waveform is another indicator of the energy of contained within a photon. To use the sound analogy once more we can imagine playing a continuous note through a speaker, if we play the note at a low volume and then a high volume the wavelength of the note wouldn’t change but the amplitude would increase with volume. The same thing happens with light, if you have a low powered red light torch and a high powered red light torch then the signal they would be outputting would be identical in wavelength but the amplitude would be higher for the high powered torch.

When a photon of light loses energy it does so primarily through amplitude rather than wavelength (ie. it does not change colour as it fades).

Light as previously mentioned is a form of energy and therefore is subject to the fundamental laws of thermodynamics, the first of which (conservation of energy) states that energy can neither be created nor destroyed, only changed.

If we take the light we receive from the sun as an example then the energy given to the photons being fired towards the earth are the result of chemical energy being released by the immense gravity of the sun smashing hydrogen atoms together so hard that they bond together to form new molecules and emit radiation as a by-product.

As the photons of light radiation energy fly through the vacuum of space they occasionally collide with a few stray molecules floating around which absorb some energy as they pass through one another, the energy lost by the photon is generally converted to thermal energy (heat) by the molecule. During the crossing of the colossal distance between the sun and earth the photons lose very little energy but once they enter the atmosphere they collide with more and more molecules in the form of oxygen, nitrogen, carbon etc. and lose more and more energy until they are entirely absorbed.

Passing from a less dense environment (space) into a more dense environment (earths gaseous atmosphere) causes the photons of light to slow down and change direction slightly in a process known as refraction. Light entering water after passing through the atmosphere goes through a much greater reduction of speed and direction as water, being a liquid, is more dense than air.

Basics of Colour and Sight:
The photons that make it down to ground level collide with objects in our surroundings, the surface of the object will absorb certain wavelengths of light and reflect others resulting in what we perceive as colour. All sight is based on the collection and analysis of reflected light, eyes can only receive the photons that are fired into them and not those going past at an angle.

An object such as an apple does not inherently have a colour, it only appears to be red to our eyes because the apples surface has absorbed all of the green and blue photons and reflected only red photons back to our eyes. If you illuminated a red apple with a purely blue light source then the apple would no longer appear red to our eyes but instead black because it absorbs all available light and reflects nothing.

An object that reflects all wavelengths of light would appear white and an object that absorbs all wavelengths appears black. Objects that reflect more than one type of light will appear as secondary or tertiary colours depending on how much of each they reflect.

The brightness of a reflected colour depends on how much light the object reflects and how much energy the photons have remaining when they reach the eye. If the reflected photons contain little energy by the time they reach the eye they will give the appearance of a darker shade, if they contain more energy then they’ll appear brighter.

Taxonomy & Basic Anatomy:

Carp as we’d commonly refer to them (cyprinus carpio) belong to the cyprinidae family of fish that forms the largest and most diverse fish family with around 3000 living and extinct species amongst its ranks. Other cyprinids found in UK waters include barbel, bream, chub, crucian carp, dace, grass carp, gudgeon, roach, rudd and tench, this relatively small cross section of examples illustrates the varied physical forms that they can take.

The defining features of cyprinids include a swim bladder, the presence of a Weberian apparatus, a lack of stomach and jaws possessing no teeth. The swim bladder is a bag of gas located near to the fishes centre of gravity that can be inflated/deflated to adjust how high/low the fish sits in the water. A Weberian device is an arrangement of small bones that connect the auditory system to the swim bladder to increase hearing sensitivity. Instead of having a stomach like ours before the intestine to break down complex foods they have an intestine that runs all the way from entry to exit, the intestine is used to absorb available nutrients from food passed through it mixed with digestive enzymes. Instead of having teeth present on the jaws to dice up food before swallowing cyprinids possess a set of pharyngeal teeth in their throats located on the specialised final gill raker, these help chew food and break down hard shelled things like seeds and snails before being passed into the intestine for digestion.

As is the case with most fish, carp are cold blooded poikilotherms meaning that they can’t manage body temperature and instead take on the temperature of their surrounding environment. They differ significantly to how we as warm blooded creatures manage heat and function.

Warm blooded creatures like mammals and birds aim to keep the inside of their bodies at a constant temperature, this is achieved by generating their own heat when they are in a cold environment and by cooling themselves when they are in a warm environment. To generate heat warm blooded animals convert the food that they eat into energy. They have to eat a lot of food compared to cold blooded animals in order to maintain a constant body temperature. Only a small amount of the food that a warm blooded animal eats is converted into body mass, the rest is used to fuel a constant body temperature.

Cold blooded creatures take on the temperature of their surrounding environment, they’re hot when their environment is hot and cold when their environment is cold. Cold blooded animals are much more active in warm environments and become very sluggish in cold environments, this is because their muscle and sensory activity depends on chemical reactions which work quickly when hot and slowly when cold. Because controlling body temperature isn’t a major energy draw cold blooded animals can convert much more of its food intake into body mass compared with a warm-blooded animal.

Carp are at their best in water between 16-24ºC, at temperatures above this the fish is forced to produce heat shock proteins to repair the damage done to overheated cells which both significantly increases the energy consumption requirements and decreases digestive efficiency. At temperatures below optimal conditions both bodily and sensory function begin to degrade, the further from the optimal range the fish is the worse the effect will be. A few degrees below optimal will have a small effect on the how much food they can turn into mass and the senses will be slightly dulled but by the time they get close to freezing temperatures they will be extremely inefficient at digesting food and the senses will be barely functioning.

 Eye Positioning and Size:

Looking at the eyes we can observe that the eyes are relatively small compared to the size of the head and this is indicative of a creature not specialised towards nocturnal or deep water activity. Creatures that are predominantly active at night or in environments where light is limited typically possess large eyes to collect as much of the available light as possible. A good example of nocturnal/deep water specialised eyes would be those of the Zander

The next clue we get to determining the visual capabilities of a carp come from the positioning of the eyes on the head. As we can see the eyes are positioned on the sides of the head facing in opposite directions, this is a common evolutionary characteristic in non-predatory species that has some advantages and disadvantages compared to the forward facing eyes that we possess. While the eyes may be on the sides of the head the positioning is more of a hybrid system offering binocular and monocular vision, this said the arrangement is more biased towards monocular vision.

The main advantage of this monocular biased arrangement is that it gives a better peripheral (side) vision allowing the carp to keep an eye on what’s going on around them while feeding and spot approaching predators/threats. The downside is that with only one eye on the target (monocular vision) the image will lack accurate depth perception (how far an item is from the eye) and have reduced visual acuity (how detailed the image is). There are areas where the left an right eye can visually overlap offering binocular vision but due to the positioning of the eyes and the location of the blind spot is unlikely that binocular vision is actively utilised to examine things closer and instead is more likely used during motion.

The lack of accurate depth perception is a result of the inability to mentally compare similar images coming from the individual eyes as you can with binocular vision, this trait does not really have a negative effect on the carp as there is very little need for the fish to estimate its relative distance to objects, this trait is far more desirable in predators who need to be able to make quick distance estimations to judge when prey is within lunging range.
Fish do utilise a mechanism similar to that which can be observed in chickens and other birds to gain a primitive form of depth perception but it only really works when the fish is in motion. By moving their heads side to side they can gain a more useful idea of how far items are from the eye or whether things are getting closer or further away. You may have noticed chickens and other birds bobbing their heads as they move and feed and this is done to help spot approaching threats which may be otherwise camouflaged beyond the capabilities of their eyes.

The reduction in visual acuity associated with monocular vision is a result of the increased mental strain of processing two wide angle images simultaneously, the relatively small brains of fish can’t take full advantage of the information available so movement/change is prioritised over ultimate detail to save energy. There is evidence suggesting that most fish are capable of temporarily shutting down or greatly reducing the signals from one eye in order to study something on the other side in more detail but how often they do this and how much improvement there is to be gained remains unclear.

Eye Anatomy:
By taking a closer look at the physical characteristics of a carps eyes we can further determine the likely capabilities based upon the understanding of how our own eyes work. We cannot say for certain that any other creature sees the world exactly as we do (we can’t even say for certain that two human beings see the world in the same way) but we can suggest with a relative degree of certainty that similar components of the eye perform the same task.

Fish were one of the earliest vertebrates (creatures possessing a backbone) to develop in the long evolutionary history of life on earth and as such formed parts of the blueprints of all vertebrate life to follow. Reptiles, amphibians, birds and mammals (a super-classification of vertebrates with legs known as tetrapods) all developed from fish ancestors and as such will share similar basic traits and components. We can clearly see these similarities within the basic construction and function of the eyes.

Above we have two images portraying the anatomy of a the eyes of a teleost* (a classification of bony fish under which carp fall) and a human eye. As we can see they are broadly the same in design comprising of roughly the same components with which to process light, there are some adaptations but the basic function remains the same.

To aid understanding going forward we will first discuss what the various parts do within the human eye and then we will continue to compare how and why fish eyes differ.

Human eye structure and function:

Both human and carp eyes belongs to a general group of eyes found in nature called ‘camera-type eyes’. Just as a camera lens focuses light onto film, a structure in the eye called the cornea focuses light onto a light-sensitive membrane called the retina

The cornea is a transparent structure found in the very front of the eye that helps to focus incoming light. Behind the cornea is a coloured, ring-shaped membrane called the iris, the iris has an adjustable circular opening called the pupil which can expand or contract to control the amount of light entering the eye.

Situated behind the pupil is a colourless, transparent structure called the crystalline lens. The cornea focuses most of the light then it passes through the lens which continues to focus the light further. Ciliary muscles surround the lens, the muscles hold the lens in place but they also play an important role in vision. When the muscles relax, they pull on and flatten the lens, allowing the eye to see objects that are far away. To see closer objects clearly, the ciliary muscle must contract in order to thicken the lens.

The interior chamber of the eyeball is filled with a jelly-like tissue called the vitreous humor. After passing through the lens, light must travel through this humor before striking the sensitive layer of cells at the back of the eye called the retina. The retina is the innermost of three tissue layers that make up the eye. Embedded in the retina are millions of light sensitive cells, which come in two main varieties: rods and cones. We will discuss the function and placement of these specialised cells in detail later.

The middle layer between the retina and sclera is called the choroid. The choroid contains blood vessels that supply the retina with nutrients and oxygen and remove its waste products. The outermost layer, called the sclera, is what gives most of the eyeball its white colour. The cornea is also a part of the outer layer.

Fish eye structure/function:

Although extremely similar to human eyes the main notable differences are that carp eyes cannot contract or relax the iris to alter the size of the pupil and thus control the amount of light entering the eye, this is a common evolutionary product of living in a relatively low light environment where exposure to bright, direct light is uncommon.
The other obvious difference is that the lens portion of the eye is different in both shape and the way it is manipulated within the eye. Compared to humans, fish lenses are generally more dense and spherical, in the aquatic environment there is not a major difference in the refractive index of the cornea and the surrounding water (compared to air on land) so the lens has to do the majority of the refraction.

Instead of changing the shape of the lens to achieve long/short range focus the lens is moved closer to/further away from the retina using a special muscle to produce an effect similar to what you observe when moving a magnifying glass closer/further away from an image (a process referred to as Visual Accommodation). In bony fishes the muscle is called the retractor lentis and is relaxed for near vision whereas for cartilaginous fishes (sharks/rays) the muscle is called the protractor lentis, and is relaxed for far vision. Thus bony fishes accommodate for distance vision by moving the lens closer to the retina, while cartilaginous fishes accommodate for near vision by moving the lens further from the retina.

The relaxed state suggests which focal range is more commonly used so we can suggest that carp are for the most part short sighted and would be unlikely to view items more than a foot away in any great detail, they would still be able to see things beyond this range the image would likely be blurry and lacking detail. Long range vision in carp is mostly used to provide early indication of potential predators moving in the vicinity but with the curious nature of the fish it is also likely be utilised for spotting food if water clarity permits. This adaptation is one born out of living in a relatively turbid (cloudy/opaque) environment where accurate long range vision is of little practical use in the same way that we’d find binoculars no use in heavy fog.

Retina Function
To shed further light on the visual capabilities of carp we need to take a closer look at the function of the retina and what part the specialised light receiving cells known as rods and cones play. By looking at the arrangement and types of cells found on the retina we can answer some fairly common questions such as whether they can see colour and how much detail they can see.

As before we will take a look at the things we know our own vision to help us understand how the carp might see the world around it.

Incoming photons are initially focused by the cornea and then further focused by the lens before being projected through the eye onto the retina at the back.

The retina is home to specialised photoreceptor cells produced by the body in the forms of rods and cones which react to incoming photons of light.

These two cell types have basically the same function in that they react when incoming photons of light hit them causing the cell to release a neural signal to the brain, the cell depletes in the process of releasing the signal and needs to be replenished or replaced. They each act like specialist filters for detecting various wavelengths of light and are found in different quantities in different locations on the retina.

Rod cells are the more basic forms of photoreceptor cells found within both human and carp eyes, they form the majority of cell numbers found upon the retina. Rod cells require significantly fewer photons to hit the cell in order to send a signal and so are far better suited to low light scenarios and are responsible for all night vision in humans and carp. The downside to these types of cells is that they take longer to replace and offer no colour vision, the slow response and replenishment of cells results in less signals being sent to the brain and a lower image clarity (could be likened to watching a video in 144p compared to 1080p). Because they only detect one part of the light spectrum they give monochrome shade differentiation rather than true colour vision.

Cone cells require more photons to strike them in order to send a signal so are much more suited to bright conditions as found in daylight hours. They have an extremely fast response time to stimuli compared to rods that allows them to perceive much finer detail but the downside is that they can only function effectively with an abundance of light, as light levels drop the cells quickly become useless for relaying signals to the brain. Within the eyes of both humans and carp cone cells come in three varieties for detecting short (blue), medium (green) and long (red) wavelengths of light. Because the detection ranges of the different varieties of cones overlap we can not only observe the three main colours but we can also observe varying shades and what are known as secondary/tertiary colours made up of combinations of the three prime colours.

Other types of cone cells do exist within nature for detecting other bandwidths of light such as Ultraviolet and Infrared, sometimes creatures will have sensitivity for bandwidths during infancy that they will not retain into maturity and vice-versa. Carp for instance have the capability to detect light in the Ultraviolet spectrum upon hatching from their egg but lack colour vision, this helps the tiny hatchlings to hunt micro organisms suspended within the water which appear mostly transparent to our eyes but do register in the UV spectrum. Pretty soon though the baby carp grows too large to sustain itself on micro organisms and this coincides with the time that the eyes stops replacing the UV sensitive cones and starts producing the RGB set instead.

The presence of three varieties of cone cells and one variety of rod found within the eyes of carp suggests that they can almost certainly observe the same range of colours that we can but we need to understand the distribution and number of cells present upon the retina to fully grasp their likely visual capabilities.

The area at the back of the eye where the light is focused onto the retina by the cornea and lens is known as the macular region, this region is home to the rod and cone cells and basically makes up what we can observe with a single eye. At the centre of this region is an indent known as the fovea and this provides further further focusing of light by increasing the number of cells available to receive stimulation, this area makes up the centre point of observation where the sharpest image will be observed.

The arrangement of cell types is similar in both fish and people with the fovea region being populated with a majority of colour/detail sensitive cone cells and the areas outside this being populated with mostly rod cells that are more attuned for spotting movement. This arrangement is why your eyes see a huge amount of detail in what ever you are looking directly at but the periphery of your vision is a bit blurry and washed out. There are cone cells present in the periphery regions to feedback information about colour but they are few and far between, rod cells are present in the fovea but in low numbers until the ambient light levels drop to the stage where cones become ineffective, at this point the number of medium and long wave sensitive cells reduce and more rods  and short wave sensitive cones are produced to make most of the available light sources.

During daylight carp have a larger number of active rod cells within the fovea compared to humans which will handicap them slightly in terms of the ultimate observable detail, this is likely an evolutionary response to living in a relatively low light environment where it’s better to make more use of the available light than use excess light to study things in detail. Reduced numbers of colour sensitive cone cells in the focal area will not only reduce the detail of the image but also number of shades and secondary/tertiary colours they are able to observe, with this in mind we can suggest that even at close range their vision isn’t too sharp and they’d be unlikely to be able to distinguish similar shades of colour.

It should be noted that like people, fish will have differences in sight capabilities from individual to individual based on genetics and life experience, some will be more sensitive to certain bandwidths of colour or have more/less visual acuity than others. It is likely that fish that live in gin clear lakes will have slightly different visual characteristics to those that live in permanently coloured/turbid waters, in terms of catching them this doesn’t necessarily mean that clear water fish will be harder to deceive than those that live in the murk as there are always trade-offs to adaptations and sight is never the only sense at play.

This concludes Part 1, in Part 2 we will start to look at the watery environment in which the carp lives and what effect various factors have on light levels and vision. The information will hopefully start to become more practical as we continue as it will help us shape the way we use vision to our advantage within fishing scenarios.

Keep an ‘EYE’ out for Part Two!

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