I've been looking into fish eyes of late and believe they are an important indicator of koi and goldfish health .
When you think it all through their eyes can be very important indeed so lets look at them .
The over all structure of koi'/goldfish eyes is similar to that of humans, there are however some major differences .
A koi/goldfish's eyes differ in that they have an adaption which is unique, the lens is attached to the back of the eye by a muscle and a special ligament. Because of this, ligament, it enables the lens to move both forwards and backwards rather than being stretched in order to focus light on the back of the retina. This is similar to the human eye in that, like us, it contains both rods and cones that enables it to process both black and white as well as color images .
Unlike us the intensity and color spectrum of light experienced by them is very different to what we humans can see .
What can the eyes tell us ?
A healthy koi will have a spherical eye with a clear and dark lens. Anything from the norm in both shape and luster will show up and can be observed by us during our routine feeds .
There are a range of health issues that will have an impact on the shape, color and structure of their eyes
1) Cloudy eye[cornea]
3) Cloudy eye [lens] possible cataract
4) Bleeding eye
5) Poor eye luster
Reasons for the above
1) a) Trauma [with excessive mucus] b) infection c) environmental d) parasite
2) a) Environmental b) Gas bubble disease [supersaturation of air] c) temperature trauma or shock d)trauma to the eye e)Infection [systemic].
3) a) Trauma b) Nutritional c) Parasite [eye flukes] d) Bright light and genetic factors have been implicated [koi can become completely overwhelmed by any sudden increase in light levels]***Note
4) a) Trauma [such as puncturing] b) [Septicemia].
5) a) Poor diet [particularly that of essential fatty acids] c) lack of vitamins
6) a) Nutritional b) environmental [toxins] c) genetic.
There are a number of factors that can cause defects damage etc to a koi/goldfishes eyes .
***Note Eye flukes are a type of parasitical flatworm that are capable of afflicting a koi's eye site opening them up to attack/ predication note seen in koi ponds that have other fish species in with them.
Goldfish can do without eyes in a tank situation thanks to all its other senses it would be unwise to place either it or a blind koi into fish pond environment as they would soon fall foul of a predator.
It is on us to check our fishes eyes every chance we get, we normally use feeding time for this giving the fish a once over not just looking for problems with our fishes eyes but its overall body shape weight etc can be noted at the same time.
Other eye facts
1) While you might always see fish with their eyes wide open, it's a myth that fish don't sleep -- they just don't have the eyelids necessary to close their eyes while they rest they normally sleep by shutting down half their brain at a time.
2) Because water has a powerful ability to refract, or bend, images, fish corneas compensate by being almost the same density as water, resulting in minor bending as images are transmitted from water into fish eyes.
3) Unlike the corneas, fish eye lenses tend to refract light a significant amount of light, their lenses being spherical, means that when pointed at an image straight on, they will have a good picture in the center, but vision becomes increasingly fuzzy and refracted at the edge of vision.
4) To focus, fish must point directly at something, meaning that while their vision exists almost in 360 degrees, their vision is clearest in the middle of the picture.
5) Fish completely lack an outer eyelid. In most fish, this translates to no eyelid at all.
6) Some fish have light-activated pigments in their eyes that shade out bright lights by becoming darker.
7) They have no lachrymal glands [tear ducts] living in water which is constantly washing their eyes thus as such they have no need of them.
8) An interesting aspect of the of the fish's eye, and its placement on the side of the fish's head is that it is bifocal, this is because, while the fish's lens is spherical, the eye itself is elliptical, this means that light entering from some directions has a short journey to the retina than light from other directions, what this means to the fish is that its eyes are close focused to light from in front, but distanced focused to light from the side, or from behind.
9) Most fish can see in ultraviolate .
10) The visual system in fish is augmented by other sensing systems with comparable or complimentary functions.