#me rapidly redesigning the cast so they have beautiful crystal eyes

[bronzewool]

The various types of vision found in fish species that has forced me to rethink the use of light and colour in my book:

Visible light is split up into various wavelengths, with ultra violet on one end of the spectrum and red on the other. Ultraviolet is the shortest wavelength and red is the longest. Water absorbs red more easily than blue, so at a depth of 1 metre, 25% of red light entering the water is already absorbed, and at 10 metre 85% of blue is absorbed.

To put that into perspective, if a fish is looking up at a red and blue object from 1 metre underwater, the red object appears grey. At 100 metres all colour is absorbed and fish can only see in black and white.

Types of Colour Vision

Tetrachromatic Vision: Fish that can see all four colours - Red, blue, green, and ultraviolet

Goldfish

Rainbow Trout

Zebrafish

Trichromatic Vision: Fish that can see three colours - Red, blue, green

Cichlids

Dichromatic Vision: Fish that are sensitive to red and see in blue-green

Catfish

Largemouth bass

Micropterus salmoides

Monochromatic vision: Fish that are colour blind

Sharks

Rays

Whales

Bonus Trivia

The lens of fish eyes is spherical, unlike human eyes. The lens is fixed in its shape, meaning it cannot be adjusted to facilitate focusing on nearer or more distant objects, instead, the lens moves forward and backwards.

Fish can see underwater AND on land, in contrast to humans who become near-sighted underwater.

Fish are better at seeing in low-light

Fish can see in the dark

Many deep-sea creatures have developed advanced vision where instead of distinguishing colours, they can see wavelengths, such as octopus.

Despite most fish seeing more colours than humans, vision is still their weakest sense. They are more sensitive to smell, sound and vibrations.

Many fish have speckling around the iris. This is due to how the iris consists of three layers, with the outermost layer made of guanine crystals. This reflects 90% of incoming blue-green light downwards rather than upwards and back towards the sun, so they're not visible to predators.