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Vision is the act of perceiving and interpreting visual information with the eyes, mind, and body. The role of light in the visual process is fundamental. Once light reaches the eye, signals are sent to the brain, where the information is deciphered. The whole process, as complex as it is, would not be possible if it were not for the presence of light. Without light, there would be no sight.
As previously explored in the Animals Eyes worksheet, many animals show weird and wonderful visual system adaptations, which enable them to gain the best type of vision in response to the light levels of their environment (especially the nocturnal animals).
Aquatic animals however experience a whole new level of light restriction and visual problems as light rays behave differently when passing through dense mediums - such as water.
It is for this reason that the visual system of the fish has been chosen as a focus, in an effort to illustrate how aquatic animals can overcome visual barriers.
The ocean is divided into different "layers."
Diagram on left: Light penetration at different water
depths
Diagram on right: Indication of oceanic regions according to depth
One of the biggest restriction on light availability is penetration. As depth
increases, light availabilty decreases.
Additionally, notice in the diagram on the left, how specific wavelengths of
light are restricted at certain depths, with blue showing the highest degree
of penetrance. As water depth increases the less light there is available for
organisms present.
The direction of a ray of light, is altered at the boundary of media having two different densities (ie - air and water). This is called refraction.
Diagram showing refraction of light (also known as
the bending of light rays).
Another factor that fish (and other aquatic animals) have to contend with is the scattering effect that results from light hitting and reflecting off particles in the water. This is especially true in turbulent waters
It creates a veil-like blinding effect which obscures vision.
The fish eye fundamentally has the same visual components as the human eye, but because fish have to make the most out of any light available to them some of these components show special modifications in order to enable the fish to respond to the level of available light and capture it most effectively.
Basic structure of the fish eye
The diagram above shows a simplified diagram of the anatomy of the fish eye.
Have you noticed how similar it is when compared to the eye of humans,
or other animals?
However there are some specialised changes common to most fish species:
The lens in fish closely resembles a glass
marble or pearl bead and is responsible for magnification of an image onto retina
(this is of vital importance as the cornea
of the fish is unable to magnify light within water).
The composition of the fish lens creates a gradient of refraction
which enables the eye to correct for spherical
abberation, and a very sharp focusing of light is achieved.
As fish have immoveable necks and lenses which are incapable of shape change (as the human eye is able to do) the distance of the lens from the retina is an important factor in determining focusing quality and size of visual field.
The lens rests against the cornea for close-up vision and drawn backwards by
retractor muscles for distance vision.
Accomodation of light onto the retina
is achieved by effectively 'swinging' the lens in and out of focus by retractor
muscles. The lens is held in position by the suspensory
ligament.
The iris and the pupil
are the optical components which serve to control the amount of light entering
the eye.
As the amount of light in the aquatic environment is restricted, most fish species
show relatively little change in pupil size in order to maximise the amount
of light entering the eye.
However - some shallow water species are able to control the amount of light entering the eye, not through pupillary dilation/contraction, but through the use of coloured lenses and cornea's.
(See Adaptation section below for more detail)
The front of the eye is protected by the thick transparent cornea,
which like the lens, also refracts (bends)
light rays.
The cornea of fish is usually very transparent to most of the visible spectrum.
Many fish in the twilight zone however show yellow or coloured cornea's - which act as intraocular filters, and cut out light of shorter wavelengths to help visual acuity.
(See Adaptation section below for more detail)
Bottom dwelling fish and those that live in shallow waters have more complex
corneas.
Often their cornea is divided into 2 parts : the cornea itself and the spectacle.
The spectacle is an additional clear covering that overlays the cornea, protecting
the eyeslit from silt and other abrasive materials. Fish with spectacles frequently
have yellow filters (see below), i.e these are fish in shallow waters and needing
extra protection for their eyes.
(See Adaptation section below for more detail)
The photoreceptors are light sensitive cell types that reside in the retina of most eye types (animal or human) - there are 2 types of photoreceptors:
The actual ratio's of rods & cones and their visual pigment proportions within retinal layers can also fluctuate due to light conditions.
The active movement of these retinal components is termed retinomotor activity and occurs in direct response to light levels. Additionally many species have evolved elaborate cone/rod patterns and structures within the retina to enhance visual sensitivity.
Cones are rare in deep sea fish, as colour vision is not a priority when the amount of light available is minimal. Their retina's are mainly composed of rods, and are structurally longer and more slender than those found in fish in higher, brighter waters. Additionally - the number of rods in the retina is vastly increased (can be from 100, 000 to several million per square inch)
All of these modifications serve to ensure that available light is utilised and captured effectively
In any given environment organisms will show specific adaptations
to increase their successfulness and productivity within the habitat.
Many aspects of fish behaviour can be seen to be visually dependant, for example:
signalling, species recognition, mate choice, feeding and predation.
For many aquatic species, survival is dependant on good vision. Species therefore
within certain photic environments show definite adaptations, which enable them
to overcome the visual restrictions imposed.
In response to high environmental light levels, and the lack of pupillary control to restrict the amount of light entering the eye, some aquatic animals have shown ingenius physiological adaptation.
Some animals have a special 'eye-flap' that can be pulled over the eye to protect it, acting like a sunshade to help block too much light damaging the eye. Basically the flap is able to drawn over the pupil when light levels become too intense.
Additionally it may serve to act as extra camoflage when necessary.
Stingrays (left), and flounder (center) are the only
fish to have eyes on the same side of the head, and interestingly both possess
a filigree-like pupillary flap that can extend
over the eye and protect from downwelling light. Additionally the pupillary
flap also protects against abrasive materials (such as sand) entering and damaging
the eye.
The Cuttlefish (left) also possess a specialised eye
flap, which enables it to physiologically control the amount of light entering
the eye in response to the amount of light in it's environment
The cornea of fish is usually very transparent to most of the visible spectrum, however many fish in the twilight zone have yellow cornea's - which are heavily pigmented lenses at the front of the eye and act as intraocular filters, and cut out light of shorter wavelengths (eg blue green).
Yellow corneas act to improve visual resolution, and contrast detection, as they strongly absorb light of a short wavelength (400-500nm), which according to the above diagram shows that this relates to mainly blue/green light.
Yellow lenses actually filter out about 80 per cent of the blue light that reaches the eye
It may seem strange that an adaptation has arisen that serves to block out the most abundant light source available within the twilight zone (see diagram above "Light penetration at different water depths") which would be a real handicap if the animal depended solely on faint downwelling sunlight. However yellow lenses can actually enhance vision at these depths, as it increases the contrast between the blue-green bioluminescent light of other species (eg prey) and the blue background illumination in the twilight zone.
Yellow lenses also help predators to break the camouflage of counterilluminated
prey.
Bioluminescence that is intended to make prey blend into the background makes
it stand out instead. This ability would be useful only in the twilight zone,
and sure enough you don't find yellow lenses in fish that live deeper than 1000
metres.
(See Camoflage section below for more detail or click here for worksheet 5 bioluminescence).
Another adaptation shown by some fish types is the migration of the yellow corneal pigment in and out of the corneal cells, depending on the photic conditions.
Iridescent eyes means that certain wavelengths
of light can pass through the eye, whereas other are strongly reflected in glittering
colours.
Hence an eye with a seemingly transparent cornea is able to have some protection
and is able to minimise the glare from downwelling light.
Iridescent eyes are common to many diurnal bony fish, particularly bottom and
shallow-reef fish.
The reflective tapetum lucidum which lies behind the retina is able to reflect back some of the light which passes through the retina, and enables a greater capture and higher absorption of light by the photoreceptors, thus enhancing sensitivity.
This mechanism does however reduce perception of detail, with the added disadvantage of being extra conspicuous when illuminated by daylight due to the reflective tapeteum reflecting back light - termed 'eyeshine'.
An example of irredescent 'eye-shine' in a bottom-dwelling
fish
Reflection
The silvery colouration of many fish is created by iridescent scales. Each scale reflects 1/3 of the spectrum - where 3 scales overlap all colours are cancelled out, leaving a mirror like effect - making them more difficult to be seen by predators, as they reflect background.
An example of the use of reflective scales for camoflage
Patterns
Some species also show a continuation of their patterning into the cornea of their eye in order to increase camoflage; such as the balloonfish and butterfly fish for example
L-R: Balloonfish and Butterflyfish both exhibit markings
which try and camoflage the real eye.
Bioluminescence
In an effort to try and blend into the environment species emit light of a similar wavelength to ambient light levels, thereby disguising the shadowing silhoutte their bodies cause when perceived against an illuminated background. The use of ventral and lateral bioluminescent photophores enables 'countershading' camoflage.
Even in the twilight zone, dimly lit by the last vestiges of sunlight, bioluminescence comes in handy. An animal looking upwards will see the shadowy silhouettes of creatures moving overhead against the dim light above. Some fish and squid make themselves invisible by counterillumination, giving out light of matching intensity from photophores along their bellies.
The actual shape of the eye is very important to the visual ability of fish, with many variations, adaptations and structural modifications occurring
A prime example of adaptive eye shape can be seen with the Anablep - often
termed the four eyed fish.
It's unusual visual modification enables these species of Carribean mudskipper
to see in both air and water.
The Anablep - often termed 'the four eyed fish'
The term four-eyed is a slight exaggeration, as they are not four distinct
eyes but rather two eye divisions.
The seperate functions of the two divisions and the beauty of the eye's adaptation
to the necessity of the Anableps life is quite remarkable.
Anableps are surface dwellers - yet it is true that they can leap remarkable
distances above water, and if required can plunge to the bottom.
Anableps are far happier to cruise along the surface, however this makes them
fairly easy for predatory birds to spot. For best survival chances Anableps
must be able to spots attackers (from both above and below) in time to take
evasive action.
Diagram illustrating the structure of the Anablep eye
and its incredible modifications
The Anablep has a double retina and double iris (though the upper retina is
larger and its corneal covering is thicker).
A band of pigment divides the eye horizontally just at the waterline. Just behind
and above that band is a golden iris flap to shield the upper pupil from glare
at the waters surface. The lens behind the upper pupil is flattened so as to
provide a view as to what's going on above the water (in air) for the lower
retina. Whilst the part of the lens that is behind the lower pupil is rounded
as in an ordinary fish eye. This provides the best image of the underwater scene
to the upper retina.
As with human eyes, all images projected onto the retina are inverted and it's up to the brain to turn them around and right side up. The amazing adaptation of the Anablep is that it is able to keep track of what's above and below at pretty much the same time! It's likely that they can only attend to one image at a time, but can obviously flip back and forth beween images. These eyes aren't as delicate as they appear. They are wellprotected by a bony socket.
The Anablep exhibits a pretty amazing and unique visual system; however some of the best examples of eye-shape modification is seen in some species of deep sea fish.
Both the twilight, abyssal, and hadal zones are considered "deep sea." .
These zones are so far below the water's surface that sunlight levels reaching
these depths is limited (if at all ) - see oceanic zoning in diagram
above).
So, how do the animals living there see without sunlight?
Most deep sea animals are bioluminescent, i.e. they can produce their own light!
Many deep sea animals have body organs that produce light (due to organs called
photophores)
Were it not for bioluminescence it is probable that most deepsea fish would be blind.
Many animals living in the twilight zone have large well-developed eyes to capture as much light from their environment as possible, unlike the animals of the abyssal zone who have poorly developed, tiny eyes, as there is no light at these depths except for that generated by bioluminescence.
A tubular formation of eye is a common feature of these species which live within very dim environments, and it is thought that this unusual eye formation is able to increase the amount of light able to enter the eye. The additional retinal regions is primarily used for peripheral vision as well as providing several focal lengths.
The eye shape has had to become tubular as there is no space left in the head
of deepsea fish for the eye to expand in a circular form.
Diagram illustrating the structure of the 'Tubular
eye'
The main 'normal' retina is responsible for near vision, whereas the smaller
acessory retina to the side is used for distant vision.
The orientation of the tubular eye within the head is seen to enhance the type
of vision required. Different species of deep sea fish can exhibit quite different
eye orientation with the head.
Cones are rare in deep sea fish, as colour vision is not a priority when the
amount of light available is minimal.
The retina's of deep sea fish therefore are mainly composed of rods,
which are structurally longer and more slender than those found in fish from
higher, brighter waters. Additionally - the retina of deepsea fish when compared
to that of fish from a well lit environments show a vast increase in the number
of rods present in the retina - it can range anywhere from 100,000 to several
million per square inch!
Deep sea fish with tubular eye configuration will have proportionally larger eyes as a result.
Examples of a deep sea fish species with well developed
eye formation (note the massive eye size in comparison to head/body).
Photo Credit: Paul Yancey, Biology Dept., Whitman College, Walla Walla WA