FISH SENSES: SIGHT

The term fish includes 33,249 different species, more than all mammals, birds, reptiles and amphibians put together!

This vast array of animals has evolved amazing survival strategies that are worth knowing!

We will begin this journey by debunking some common myths and legends; we will put ourselves in the shoes, or rather in the fins of these extraordinary animals, knowing them better!

Let's start with their senses and in particular with sight!

“Our sensory world is very different from yours, simply because we evolved in water and you on land.”

The sight of the fish

Fish eyes are structurally similar to ours. In fact, since our evolutionary history derives from them, we can think that it was fish that evolved this visual system!

Fish are able to focus and rotate their eyes in all directions. Thanks to their spherical lenses, they can see as clearly underwater as we can through the air.

“Unlike you, we do not have eyelids or tear glands, because living in water there is no need for us to lubricate our eyes!”

Independent eyes

Some fish, such as seahorses, are even able to rotate their eyes independently!

Ma questo cosa significa? ci avete mai pensato?

These animals have two independent fields of vision to process, so they think about two things at the same time and quite distinctly: how to search for a mate with one eye and control a lurking predator with the other!

“Are you able to think about two things at the same time?!”

Binocular vision

What is it? The ability to convert two images, from each eye, into one.

This vision is very useful to animals that live in ambush on the bottom such as flatfish (sole, turbot, plaice...), perfectly camouflaged, while waiting for a potential passing prey.

Binocular vision ensures effective perception of depth and therefore of the distance between prey and predator!

Independently moving eyes, very close together and located on the same side of the body, are capable of creating binocular vision.

“When you pass over us curiously with all your bubbles, we have already been watching you for a while!”

Solea

Warm eyes and cold body

Depth is not just for divers! Some large open-sea predators, such as sharks and swordfish, can be found at very great depths.

What is at the bottom of the sea?

The dark and the cold!

These two elements combined are not exactly eye-catching but someone has learned to manage them!

“The cold slows down movements and reasoning, lengthening reaction times. If we are not used to it and do not know how to counteract it, we could run big risks.”

Large predators exploit the heat produced by their swimming muscles by carrying it to their sensory organs, allowing them to work better!

The swordfish is able to heat its large eyes up to 10-15 degrees above the water temperature. If you consider that in water heat is dispersed much faster than in air, this is certainly an excellent result!

Tapetum lucidum

The tapetum lucidum is a layer of reflective cells behind the retina that doubles the shark's night vision. A very effective strategy adopted by sharks to hunt at night, when even a very small source of light can be useful. When the little light hits this layer of cells, a glow is produced just like in the eyes of cats and other nocturnal land predators when they are illuminated in the dark.

Color vision

In 2014, scientists found evidence of rods and cones like ours in a 300 million year old shark-like creature!

A sensational discovery! But what are they?

They are photoreceptors that translate external light into information for the brain: the cones are responsible for color vision and image sharpness, while the rods are more sensitive to moving objects.

“This is confirmation that color vision evolved in an aquatic environment, before you even set foot on Earth.”

Furthermore, fish have a superior visual capacity to ours; in fact…

“We are tetrachromats and see colors more vividly than you who are trichromats and therefore see a more limited spectrum of colors, which is also why the coral reef is so extraordinarily colorful!”

Coral reef

Some species also see light in the ultraviolet (UV) spectrum, especially in coral reef environments. The skin of more than 100 species of fish, belonging to different families, reflects large amounts of UV light. What does this mean?

This means that there is a communication based precisely on ultraviolet vision… For example:

• Two species of damselfish (a small fish typical of the coral reef) coexist in the coral reefs of the Western Pacific and to the human eye they are identical, except that one of them is very territorial and defends its territory against members of the same species. But if they are identical, how can it tell if it is an intruder of the same species or the other? The two species have very different facial features, visible only in the ultraviolet light spectrum!
  

“We are too smart!”

Optical illusions

The fight for survival has led to the development of exceptional strategies, including optical illusions!

Fish that deceive or are deceived demonstrate their ability to see the world as we see it and therefore have a precise perception of what is around them.

Let's see some optical illusions invented by our friends:

Predators tend to attack their prey's head to ensure a lethal attack, aiming directly for the eyes. So...

Some families of fish such as butterflyfish, angelfish and pufferfish, have developed an optimal solution to escape from predators, that is, a fake eye drawn on the final part of the body, right near the tail!

“We will make this eye very showy with bright and flashy colors, so as to steal the attention of our unwary predator!”

At the moment of attack, the predator confuses itself by attacking towards the tail and not towards the head of the fish, giving it the opportunity to escape skillfully!

“As if that wasn’t enough, some of us are able to swim backwards to further confuse matters!”

Change color

In nature, color is given by pigments, substances capable of absorbing light and reflecting it, taking on a particular color.

• Just to clarify: each pigment is able to absorb a specific wavelength of the solar spectrum and therefore give a specific color. They are found inside cells called CHROMATOPHORES.

• Where are these cells? They are mostly found in the skin of animals, and by contracting or relaxing, they are able to change the color of the animal itself!
  

“If wearing brightly colored clothes is useful, having the ability to change them is even more interesting!”

Chromatophores are often used to intensify the color of the skin for communication purposes such as courtship or competition; but also to tone it down, going unnoticed by a predator or to stop some dispute!

Camouflage

Without a doubt, the masters of color change are the animals capable of camouflage!

This extraordinary ability to mimic the underlying background by changing the arrangement of pigments on the skin is a complicated process involving hormones and vision.

But what does this mean?

These animals have an absolute perception of the environment around them and thanks to their sight they replicate it on their body!

“Simply extraordinary!”

Scorpionfish

What we know about fish is only a fraction of what they know!

Cit: Jonathan Balcombe