These two green moon wrasses (Thalassoma lutescens) (right) were playing together in Emily Bay. In the photo, below, which is taken from above, you can see that one eye is looking up at me, while the other one looks sideways. Which got me wondering. Do fish have eyes that move independently? Well, no, not really, but, yes, sort of, in some species, sometimes!
I'd better explain (very broadly!).
The left eye is looking up at me, while the right one is surveying the locality
We, as predators, have eyes that work together – they are coordinated, or closely ‘yoked’. This enables us to judge distance (binocularity), which is handy as we are predators.
Fish have eyes on the side of their heads because they can be both prey and predator, meaning they can judge distance and still have a relatively wide range of vision, which is good when you want to see who is coming at you looking for a snack.
Independent, asynchronous (or uncoupled) optokinetic movement (for that read, the eyes moving independently of each other) is a phenomenon seen at one extreme in chameleons, giving them an unparalleled range of vision. Humans, cats, and other predators must move their eyes together for them to be effective at their job, so they are yoked. They move synchronously.
Fish, meanwhile, are somewhere in the middle of this continuum. Their eyes can’t be used to measure distance because of where they are placed on their heads, so they don’t need to be so closely yoked. Different species of fish can have different amounts of asynchronous eye movement, so there is a range, from tightly yoked to being able to move independently (but still not quite amazing as our friend the chameleon!).
As can be seen in this wrasse, they are capable of some independent movement. He’s looking at me and he’s surveying the environment. Clever!
Let’s focus
While we are talking about fish eyes, I thought a brief explanation of how a fish focuses on objects might be helpful. Mammals and birds will change the shape of their lenses (deform them) in order to focus, whereas fish will move their lenses, either nearer to the retina, which is at the back of the eye, or further away, towards the front of the eye.
Other fast facts:
Their lenses are more spherical than ours.
Most fishes can see in colour and some can see ultraviolet and polarised light, too.
Each species has eyes adapted to its habitat, for example if they live in deep, gloomy places, they will often have larger eyes. Likewise if a fish is more active at night.
And some trivia:
In 1906, American physicist and inventor conducted an experiment to find out how a fish would see beneath the water.#
Physicist Robert W. Wood invented what would become known as the fisheye lens in his lab at Johns Hopkins University by using ‘a bucket full of water, a pinhole camera, mirrored glass, and a lot of light’.*
Today, mass-produced fisheye lenses are a favourite of many photographers.
A notch-head marblefish (Aplodactylus etheridgii), Emily Bay, Norfolk Island. You can read more about this smiling chracter, here: The smiling notch-head marblefish
References
Fritsches, A & Marshall, NJ 2002, ‘Independent and conjugate eye movements during optokinesis in teleost fish’, Journal of Experimental Biology, vol. 205, pp. 1241-1252.
Land, M 2019, ‘Eye movements in man and other animals’, Vision Research, vol. 162, pp. 1-7.
*PetaPixel: A Brief History of the Fisheye Lens and How it Became a Music Photography Icon
#Wikipedia: Fisheye lens.
NB This blog post is very general. Fish eyes, like all other animals’ eyes, are complex and vary from one species to another.